Gestational diabetes mellitus is defined as carbohydrate intolerance of variable severity with onset or first recognition during pregnancy. This definition applies regardless of whether or not insulin is used for treatment. The diagnostic term gestational diabetes implies that this disorder is induced by pregnancy, perhaps due to exaggerated physiological changes in glucose metabolism. An alternative explanation is that gestational diabetes is maturity-onset or type 2 diabetes, unmasked or discovered during pregnancy. A majority (90 percent) of pregnancies complicated by diabetes are due to gestational diabetes. Those pregnant women diagnosed to have gestational diabetes and whose fasting glucose values are less than 105 mg/dL are often identified as having class A1 gestational diabetes. Women with fasting hyperglycemia (105 mg/dL or greater) are placed into class A2. Approximately 15 percent of women with gestational diabetes will exhibit such fasting hyperglycemia. Undoubtedly, the earlier in pregnancy fasting hyperglycemia is diagnosed, the greater the likelihood of preexisting diabetes.
The most important perinatal concern in women diagnosed with gestational diabetes is excessive fetal growth, which may result in birth trauma. Unlike women with overt diabetes, fetal anomalies are not increased in those women diagnosed with gestational diabetes. Similarly, whereas pregnancies in women with overt diabetes are at greater risk for fetal death, this danger is not apparent for those with gestational diabetes treated with diet alone. In contrast, gestational diabetes with elevated fasting glucose, has been associated with unexplained stillbirth similar to overt diabetes. Adverse maternal effects include an increased frequency of hypertension and the need for cesarean delivery.
The perinatal focal point for gestational diabetes is avoidance of difficult delivery due to macrosomia, with concomitant birth trauma due to shoulder dystocia. Macrosomic infants of diabetic mothers are anthropometrically different from other large-for-age infants. Specifically, there is excessive fat deposition on the shoulders and trunk predisposing these fetuses to shoulder dystocia (Figure 71-1). Fortunately however, shoulder dystocia is uncommon, even in women with gestational diabetes (3 percent). Infants of diabetic women also more frequently require cesarean delivery for cephalopelvic disproportion.
FIGURE 71-1 This macrosomic infant who weighed 6050 g was born to a woman with gestational diabetes. (Reproduced, with permission, from Cunningham FG, Leveno KJ, Bloom SL, et al (eds). Williams Obstetrics. 23rd ed. New York, NY: McGraw-Hill; 2010.)
Macrosomia in these infants is compatible with the long-recognized association between fetal hyperinsulinemia resulting from maternal hyperglycemia, which in turn stimulates excessive somatic growth. Similarly, neonatal hyperinsulinemia may provoke hypoglycemia.
Despite more than 40 years of research, there is no consensus regarding the optimal approach to screening for gestational diabetes. In 1997, the American Diabetes Association changed its prior recommendations for universal screening and now recommends selective screening using the guidelines shown in Table 71-1. The American College of Obstetricians and Gynecologists (2001) has concluded that it may be appropriate to use selective screening in some clinical settings and universal screening in others. It is recommended that screening be performed between 24 and 28 weeks in those women not known to have glucose intolerance earlier in pregnancy. Screening is performed using a 50-g oral glucose challenge test. Women with values of 140 mg/dL or greater are then tested with a diagnostic 100-g oral glucose tolerance test (see following discussion). It is recommended that plasma be used for screening and that testing using fingerstick glucometers be avoided.
TABLE 71-1. Fifth International Workshop-Conference on Gestational Diabetes: Recommended Screening Strategy Based on Risk Assessment for Detecting Gestational Diabetes (GDM)
There is international disagreement as to the optimal glucose tolerance test for the definitive diagnosis of gestational diabetes. The World Health Organization recommends a 75-g 2-hour oral glucose tolerance test, and this approach is often used in Europe. In the United States, the 100-g 3-hour oral glucose tolerance test performed after an overnight fast remains the standard. There is also not a consensus on which glucose threshold values to use for the diagnosis of gestational diabetes. Plasma values suggested by the Fifth International Workshop Conference on Gestational Diabetes are shown in Table 71-2. Also shown are the criteria for the 75-g test most often used outside the United States, but increasingly used in this country.
TABLE 71-2. Fifth International Workshop-Conference on Gestational Diabetes: Diagnosis of Gestational Diabetes by Oral Glucose Tolerance Testinga
According to the American College of Obstetricians and Gynecologists (2001), insulin therapy is usually recommended when standard dietary management does not consistently maintain the fasting plasma glucose at less than 95 mg/dL or the 2-hour postprandial plasma glucose at less than 120 mg/dL. Whether insulin should be used in women with lesser degrees of fasting hyperglycemia is unclear. The Fifth International Workshop Conference (2007) on gestational diabetes recommended that maternal capillary glucose levels be kept at or below 95 mg/dL in the fasting state.
Nutritional counseling is a cornerstone in management. The goals of such therapy are (1) to provide the necessary nutrients for the mother and fetus, (2) to control glucose levels, and (3) to prevent starvation ketosis. The American Diabetes Association (2000) has recommended nutritional counseling with individualization based on height and weight and a diet that provides an average of 30 kcal/kg/day based on ideal body weight. These recommendations pertain to women treated with insulin as well as dietary restrictions. Obese women with a body mass index greater than 30 kg/m2 may benefit from further caloric restriction.
A liberal exercise program is encouraged. Appropriate exercises are those that use the upper-body muscles or place little mechanical stress on the trunk region during exercise. Such upper body cardiovascular training may result in lower glucose levels and reduce the likelihood of insulin therapy.
Most practitioners initiate insulin therapy in women with gestational diabetes if fasting hyperglycemia greater than 105 mg/dL persists despite diet therapy. Use of insulin in women with lower fasting glucose levels is controversial. Institution of insulin therapy usually can be accomplished in the outpatient setting, but, occasionally hospitalization is necessary. A total dose of 20 to 30 units given once daily, before breakfast, is commonly used to initiate therapy. The total dose is usually divided into two-thirds intermediate-acting insulin and one-third short-acting insulin.
Oral Hypoglycemic Agents
Oral glucose-lowering agents are currently not recommended by the American College of Obstetricians and Gynecologists (2001) during pregnancy. Fetal hyper-insulinemia and increased rates of congenital malformations are the main concerns.
In general, women with gestational diabetes who do not require insulin seldom require early delivery or other interventions during pregnancy. There is no consensus regarding whether antepartum fetal testing (see Chapter 12) is necessary, and if so, when to begin such testing in women without severe hyperglycemia. Elective induction to curtail fetal growth and prevent shoulder dystocia is controversial and may lead to an unnecessary increase in cesarean delivery. Women who require insulin therapy for fasting hyperglycemia, however, typically receive antepartum fetal testing and are managed as if they had overt diabetes (see Chapter 72).
There is a 50-percent likelihood of women with gestational diabetes developing overt diabetes within 20 years of the diagnosis of gestational diabetes. Therefore, women diagnosed as having gestational diabetes should undergo periodic glucose evaluation after delivery. A 75-g oral glucose tolerance test is recommended, and the criteria for interpretation are shown in Table 71-3.
TABLE 71-3. Fifth International Workshop-Conference: Metabolic Assessments Recommended After Pregnancy with Gestational Diabetes
Women whose 75-g test is normal should be reassessed at a minimum of 3-year intervals.
Recurrence of gestational diabetes in subsequent pregnancies may occur in up to two-thirds of women. Obese women are more likely to have impaired glucose tolerance in subsequent pregnancies. Thus, lifestyle behavioral changes, including weight control and exercise between pregnancies, could be a valuable strategy to prevent recurrence of gestational diabetes as well as type-2 diabetes later in life. Low-dose oral contraceptives may be used safely by women with recent gestational diabetes.
For further reading in Williams Obstetrics, 23rd ed.,
see Chapter 52, “Diabetes.”