The Active Female: Health Issues Throughout the Lifespan 2008th Edition

26. Exercise Prescription and Pregnancy

Claudia Cardona Gonzalez1, 2  Elvis Álvarez Carnero  and Jacalyn J. Robert-McComb 

(1)

Department of Sports Fundamentals, Universidad Europea de Madrid, Madrid, Spain

(2)

Department Fundamentos de la moticidad y el Entrenamiento Deportivo, Universidad Europea de Madrid, Madrid, Spain

(3)

Biodynamic and Body Composition Laboratory, Sports and Physical Activity Sciences, University of Malaga, Málaga, Málaga, Spain

(4)

Department of Health, Exercise, and Sports Sciences, Texas Tech University, Lubbock, TX 79409, USA

Claudia Cardona Gonzalez

Email: claudiaandrea.cardona@gmail.com

Elvis Álvarez Carnero

Email: ecarnero@uma.es

Jacalyn J. Robert-McComb (Corresponding author)

Email: jacalyn.mccomb@ttu.edu

Abstract

Sports and physical activity participation are an increasingly widespread habit among women of reproductive age. It is interesting that a pregnant woman has the opportunity to continue her exercise program without having to postpone it until after the puerperium. The knowledge about physiological adaptations either pregnancy or exercise must be the cornerstone which drive the exercise prescription on this specific population. Before implementing an exercise program, physical activity professionals need a deep knowledge about the physiological changes and the effects of exercise during the pregnancy either for women and fetus. This chapter focuses mainly on the effects of physical activity in pregnancy which would promote positive adaptations without increasing the risk of mother and the fetus. Additionally, we offer a review of the main exercise prescription guidelines from the most important institutions in physical activity area. Finally, practical ideas for exercises are suggested.

Keywords

Gravid womenExercising during pregnancyExercise guidelines during pregnancy

An erratum to this chapter can be found at http://​dx.​doi.​org/​10.​1007/​978-1-4614-8884-2_​33

26.1 Learning Objectives

After completing this chapter, you should have an understanding of:

·               The most important acute and chronic adaptations during pregnancy as a consequence of physical activity practice and/or exercise training;

·               The benefits of regular physical activity for gravid women and fetus;

·               How to avoid risks for fetus and mother associated with exercise training;

·               Recommendations and guidelines to prescribe rational exercise programs for pregnant women during pregnancy and post-delivery.

26.2 Introduction

While pregnancy involves changes in women’s physiology, several adaptations from regular physical activity (PA) practice must occur during this period of a life cycle. Although there has been a large discrepancy about the effect/consequences of exercise in the past, research has highlighted evidence about the benefits of exercise and how to decrease risks. So PA and exercise training must provide several benefits for both the mother and the fetus: blood pressure control, blood glucose regulation, healthy birth weight, and prevention of obesity and maintenance of pre-gravid physical fitness. Nevertheless, there appears to be some risks (hyperthermia, hypoglycemia, chronic fatigue, abortion, etc.) associated with excessive and poorly planned exercise regimens. These risks occur mainly when environmental conditions, volume and intensity thresholds of exercise load, and contraindications are not respected. Associations of obstetricians and gynecologists from different countries have collected scientific knowledge since the last century to develop useful guidelines to design safe and effective PA and exercise training programs.

26.3 Research Findings and Exercise Physiology During Pregnancy

26.3.1 Physiology of Pregnancy

The pregnancy period is a vital life cycle of the women that last 40 weeks in order to gestate a new human body. Parturition is the rather formal term for birth, and labor is the sequence of events that occur during birth. Classical pregnancy is divided into trimesters, each of them characterized by milestones. Weeks are also used to quantify gestational age at delivery, anatomical and physiological development of the fetus, or an assessment agenda (ultrasound or blood samples). In order to support the life growing inside her body, the pregnant woman undergoes several organic morphologic and functional changes (Table 26.1). The purpose of this section is to describe the most important physiological changes and mechanisms operating during pregnancy which are relevant to physical performance and health.

Table 26.1

Physiological changes during pregnancy

System

Function

Mechanism

Cardiovascular

Higher peripheral oxygen demands (from 50 to 500 ml/min)

Cardiac Output increases by 40 % (Heart rate increases at the beginning up to 10–15 bpm and systolic volume by 10–12 %)

Peripheral resistance decreases from week 12 to week 24, becoming normal later

Blood returning to the heart is more oxygenated

Resting respiratory rate is reduced whilst vital capacity is preserved

Oxygen uptake increases from 15 to 20 % during the second half of pregnancy. This is caused by growing oxygen uptake from the uterus, placenta, and fetus

Resting HR in the pregnant woman increases by the increase in gonadotropin hormone, the lower activity of the parasympathetic system, and reduced concentration of blood catecholamine

This is caused by vasodilatation produced by hormones

Minute volume increases more than oxygen uptake

This is caused by a slight increase in inspiratory capacity

Blood

Plasma volume increases gradually until the 32 weeks (30–60 %)

Red blood cells number and size increase

The veins increase their capacity and peripheral vascular resistance decreases

This causes a hemodilution of the blood causing the physiological anemia of pregnancy

Produced by increased renal erythropoietin

Produced by effect of progesterone

Respiratory

Resting hyperventilation to compensate alkalosis (increased ventilation from 6 to 9 L/min)

This is produced by the increment of the tidal volume, which removes more CO2 from blood, this raises PH. Also, it helped by chemoreceptors enhanced sensitivity to CO2 in order to prevent fetal ischemia and acidosis

Renal and urinary

Dilated ureters and renal pelvis producing an increase of the dead space and a delay in the elimination of urine

Increased kidney size

Diastolic decreases 5–10 mmHg

Increased renal plasma flow, in the first quarter (600 to 836 ml/min)

This is caused by aldosterone and estrogen release, which balances progesterone

Caused by progesterone activity

Increased renin secretion and activation of the axis renin–angiotensin–aldosterone

The increased glomerular filtration. Later, it decreases

Gastrointestinal

Nausea, vomiting

Predisposition to tooth decay and gum hyperemia

Delay in time for gastric evacuation producing obstipation

Pyrosis

Associated with hormone secretion (gonadotropins and estrogens)

Related to hormone concentration in saliva

The growing of the uterus, moves bowel and stomach

Cardiac sphincter relaxation causes the hydrochloric acid in the stomach to reflux into the esophagus

Metabolic

Diabetogenic effect of pregnancy

Change in blood lipid profile

Increased resting metabolic rate

This is due to some hormones like cortisol, estrogens, and lactogen from placenta can have blocking effects on insulin (insulin resistance). Pancreas can naturally produce more insulin, causing gestational diabetes

Lipids increase from 600 to 900 mg/ml. Produced by the influence of estrogens and cortisol

Caused by the increased demands from gestational state

Water metabolism

Increment in total body water

Hydrostatic vessels pressure

Increase in lower limb blood flow return

Capillary permeability

Sodium retention

Dermatological

Increased pigmentation

Possible appearance of stretch marks

Increased sweat secretion

Caused by estrogen activity

Hormonal activity produces muscle distension and low ligament elasticity

Sweating glands tend to have a higher activity due to elevated hormonal secretion

Skeletal system

Ligaments become more relaxed (Sacroiliac, sacrococcygeal, and pubic joints)

Increased lumbar dorsal curvature (lordosis)

Pain in zones around peripheral innervations

Frequent muscle cramps in the third term, especially in legs

Caused by relaxin

Produced by the displacement of the center of mass

Produced by liquid retention and relaxation of ligaments by hormonal increase

Related to sodium depletion

Hormonal changes

Human chorionic gonadotropin

Estrogens

Progesterone

Human chorionic gonadotropin develops the placenta

Estrogens increase the size of the uterus and prepare milk ducts for breastfeeding

Progesterone retains pregnancy and develops the lobules of the breast

Body weight

Increase between 9 and 12 kg

This is due to fetus growth; also mother gains fat mass, liquid, uterus blood volume, amniotic liquid, and placenta and breast tissue

26.3.1.1 Cardiovascular Function

The most striking changes occur at the level of the cardiovascular system. The excess heart load (caused by increased-weight bearing) should promote myocardium hypertrophy, either volumetric or wall thickness [1]. By the end of the first trimester, the raising of the diaphragm increases resting cardiac output throughout the pregnancy and peaks around the 20th week (40 % more than in the nonpregnant state), remaining steady during the final 3 months. The latter adaptation induces several hemodynamic changes at rest [2]. Systolic blood pressure (SBP) during pregnancy is completely stable, while the diastolic blood pressure (DBP) falls from 5 to 10 mmHg; this may be a consequence of a reduction in peripheral resistance and the development of circulation in the uterus and placenta up to 22 weeks.

The blood volume increases up to 40 %, peaking during weeks 32–34. However, plasma volume increases more than the blood cells, which produces hemodilution and less blood viscosity; as a consequence the circulation time is reduced, resulting in physiological anemia. Even the oxygen content difference between arterial and mixed venous blood (a-V O2 difference) decreases during the first few months. Oxygen uptake (VO2) increases between 15 and 20 % at the second half of pregnancy, mainly by a larger amount of oxygen consumed by the uterus, placenta, and growing fetus [3]. The a-V O2 difference returns to previous levels (normally) in the third quarter.

26.3.1.2 Pulmonary Function

During pregnancy higher ventilation (VE, L/min) is observed at rest and during incremental exercise than during the nonpregnant status [4]. Residual respiratory capacity is reduced while vital capacity is not altered as a consequence of a slight increase in inspiratory capacity. Tidal volume increases, even though the respiratory rate is held constant. Gravidarum hyperventilation leads to a compensated respiratory alkalosis, which decreases the concentration of carbon dioxide in the blood and increases pH slightly [3].

26.3.1.3 Endocrine System

The endocrine system of pregnant women undergoes significant changes due to hormone production in the placenta. So the stimulation of the pituitary–adrenal axis raises the production of corticotropin (ACTH). This is associated with an increase of total and free cortisol [5]. The thyroid function is relatively stable during pregnancy, although the concentrations of T3 and T4 in plasma are increased [6]. However, thyrotropin (TSH) secretion may be reduced in early pregnancy.

PA promotes positive acute and chronic effects on insulin resistance syndrome, and pregnant women are not an exception [7]. Under resting conditions, catecholamine levels remain as before pregnancy; however, a misbalance between hormones from the endocrine pancreas can be observed (glucagon and insulin). This is due to the antagonized diabetogenic hormones of the placenta (human lactogen). Both alterations would contribute to insulin insensitivity, commonly diagnosed during pregnancy. Peak concentrations of glucose and insulin are progressively higher, and insulin sensitivity reduces up to 80 % accordingly to the tolerance test glucose (TTG). This previous alteration would state a grade of insulin resistance during the late phase of pregnancy.

26.3.1.4 Metabolism, Energy Expenditure, and Weight Control

A physiological weight gain must happen during pregnancy to support the growth of the fetus at the beginning of the third trimester, so there is an increase between 3.5 and 5 kg of fat deposits (mainly from the 10th to 30th week). This alteration is associated with a normal range of increased weight, which is suggested to be dependent of the pre-pregnant body mass index (BMI [see Table 26.2]).

Table 26.2

Rates of weight gain during pregnancy, as related with the pre-gravid (pre-pregnancy) weight status

Pre-gravid BMI groups

Recommended total weight gain (kg)

Recommended rate of weight gain (kg/month)a

Underweight (<19.8 kg/m2)

12.5–18

2.3

Normal weight (19.8–26.0 kg/m2)

11.5–16

1.8

Overweight (>26.0–29.0 kg/m2)

7–11.5

1.2

Obese (>29.0 kg/m2)

7.0 minimum

2.0–0.9

Adapted from Gunderson, E. P. (2003). Nutrition during pregnancy for the physically active woman. Clin Obstet Gynecol46(2), 390–402

BMI body mass index

aRate of gain applies to gain during the second and third trimesters

A proportional increase of energy intake from a balanced diet must be ensured. There must be enough of a weight gain to support the needs of fetus. During pregnancy, a daily intake around 300 kcal of additional energy is required to maintain metabolic homeostasis. Pregnant women performing exercise should eat a proper diet well balanced in energy [8] and micronutrients [9]. Women who are exercise training should increase their energy intake proportionately to meet the energy costs of the exercise [8].

Connected with weight and body composition modifications, there are some normal alterations in energy expenditure, fat and carbohydrate metabolism [10]. Regarding the components of total daily energy expenditure (TDEE), basal metabolic rate is elevated during pregnancy. The increase in TDEE could be related to the additional cardiac and renal costs observed during the first part of pregnancy, in addition to the growth of the placenta, uterus, and fetus during the second part of pregnancy [11]. Both of these factors contribute to the increased TDEE described in women under gestation [10]. Other mechanisms which could explain body composition and TDEE alterations are related to changes in fat metabolism. There is an increase in the production of triglycerides in the plasma up to 3 times normal. The increased concentration of free fatty acids during late pregnancy is 2–4 times above normal [5]. This is a consequence of protective mechanism, where the mother stores carbohydrates and uses more fat as an energy source in order to preserve placental, fetus, and uterus demands. The increment in cortisol and impairment of insulin sensitivity help to meet the glucose needs for a healthy growth of the fetus [5]. So during the third trimester acetone in the plasma is increased and fasting plasma glucose is decreased, a metabolic state characterized by hypoglycemia, hypoinsulinemia, and hypoketonemia can occur [1].

PA energy expenditure may not be a primary cause to increase energy intake, since improved daily task efficiency has been well documented in gestating [10].

26.3.1.5 Musculoskeletal System

Several adaptations during pregnancy and labor will promote changes in musculoskeletal tissues. The action of the hormone relaxin progressively leads a softening of the ligaments, particularly at the region of the pubic symphysis and sacroiliac joints. This softening reaches the peak at the beginning of the third trimester. So the growth of uterus size induces great pressure against the lumbar spine. This increases lordosis, joint angle changes, and joint relaxation, which lead to lumbar pain. Finally, relaxed pubic symphysis can move a few millimeters causing pain when walking or standing. All of these adaptations will be important concerns when selecting activity to prescribe exercise during pregnancy.

26.4 Contemporary Understanding of the Issues

26.4.1 Benefits of Exercise During Pregnancy

Since practices of PA and exercise training have positive effects on the health of nonpregnant women, the outcomes highlighted in Table 26.3 could be expected during pregnancy (see Table 26.3). Sadly, a decrease in daily PA has been widely reported along gestation [10]. Misperceptions about the risks of exercise during pregnancy might be one of the most important reasons explaining this behavior [12]. For example, Zhang and Savitz [13] have shown that 60 % of pregnant women were sedentary, which represented twice the sedentarism of the US adult population.

Table 26.3

Possible benefits of exercise during pregnancy

Improvement of cardiovascular fitness

• It decelerates heart rate

• It improves circulation

• It helps prevent varicosities

• It help to regulate blood pressure

Improvement of muscle fitness

• It improves muscle tone

• It reduces cramps

• It corrects posture

• It eases back pain

Prevention of excessive weight gain

• It improves the general physical condition of the pregnant woman and it reduces the risks of pregnancy and labor

• It helps to manage cellulite

• It reduces fluid retention

Digestive system regulation

• It reduces digestive discomfort

• It reduces obstipation

Psychological well-being enhanced

• It reduces: Fatigue, depression, and insomnia

• It helps control anxiety

• It helps to reduce stress

• It creates healthy lifestyle habits

Prevention of gestational diabetes

• It helps to regulate glucose and insulin needs

• It prevents excessive weight gain

Enhancement of postpartum recovery

• It reduces hospitalization time

• It reduces cesarean section risk

• It helps restore the physical appearance

Despite old beliefs about the harmful consequences of exercise during pregnancy, there is more than ample evidence about the healthy outcomes of exercise for both the mother and the fetus [14]. The risks associated with exercise training are well known and easily managed [1415]. Table 26.3 highlights the benefits associated with regular PA or exercise training during gestation.

26.4.1.1 Improved Aerobic Fitness

Hormonal and physiological changes occurring in pregnancy will affect the cardiovascular system during exercise [16]. Cross-sectional evidence suggests that aerobic fitness as assessed by VO2max can be reduced in women who do not practice aerobic exercise during pregnancy. However, if women continue to exercise during their pregnancy, aerobic fitness should be maintained as long as they are active during their pregnancy [17]. Female athletes (athletes vs. sedentary) are generally able to maintain their level of fitness (VO2max, power output, heart rate [HR] at anaerobic thresholds) during pregnancy if they continue to train [18]. As a general rule, VO2max will not be reduced during pregnancy if women maintain their exercise training, at least when expressed by L/min [19]. However, if the unit is expressed in relative terms (ml/kg/min) rather than absolute terms (L/min), a slight reduction of 9 % would be observed during the first weeks of postpartum. In athletes, this diminution might be recovered 4 months after delivery [20].

A well-reported adaptation is improved efficiency during pregnancy for both weight-bearing and weight supported activities. This effect is correlated with the enlargement of non-consuming-oxygen tissues during exercise such as fat mass, placenta or extracellular fluid. Therefore, absolute VO2 (L/min) remains unchanged during gestation [3]. During pregnancy it is unusual to see improvements in VO2max or aerobic power; therefore, the purpose of aerobic training should be to induce healthy physiologic outcomes in child bearing women that will augment the welfare of the mother and the fetus, such as a reduction of insulin resistance [21]. Another example is that trained pregnant women have a lower resting HR than untrained pregnant women which allows active gravid women to have a greater cardiac reserve than untrained gravid women [22]. However, during pregnancy there is an increase in HR (about 15 beats per minute [bpm]) compared to the nonpregnant state, regardless of the exercise regimen.

26.4.1.2 Decreased Lumbar Pain

Lumbar pain is a common event of pregnancy. At least 50 % of pregnant women suffer lumbar pain [23]. It seems that this incidence is lower in female athletes, due to improved muscular fitness. However, this does not mean that exercise prevents all lower back pain in pregnancy [24]. It does seem that exercise during the second half of pregnancy reduces the intensity of back pain; the mechanism proposed was an enhanced flexibility of the spine without altering the lordosis.

A second paradigm related with the etiology of lumbar pain has been focused on weight gain and the parallel loss of stability of the pelvic girdle. This loss of stability of the pelvic girdle is due to hormonal changes which causes weight gain in the abdominal region. During pregnancy, women experience hormonal changes that occur in preparation for delivery. The hormone relaxin is released, which loosens joints and ligaments, which can cause hip pain and other aches. As the baby grows and extra weight is placed on the pelvis, the pelvis can shift, which causes pain. Changes in a woman’s posture can also contribute to hip pain, as her back and muscles are pulled in a different way to carry the baby. This places strain on the muscles and causes the pelvis to tilt out of alignment. Back exercise treatment has implications not only for pregnant women, but also for health care costs and labor productivity. Since one goal of the exercise program is to restore joint biomechanics, lumbo-pelvic stabilization following proper posture training must be a cornerstone strategy for exercise prescription during gestation. A study published in 2005 reported significant reductions in the intensity of lumbar pain and improved mobility of the spine, in spite of no observed changes in lordosis [23]. So it appears that internal mechanisms, more than the lordosis angle, are responsible for low back and pelvic pains; the hormone relaxin has been suggested as an important factor [25].

26.4.1.3 Weight Control

Exercise promotes important benefits for weight control during gestation and postpartum. Preventing a body weight increase >10 % of gestational mass reduces the risk of diabetes or hypertension, and the probability of delivering a macrosomic baby [14]. In accordance with traditional studies, exercise combined with diet is the best way to control weight gain during pregnancy. Thus, gravid women who performed physical exercise gained less weight, without impacting negatively on the fetus [26]. However, it seems that only individualized nutrition and PA programs are successful to avoid increasing body weight during pregnancy. Some examples of successful interventions were aqua-aerobics 1–2 days/week; supervised walking/biking at 60 % of VO2max; walking 3–4 days/week at 30 % of HR reserve; or resistance training with a personal trainer [14].

Additionally, controlling weight prevents gestational diabetes and possible future obesity (next paragraph). Also, a healthy weight facilitates the delivery; hence, women who exercise during pregnancy had shorter labors as well as labors that were faster and easier [2728].

26.4.1.4 Prevention of Gestational Diabetes

Gestational diabetes is a temporary condition which occurs around the end of pregnancy as a result of the action of insulin and placental hormones. These women develop a physiological insulin resistance state and generate macrosomias in the fetus. A lack of PA as well as being overweight or obese can lead to diabetes mellitus and an increased risk of preeclampsia [29].

Some studies have indicated that women involved in 30 min (min) of moderate PA on most days of the week during pregnancy reduced the risk of gestational diabetes compared to sedentary women 50–75 % [30]. Even, among overweight and obese gravid women with gestational diabetes mellitus, a simple exercise program (25 min 3–4 days/week, and increments of 2 min/week until 40 min) showed important benefits in glucose and insulin regulation [14]. Likewise, exercise has been shown to improve the lipid profile, hence lowering the risk of diabetes [17].

26.4.1.5 Hypertension and Preeclampsia

There are several disorders related with high blood pressure during gestation; the most prevalent are gestational hypertension, chronic hypertension, and preeclampsia/eclampsia [31]. Preeclampsia is a disorder related to hypertension, which occurs in 3–7 % of pregnancies. Women with preeclampsia may have glucose intolerance, hypertriglyceridemia, systemic chronic inflammation, and endothelial dysfunction. Furthermore, preeclampsia has been associated with perinatal complications and it is one of the most important reasons for maternal mortality [32]. The risk of preeclampsia appears to be reduced about 30 % when performing PA before and during gestation [3334]. Moreover, it seems that pregnant women who suffer anxiety or depression are 3 times more at risk for preeclampsia. Therefore, regular PA would be an additional benefit for them [2634]. Although several studies have shown the positive effect of PA on blood pressure regulation during pregnancy, several variables could influence the results. A recent review analyzed the influence of exercise on hypertensive disorders and it concluded that exercise seems to protect against these complications, but more randomized control trials must be done to confirm a causal relation. As a suggestion, data from large cohort studies suggest that >25 times/month or 270–419 min/week of leisure PA can help gravid women reduce the likelihood of suffering preeclampsia [14].

26.4.1.6 Psychological Benefits

Some psychological dimensions have been studied in pregnant women undergoing exercise training: Body image was shown to improve, symptoms of depression were reduced [35], self-esteem increased [36], and level of stress may be lowered in exercising pregnant women [37].

Regarding female athletes, additional psychological benefits could be attained among well-trained women, namely, it seems that they return to competition sooner and they have more confidence and self-motivation [38]. Improved performances observed after having children have been attributed to either physiological or psychological reasons [39]. Table 26.3 highlights both the physiological and psychological benefits of exercise for pregnant women.

26.4.1.7 Benefits for the Fetus

Less Complications During Labor

A research conducted in 1984 showed that women who performed resistance exercises at the same level as before pregnancy and continued to exercise until the third trimester of the gestation period, gained less weight, and gave birth faster. Additionally, their babies were thinner than those from women who quit their exercise regimen before 28 weeks [40]. It seems that women who trained at medium intensity gave birth later than those who did exercise at high intensity exercise, although the training duration was less in the first (training at medium intensity) than in the second group of active women (training at high intensity). In addition to this, most studies concluded exercise reduces the duration of the active stage of labor and diminishes the incidence of obstetric difficulties during labor [41]. However, a meta-analysis on the incidence of obstetric difficulties during labor found no difference between mothers who did exercise programs and control groups in duration of labor, birth weight or APGAR score [15]. The APGAR score is a system of assessing the general physical condition of a newborn infant based on a rating of 0, 1, or 2 for five criteria: HR, respiration, muscle tone, skin color, and response to stimuli. The five scores are added together, with a perfect score being 10. Nonetheless, it must be difficult to isolate the effects of exercise on the characteristics of the fetus at birth, since it must be influenced by many other factors such as genetics, nutrition, socioeconomic elements, and environmental factors [42].

More Active Children

Children activity must be a key component for a healthy development. An interesting study examined children between 1 and 5 years and analyzed the mother’s level of PA influence on motor and intellectual capacities of their children. The authors concluded that at 1 year of age, children whose mothers exercised during pregnancy showed improved motor skills, but mental abilities and morphological characteristics were identical to those of mothers who were not involved in training. When 5-year-old children were assessed the children of exercising women were much thinner and had much better levels of intelligence (mainly in oral skills) than the children whose mothers were not involved in training. The consequences of these data on future life remains to be elucidated [26].

Studies conducted for 5 days in children of mothers who maintained their exercise programs during pregnancy resulted in differences in the profiles of babies compared to sedentary mothers: the babies of exercising mothers were more responsive to environmental stimuli and bright light, with better motor organization according to the scale of humor [43]. However, all of these positive outcomes need to be confirmed by other studies with different samples.

26.4.2 Risks of Exercise in Pregnant Women

As pointed out in the previous paragraphs, being physically active during pregnancy can result in several positive outcomes. Nevertheless, there are many ways in which sport and PA during pregnancy may induce risk to the mother and fetus. Therefore, certain exercises must be avoided or strenuous PA when specific conditions appear. The American College of Obstetricians and Gynecologists (ACOG) has delineated several guidelines in order to guide pregnant women and PA professionals to prevent risks associated with exercise and PA practice [4445] (see Fig. 26.1).

A145875_2_En_26_Fig1_HTML.gif

Fig. 26.1

Absolute and relative contraindications for and during practice exercise. Adapted from Artal, R., & O’Toole, M. (2003). Guidelines of the American College of Obstetricians and Gynecologists for exercise during pregnancy and the postpartum period. Br J Sports Med37(1), 6–12; discussion 12

Additionally, complications related with poorly planned exercise have also been reported. For example, severe hypoglycemia can overcome gravid women after a sharp and intense exercise, which if repeated chronically can prompt malnutrition and low birth weight in the fetus [46]. Chronic Fatigue is other common symptom associated with erroneous exercise prescriptions; this must be a main concern when planning PA for gravid women, because the physiological characteristics of pregnancy can induce early fatigue. The excess body weight gained during gestation must be a factor inducing fatigue from low workloads. Also, due to increasing human chorionic gonadotropin hormone (better known as hCG), the hemodynamic changes and lower parasympathetic activity of pregnant women induce a higher HR of about 15 bpm than while nonpregnant; hence, it should be normal that gravid females got more tired than in the normal state [47]. The hormone hCG is made by cells that form the placenta, which nourishes the egg after it has been fertilized and becomes attached to the uterine wall. Moreover, all strenuous activities performed mainly during the third trimester of pregnancy can lead to chronic fatigue, thus special care must be used when prescribing weight bearing activities. Musculoskeletal injury is other risk related with the augment of 15–30 % body mass during pregnancy. Also, biomechanical modifications happening in the pelvic/abdominal region, greater elasticity of the ligaments, and changes in the musculature (abdominal diastasis) all can promote inefficiency of movement skills [48]. So all these factors together can make it easier to get a musculoskeletal injury while performing basic movements such as walking for a long time or of moderate intensity [49].

26.4.3 Risks of Exercise During Pregnancy for the Fetus

Similar with the mother, if PA were not well conducted the fetus would suffer risks related with exercise. This section describes events affecting the fetus as a consequence of excess or wrong exercise.

26.4.3.1 Acute Hypoxia

It has been hypothesized that the fetus may have hypoxia during aerobic exercise by the redistribution of blood flow, and thus more oxygen goes to the muscles instead of going to the uterus. The fetal HR, which reflects cardiac output, is usually between 120 and 160 bpm. An exercise event that induces a HR elevation higher than 160 bpm for 10 min is designated as tachycardia, and lower than 120-bpm as bradycardia. Parer [50] reported an increase of 10–30 bpm in the fetus during maternal exercise. The HR responses in the fetus could reflect tissue oxygenation. The a-V O2 difference (indicating the difference of the oxygen content of her arterial blood and her venous blood) was improved in the active mother that suggests more oxygen was being delivered to her tissues [36].

26.4.3.2 Acute Hyperthermia

Excessive elevation of temperature primarily during the first weeks of pregnancy may be a risk to development defects and fetal death [51]. Also, the fetal temperature is about 0.5 °C higher than the maternal temperature [52]. However, the pregnant woman has thermoregulatory mechanisms that increase the circulation to the skin to lose heat, so the increase in temperature of the fetus is tightly regulated to prevent fetus hyperthermia. Regardless, it is not advisable to perform exercise with high environmental temperatures (above 40 °C). Also it is mandatory to drink enough water in order to avoid dehydration and internal warming [5354].

26.4.3.3 Low Glucose Availability

The use of carbohydrates by skeletal muscle in pregnant women increases significantly during strenuous exercise [55]. This may limit the ability to extend vigorous exercise, and may predispose pregnant women to hypoglycemia [56]. This effect may be the result of the insulin resistance that develops in the latter half of pregnancy [29]. However, a drop in blood glucose levels, which can limit the consumption of glucose by the fetus, may be a consequence more probable of long-term nutritional mistakes than exercise [57]. Nonetheless, this should induce low weight or alterations in the growth of fetal organs and tissues, so carbohydrate intake after exercise training should be considered, mainly gravid women training longer than 60 min.

26.4.3.4 Abortion in the First Quarter

Beliefs of PA as a promoter of abortion have not been supported by literature [54]. Clearly, the risk of spontaneous abortion was not found to be higher in athletes than in healthy controls [58]. However, exercise in the first trimester can lead to an early abortion, so avoiding strenuous PA is one of the most important restrictions to avoid fetus death prematurely [59].

26.4.3.5 Risk of Preterm Delivery

Acute exercise may induce premature birth because it increases the secretion of catecholamines, especially norepinephrine, which in its turn causes uterine contractions after exercise [60]. This hypothesis was analyzed in a study that recruited more than 7,000 women. The researchers found that 8 h a day in a standing position increased the risk of preterm delivery. However, 4 h a day of work or exercise was not associated with preterm delivery. Also, there were no significant differences between sedentary and physically active jobs for the percentage of premature babies. Moreover, the time of PA was not associated with a higher risk of preterm delivery [61]. However, women at risk of preterm birth are advised to avoid exercise training [44].

26.4.3.6 Reduced Birth Weight

As previously explained, there may be a possible mechanism related with fetal hypoxia, which explains low birth weight of the newborn. In accordance with this hypothesis, there appears to be a dose–response relationship between days/energy expenditure per week of training and low birth weight among athletes. As a suggestion, exercise energy expenditure less than 2,000 kcal/week or intense exercise 1 h a day performed 5–7 days/week, must avoided in order to reduce the probability to deliver a low-weight baby, mainly after 28th week of gestation [20]. Conversely, recreational exercisers or those who meet the American College of Obstetricians and Gynecologists (ACOG) guidelines gave birth to babies with normal weight, even when vigorous intensity was performed [1462]. Thus, children of high-level female athletes (and ex-participants in the Olympic Games) have been of normal weight [63]. It seems that individual exercise and nutritional prescriptions need to be followed for a healthy birth weight. There seems to be a lower (>120 min/week) and upper thresholds (intense exercise 1 h a day performed 5–7 days/week) for the quantity of exercise to deliver a healthy baby [1420].

26.4.4 Recommendations for Exercise During Pregnancy

26.4.4.1 General Recommendations for Programming Exercise During Pregnancy

As pointed out in the previous sections, PA has several positive effects for both the pregnant women and fetus. A recent review focused on daily PA interventions (exercise training and unstructured PA) analyzed the positive and negative effects of PA during pregnancy. The results of this review suggested that PA performed without supervision or prescription could result in lower benefits than prescribed exercise [14]. Furthermore, the analysis of gestational physiology and possible complications as consequence of excessive workload makes us think that care must be put into designing exercise or PA advice for pregnant women. In the previous sections, we have given some specific guidelines for particular cases; however, exercise prescription requires an integration of all determinants of the workout.

Load (duration and intensity), mode (contraction pattern and metabolic pathway), type (activities), periodization, nutrition, and environment have been the variables most commonly studied in order to define specific guidelines by the representative professional/academic associations of pregnancy or exercise [4445596468]. These recommendations have been updated during the last decades. We have summarized the most recent guidelines (see Table 26.4) which have been synthesized from the ACOG guidelines and the Physical Activity Readiness Examination (PARmed-X for Pregnancy).

Table 26.4

Recommendations for exercise prescription during pregnancy

Exercise prescription variables

Level of performance/practice

General

Sedentary

Recreational

Elite/athlete

Bout volume

At least 15 min

30 min

30–60 min

Strength: high repetitions (15–20)

60–90 min

Bout intensity

140–160 bpm (safe for walking and biking)

%HRmax: 70 % (bike and aerobics)

%VO2max: 70 % (swimming and water exercises); lower HR than biking

“Talk Test”

High intensitya

RPE: moderately hard

%HRmax: 65–75 %

RPE: moderately hard to hard

%HRmax: 65–80 %

Strength: Light weights

RPE: hard

%HRmax: 75–80 %

Frequency

3 d/wk

3 d/wk

3–5 d/wk

3–5 d/wk

Mode

Low impact

Low impact

Extensive isometric contractions, anaerobic exercisesa

Strength training without Valsalva maneuver

Low and moderate impact

Low and moderate impact

Maximum and isometric strength exercisesa

Exercise in supine position first quartera

Type (activities)

Childbirth preparation (minimum)

Start with no weight-bearing exercises (cycling, swimming, etc.)

Walking and brisk walking

Aerobics

Water exercises are recommended to relieve back pain. Exercise increases blood mobilization and reduction of edema

Pilates under individual supervision

Walking, cycling, swimming, and water aerobics

Low impact, and progress to moderate as jogging/running, tennis

Jogging/running, tennis, and similar, progress to racing activities

Change races by elliptical device

Resistances machines

Water exercises to prevent back pain

Avoida

Participating in competitions, contact sports or risk of traumaa

Exercises that could overload the lower backa

Exercise at moderate altitude (2,500 m over the sea)a

Frequently shallow diving (never deep)a

Sport contacts (team sports or martial arts)a

Horse riding, skating, skiing, climbing and others, which increase fall riska

   

Training with infection, fever, or fatiguea

Competition eventsa

Contact sportsa

Quick changes of direction (ligamentous laxity)a

Anaerobic exercisesa

Stop training with symptoms such as pain, bleeding, etc.

Environment

Avoid high temperaturesa

   

Avoid high temperaturesa

Nutrition and supplementation

Adequate nutrition and hydration

   

Dehydrationa

Periodization

Supine position during the first quarter; start the training program first quartera

Begin with 15 min and progress to 30 min

From 3 to 5 d/wk

 

3 d/wk in the first and third quarter

5 d/wk in the second quarter

Adapted from: (a) Artal, R., & O’Toole, M. (2003). Guidelines of the American College of Obstetricians and Gynecologists for exercise during pregnancy and the postpartum period. Br J Sports Med37(1), 6–12; discussion 12; (b) Paisley, T. S., Joy, E. A., & Price, R. J., Jr. (2003). Exercise during pregnancy: a practical approach. Curr Sports Med Rep2(6), 325–330; (c)Wolfe, L. A., &Weissgerber, T. L. (2003). Clinical physiology of exercise in pregnancy: a literature review. J Obstet Gynaecol Can25(6), 473–483

HR max maximal heart rate, VO2max maximal oxygen uptake, RPE rate of perceived exertion, bpm beats per minute (heart rate), d/wk days/week

aIndicate practices that must be avoided

Before starting any exercise program, the general state of the individual pregnant woman should be kept in mind. It is therefore necessary to consider the contraindications that exercise may have, both absolutely and relatively—Meaning an absolute contradiction to exercise and a relative contradiction to exercise: Absolute means DO NOT EXERCISE UNDER ANY CONDITIONS and a relative contraindication means that a case-by-case decision must be made depending on the pros and cons of exercise for this situation. Additionally, several physical signs must be under control in order to stop the practice and proceed with an emergency protocol if necessary (see Fig. 26.1).

The most important aspect of exercise prescription in pregnancy is intensity. Aerobic training appears to be the mode of training most studied and used when prescribing exercise for gravid women. The HR is the classical parameter used to quantify the intensity of continuous aerobic activities; so several thresholds or percentages of maximum HR have been proposed to get benefits and diminish the risks. The main physiological variable determining HR values during PA must be VO2; hence, estimation of VO2max has been an important concern in order to establish training goals.

A meta-analysis on the effects of exercise on pregnancy outcomes did not find evidence of harm to the mother or the fetus with training up to 80 % of HRmax (144 beats in women around 26 years old), 43 min/session, up to 3 days/week [15]. Nevertheless, these values can be influenced by the level of sport and activity practice before gestation.

Athletes can continue with exercise training during pregnancy, but intensity should be lowered [69]. They also might be able to perform PA with lower HR than sedentary women. However, it is difficult to define the limits of training with pre-gravid athletes participating in different sports, so more research is needed to find HR ranges for specific modalities [20].

Sedentary pregnant women who performed an exercise training program during quarter 2 and 3 (140–150 beats, 25 min/session, 3 times/week) had decreased submaximal HR when compared to a control group with a sedentary lifestyle [15].

An attenuated response of the sympathetic system during pregnancy induces reductions of HRmax in response to exercise. This has lead to a reduction in the importance of HR as a sensitive indicator of intensity. Likewise ACOG recommends the use of RPE to monitor exercise intensity, since HR is affected by the hemodynamic changes during gestation [4445].

Methods to estimate VO2max are traditionally useful in clinical and field settings to avoid time consuming and expensive laboratory tests. Bike tests using external load and the Astrand nomogram are widely applied by exercise physiologists, clinicians, and coaches [70]. However, the Astrand nomogram has been shown to overestimate the VO2max by about 9 %, in pregnant women. Other methods such as the linear regression from the relationship between VO2-HR using submaximal loads (such as the YMCA protocol), overestimated VO2max by 6 %. These regression methods are procedures well fitted to non-gravid women. However, the necessary assumptions to make valid measurements of VO2max are not met during gestation, mainly when submaximal VO2 is estimated and not measured.

Currently, there are not many well-validated methods to estimate VO2max during pregnancy. One of the few methods for pregnancy utilizes a single constant workload protocol. It requires the measurement of HR at the end of a 6 min steady-state constant exercise on bike. Afterwards, 26.1 and 26.2 equations are used to estimate VO2max [39].

 $$ \% V{O}_{2 \max }=\left(0.634\times FC\right)-30 $$

(26.1)

 $$ V{O}_{2 \max }=V{O}_2/\% V{O}_{2 \max}\times 100 $$

(26.2)

Another key element of programming exercise is the mode or modality of the activities. There are many aspects to consider when selecting safe exercises; these are summarized in Table 26.4. A basic training circuit is illustrated in panel 26.1

A145875_2_En_26_Figa_HTML.jpg

26.4.4.2 Post-delivery

Although there are other important concerns associated with PA, for example, the reestablishment of muscular fitness, the quality of breast-feeding, and the mother’s weight gain, however, the first aim should be to perform exercise focused on the recovery of the strength of perineal muscles and later in the abdominal region. Nevertheless, several physiological modifications persist at least during the first 4 weeks, especially the cardiorespiratory system, and thus some specific temporal guidelines have been suggested [6465]. Few hours of delivery, pelvic floor exercises might be initiated.

·               The first 3 weeks, exercises to recover abdominal wall tonus. Limit exercise training.

·               After 40 days of delivery, moderate aerobic activity outdoors jumping or running must be delayed until after 8–12 weeks after birth (risk of trauma for the pelvic floor).

·               Hypotension is common. Restrain sudden changes of position.

·               Restart strenuous or competition activity only 8 weeks after delivery.

·               The first aim is to perform exercise focused on recovering first strength of perineal muscles and later in the abdominal region.

Regarding weight control, gestational excessive weight gain is the strongest predictor of postpartum weight retention. Also, it has been reported that weight retention after delivery and low physical activity, may also contribute to obesity [70]. So it has recently been suggested that individualized diet and exercise training plans are needed in order to manage a healthy weight loss [71]. However, more important than weight loss is an enhanced body composition profile, since exercise training preserves fat free mass (FFM), while dieting alone reduces fat mass and FFM. The best results are ensured incorporating both diet and exercise.

In addition, a balance between training energy expenditure and diet must be maintained in order to ensure healthy baby growth and development. Excessive PA and poor energy intake would impoverish milk production and quality, which does not allow the infant to gain weight. General guidelines encourage mothers to intake enough liquids and nutrients to meet increased energy requirements as a consequence of physical activity. Also, nursing women are advised to feed babies before exercise practice; this procedure helps the mother to diminish the discomfort of engorged breasts and balance the acidity of her milk [64]. Reduction of postpartum depression symptoms is an additional benefit of exercise. This has been widely reported, but evidence has not been demonstrated without revocation [72].

26.5 Future Directions

Although the knowledge of exercise-applied pregnancy has been improved widely during the last few decades, several concerns remain to be defined. The effects of exercise training on the physiological function and body composition of children are not completely elucidated, so longitudinal studies need to be designed in order to establish this relationship. Also, data of the dose–response relationship between exercise-training load and health outcomes have not been massively collected. Finally, improved methods to assess physical fitness and body composition for gravids need to be developed in order to improve exercise prescriptions in field settings.

26.6 Concluding Remarks

The guidelines presented in this chapter should only be used as a general rule. The pregnant woman should be monitored closely to adjust training loads as a function of body mass, blood markers, and the fetus developmental changes. Individual prescriptions should be based on assessing energy requirements and capacity as tight as possible. A flow networking must be established between gynecologist, nutritionist, and exercise physiologist in order to maximize benefits to both the fetus and the mother.

The first aim of exercise program must be to guarantee the safety of the mother and the fetus more than performance or esthetic outcomes during pregnancy and after-delivery. Hence, a checklist of risks must be kept in mind when planning goals of PA before prescribing training loads.

While there are no well-developed methods to perform the assessment of physical fitness, these assessments must be carried out cautiously. Regarding body composition estimations, we have not suggested any models or techniques because of changes of FFM hydration and total body water during pregnancy. The change in these variables during pregnancy invalidates the traditional methods to estimated fat mass, FFM or skeletal muscle mass [73]. Given the choice, we would suggest measuring accurately limb skinfolds and circumferences in order to obtain a general idea of changes in fat mass and FFM.

A precise recall of volume, intensity, and type of exercise, in addition to physical and clinical assessment outcomes, is the best way to upgrade and modify PA programs for gravid women rationally. This will allow professionals to understand the individual dose–response relationship in each specific case and the need for individualized exercise prescription in order to maximize the health benefits for both the mother and the fetus.

References

1.

Revelli A, Durando A, Massobrio M. Exercise and pregnancy: a review of maternal and fetal effects. Obstet Gynecol Surv. 1992;47(6):355–67.PubMedCrossRef

2.

Capeless EL, Clapp JF. Cardiovascular changes in early phase of pregnancy. Am J Obstet Gynecol. 1989;161(6 Pt 1):1449–53.PubMedCrossRef

3.

O’Toole ML. Physiologic aspects of exercise in pregnancy. Clin Obstet Gynecol. 2003;46(2):379–89.PubMedCrossRef

4.

Jensen D, Webb KA, Wolfe LA, O’Donnell DE. Effects of human pregnancy and advancing gestation on respiratory discomfort during exercise. Respir Physiol Neurobiol. 2007;156(1):85–93.PubMedCrossRef

5.

Bessinger RC, McMurray RG. Substrate utilization and hormonal responses to exercise in pregnancy. Clin Obstet Gynecol. 2003;46(2):467–78.PubMedCrossRef

6.

Bell R, O’Neill M. Exercise and pregnancy: a review. Birth. 1994;21(2):85–95.PubMedCrossRef

7.

Artal R. Exercise and diabetes mellitus in pregnancy. A brief review. Sports Med. 1990;9(5):261–5.PubMedCrossRef

8.

Gunderson EP. Nutrition during pregnancy for the physically active woman. Clin Obstet Gynecol. 2003;46(2):390–402.PubMedCrossRef

9.

Ladipo OA. Nutrition in pregnancy: mineral and vitamin supplements. Am J Clin Nutr. 2000;72(1 Suppl):280S–90.PubMed

10.

Forsum E, Lof M. Energy metabolism during human pregnancy. Annu Rev Nutr. 2007;27:277–92.PubMedCrossRef

11.

Butte NF, Wong WW, Treuth MS, Ellis KJ, O’Brian SE. Energy requirements during pregnancy based on total energy expenditure and energy deposition. Am J Clin Nutr. 2004;79(6):1078–87.PubMed

12.

Clarke PE, Gross H. Women’s behaviour, beliefs and information sources about physical exercise in pregnancy. Midwifery. 2004;20(2):133–41.PubMedCrossRef

13.

Zhang J, Savitz DA. Exercise during pregnancy among US women. Ann Epidemiol. 1996;6(1):53–9.PubMedCrossRef

14.

Mudd LM, Owe KM, Mottola MF, Pivarnik JM. Health benefits of physical activity during pregnancy: an international perspective. Med Sci Sports Exerc. 2013;45(2):268–77.PubMedCrossRef

15.

Lokey EA, Tran ZV, Wells CL, Myers BC, Tran AC. Effects of physical exercise on pregnancy outcomes: a meta-analytic review. Med Sci Sports Exerc. 1991;23(11):1234–9.PubMedCrossRef

16.

Pivarnik JM. Cardiovascular responses to aerobic exercise during pregnancy and postpartum. Semin Perinatol. 1996;20(4):242–9.PubMedCrossRef

17.

Gorski J. Exercise during pregnancy: maternal and fetal responses. A brief review. Med Sci Sports Exerc. 1985;17(4):407–16.PubMedCrossRef

18.

Lotgering FK, Struijk PC, van Doorn MB, Spinnewijn WE, Wallenburg HC. Anaerobic threshold and respiratory compensation in pregnant women. J Appl Physiol. 1995;78(5):1772–7.PubMed

19.

Kramer MS, McDonald SW. Aerobic exercise for women during pregnancy. Cochrane Database Syst Rev. 2006;3, CD000180.PubMed

20.

Pivarnik JM, Perkins CD, Moyerbrailean T. Athletes and pregnancy. Clin Obstet Gynecol. 2003;46(2):403–14.PubMedCrossRef

21.

Wolfe LA, Weissgerber TL. Clinical physiology of exercise in pregnancy: a literature review. J Obstet Gynaecol Can. 2003;25(6):473–83.PubMed

22.

McAuley SE, Jensen D, McGrath MJ, Wolfe LA. Effects of human pregnancy and aerobic conditioning on alveolar gas exchange during exercise. Can J Physiol Pharmacol. 2005;83(7):625–33.PubMedCrossRef

23.

Garshasbi A, Faghih ZS. The effect of exercise on the intensity of low back pain in pregnant women. Int J Gynaecol Obstet. 2005;88(3):271–5.PubMedCrossRef

24.

Ostgaard HC, Zetherstrom G, Roos-Hansson E. Back pain in relation to pregnancy: a 6-year follow-up. Spine (Phila Pa 1976). 1997;22(24):2945–50.CrossRef

25.

Kristiansson P, Svardsudd K, von Schoultz B. Serum relaxin, symphyseal pain, and back pain during pregnancy. Am J Obstet Gynecol. 1996;175(5):1342–7.PubMedCrossRef

26.

Clapp 3rd JF. Exercise during pregnancy. A clinical update. Clin Sports Med. 2000;19(2):273–86.PubMedCrossRef

27.

Clapp 3rd JF. The course of labor after endurance exercise during pregnancy. Am J Obstet Gynecol. 1990;163(6 Pt 1):1799–805.PubMedCrossRef

28.

Rice PL, Fort IL. The relationship of maternal exercise on labor, delivery and health of the newborn. J Sports Med Phys Fitness. 1991;31(1):95–9.PubMed

29.

Damm P, Breitowicz B, Hegaard H. Exercise, pregnancy, and insulin sensitivity–what is new? Appl Physiol Nutr Metab. 2007;32(3):537–40.PubMedCrossRef

30.

Dempsey JC, Sorensen TK, Williams MA, et al. Prospective study of gestational diabetes mellitus risk in relation to maternal recreational physical activity before and during pregnancy. Am J Epidemiol. 2004;159(7):663–70.PubMedCrossRef

31.

Gaillard R, Bakker R, Willemsen SP, Hofman A, Steegers EA, Jaddoe VW. Blood pressure tracking during pregnancy and the risk of gestational hypertensive disorders: the Generation R Study. Eur Heart J. 2011;32(24):3088–97.PubMedCrossRef

32.

Ghulmiyyah L, Sibai B. Maternal mortality from preeclampsia/eclampsia. Semin Perinatol. 2012;36(1):56–9.PubMedCrossRef

33.

Saftlas AF, Logsden-Sackett N, Wang W, Woolson R, Bracken MB. Work, leisure-time physical activity, and risk of preeclampsia and gestational hypertension. Am J Epidemiol. 2004;160(8):758–65.PubMedCrossRef

34.

Sorensen TK, Williams MA, Lee IM, Dashow EE, Thompson ML, Luthy DA. Recreational physical activity during pregnancy and risk of preeclampsia. Hypertension. 2003;41(6):1273–80.PubMedCrossRef

35.

Demissie Z, Siega-Riz AM, Evenson KR, Herring AH, Dole N, Gaynes BN. Associations between physical activity and postpartum depressive symptoms. J Womens Health (Larchmt). 2011;20(7):1025–34.CrossRef

36.

Wallace AM, Engstrom JL. The effects of aerobic exercise on the pregnant woman, fetus, and pregnancy outcome. A review. J Nurse Midwifery. 1987;32(5):277–90.PubMedCrossRef

37.

Parker KM, Smith SA. Aquatic-aerobic exercise as a means of stress reduction during pregnancy. J Perinat Educ. 2003;12(1):6–17.PubMedPubMedCentral

38.

Beilock SL, Feltz DL, Pivarnik JM. Training patterns of athletes during pregnancy and postpartum. Res Q Exerc Sport. 2001;72(1):39–46.PubMedCrossRef

39.

Sady SP, Carpenter MW. Aerobic exercise during pregnancy. Special considerations. Sports Med. 1989;7(6):357–75.PubMedCrossRef

40.

Clapp 3rd JF, Dickstein S. Endurance exercise and pregnancy outcome. Med Sci Sports Exerc. 1984;16(6):556–62.PubMedCrossRef

41.

Hartmann S, Bung P, Schlebusch H, Hollmann W. The analgesic effect of exercise during labor. Z Geburtshilfe Neonatol. 2005;209(4):144–50.PubMedCrossRef

42.

Lotgering FK, Gilbert RD, Longo LD. The interactions of exercise and pregnancy: a review. Am J Obstet Gynecol. 1984;149(5):560–8.PubMedCrossRef

43.

Clapp 3rd JF, Lopez B, Harcar-Sevcik R. Neonatal behavioral profile of the offspring of women who continued to exercise regularly throughout pregnancy. Am J Obstet Gynecol. 1999;180(1 Pt 1):91–4.PubMedCrossRef

44.

ACOG committee opinion. Exercise during pregnancy and the postpartum period. Number 267, January 2002. American College of Obstetricians and Gynecologists. Int J Gynaecol Obstet. 2002;77(1):79–81.CrossRef

45.

ACOG Committee opinion. Number 267, January 2002: exercise during pregnancy and the postpartum period. Obstet Gynecol. 2002;99(1):171–3.CrossRef

46.

Vadakekut ES, McCoy SJ, Payton ME. Association of maternal hypoglycemia with low birth weight and low placental weight: a retrospective investigation. J Am Osteopath Assoc. 2011;111(3):148–52.PubMed

47.

Lotgering FK, van Doorn MB, Struijk PC, Pool J, Wallenburg HC. Maximal aerobic exercise in pregnant women: heart rate, O2 consumption, CO2 production, and ventilation. J Appl Physiol. 1991;70(3):1016–23.PubMed

48.

Mullinax KM, Dale E. Some considerations of exercise during pregnancy. Clin Sports Med. 1986;5(3):559–70.PubMed

49.

Borg-Stein JP, Fogelman DJ, Ackerman KE. Exercise, sports participation, and musculoskeletal disorders of pregnancy and postpartum. Semin Neurol. 2011;31(4):413–22.PubMedCrossRef

50.

Parer JT, Court DJ, Block BS, Llanos AJ. Variability of basal oxygenation of the fetus–causes and associations. Eur J Obstet Gynecol Reprod Biol. 1984;18(1–2):1–9.PubMedCrossRef

51.

Edwards MJ. Review: hyperthermia and fever during pregnancy. Birth Defects Res A Clin Mol Teratol. 2006;76(7):507–16.PubMedCrossRef

52.

Power GG. Biology of temperature: the mammalian fetus. J Dev Physiol. 1989;12(6):295–304.PubMed

53.

Soultanakis-Aligianni HN. Thermoregulation during exercise in pregnancy. Clin Obstet Gynecol. 2003;46(2):442–55.PubMedCrossRef

54.

Artal R, Sherman C. Exercise during pregnancy: safe and beneficial for most. Phys Sportsmed. 1999;27(8):51–75.PubMedCrossRef

55.

Soultanakis HN, Artal R, Wiswell RA. Prolonged exercise in pregnancy: glucose homeostasis, ventilatory and cardiovascular responses. Semin Perinatol. 1996;20(4):315–27.PubMedCrossRef

56.

Clapp 3rd JF, Wesley M, Sleamaker RH. Thermoregulatory and metabolic responses to jogging prior to and during pregnancy. Med Sci Sports Exerc. 1987;19(2):124–30.PubMedCrossRef

57.

Ebbeling CB, Pearson MN, Sorensen G, et al. Conceptualization and development of a theory-based healthful eating and physical activity intervention for postpartum women who are low income. Health Promot Pract. 2007;8(1):50–9.PubMedCrossRef

58.

Clapp 3rd JF. The effects of maternal exercise on early pregnancy outcome. Am J Obstet Gynecol. 1989;161(6 Pt 1):1453–7.PubMedCrossRef

59.

Wolfe LA, Davies GA. Canadian guidelines for exercise in pregnancy. Clin Obstet Gynecol. 2003;46(2):488–95.PubMedCrossRef

60.

Artal R, Platt LD, Sperling M, Kammula RK, Jilek J, Nakamura RI. Maternal cardiovascular and metabolic responses in normal pregnancy. Am J Obstet Gynecol. 1981;140(2):123–7.PubMed

61.

Klebanoff MA, Shiono PH, Carey JC. The effect of physical activity during pregnancy on preterm delivery and birth weight. Am J Obstet Gynecol. 1990;163(5 Pt 1):1450–6.PubMedCrossRef

62.

Duncombe D, Skouteris H, Wertheim EH, Kelly L, Fraser V, Paxton SJ. Vigorous exercise and birth outcomes in a sample of recreational exercisers: a prospective study across pregnancy. Aust N Z J Obstet Gynaecol. 2006;46(4):288–92.PubMedCrossRef

63.

Zaharieva E. Olympic participation by women. Effects on pregnancy and childbirth. JAMA. 1972;221(9):992–5.PubMedCrossRef

64.

Artal R, O’Toole M. Guidelines of the American College of Obstetricians and Gynecologists for exercise during pregnancy and the postpartum period. Br J Sports Med. 2003;37(1):6–12. discussion 12.PubMedCrossRefPubMedCentral

65.

Davies GA, Wolfe LA, Mottola MF, MacKinnon C. Joint SOGC/CSEP clinical practice guideline: exercise in pregnancy and the postpartum period. Can J Appl Physiol. 2003;28(3):330–41.PubMedCrossRef

66.

Fazlani SA. Protocols for exercise during pregnancy. J Pak Med Assoc. 2004;54(4):226–9.PubMed

67.

Paisley TS, Joy EA, Price Jr RJ. Exercise during pregnancy: a practical approach. Curr Sports Med Rep. 2003;2(6):325–30.PubMedCrossRef

68.

Snyder S, Pendergraph B. Exercise during pregnancy: what do we really know? Am Fam Physician. 2004;69(5):1053. 1056.PubMed

69.

Davies B, Bailey DM, Budgett R, Sanderson DC, Griffin D. Intensive training during a twin pregnancy. A case report. Int J Sports Med. 1999;20(6):415–8.PubMedCrossRef

70.

Evenson KR, Herring AH, Wen F. Self-Reported and objectively measured physical activity among a cohort of postpartum women: the PIN Postpartum Study. J Phys Act Health. 2012;9(1):5–20.PubMedPubMedCentral

71.

Choi J, Fukuoka Y, Lee JH. The effects of physical activity and physical activity plus diet interventions on body weight in overweight or obese women who are pregnant or in postpartum: a systematic review and meta-analysis of randomized controlled trials. Prev Med. 2013;56:351–64.PubMedCrossRefPubMedCentral

72.

Daley AJ, Macarthur C, Winter H. The role of exercise in treating postpartum depression: a review of the literature. J Midwifery Womens Health. 2007;52(1):56–62.PubMedCrossRef

73.

Lederman SA. Pregnancy. In: Heymsfield SBL, Lohman T, Wang ZM, Going S, editors. Human body composition, vol. I. 2nd ed. Human Kinetics: Champaign; 2007. p. 299–312.