This chapter deals primarily with APGO Educational Topic Area:
TOPIC 30 POSTTERM PREGNANCY
Students should be able to define postterm pregnancy and list possible maternal and fetal complications associated with postterm pregnancy. They should be able to outline a basic approach to evaluation and management options for postterm pregnancy.
Clinical Case
A 26-year-old patient with lifelong irregular and infrequent periods presents with increasing abdominal girth and fatigue. She does not think she is pregnant because she had been told by a previous physician that she would have a hard time conceiving. Her pregnancy test is positive, and ultrasound of the abdomen shows a 39-week intrauterine pregnancy with an estimated fetal weight of 7 lb. She is unsure when she would have conceived. In addition to having routine prenatal laboratory tests performed, she asks about plans for timing of delivery.
Normal full-term pregnancy lasts from 38 to 42 weeks. The “due date” or estimated date of delivery (EDD) is calculated to be 40 weeks from the first day of the last menstrual period (LMP), presuming regular, 28-day cycles and without recent, prior use of oral contraceptives. Postterm pregnancy is a pregnancy lasting 42 weeks of gestation or beyond. This condition occurs in approximately 10% of pregnancies and carries with it an increased risk of adverse outcome. The increased morbidity and mortality in a small percentage of cases, however, warrant careful evaluation of all postterm pregnancies. In addition, postterm pregnancies can create significant stress for the patient, her family, and those caring for her. Therefore, the physician should understand the condition and the options for management. “Postdates” is a commonly used, but misleading synonym and should be avoided.
CAUSE
The most common “cause” of postterm pregnancy is inaccurate estimation of gestational age (dating). Inaccurate dating is more likely in women with irregular menses and, thus, inconsistent ovulation; women who seek prenatal care later in pregnancy; women with delayed ovulation (e.g., women who have recently discontinued oral contraceptives); and women who inaccurately recall their LMP. Inaccurate dating that leads to the erroneous classification of a pregnancy as postterm has important sequelae. These pregnancies are erroneously labeled “high risk,” resulting in the use of costly and unnecessary evaluations. This, in turn, increases the likelihood of intervention, specifically, induction of labor and cesarean delivery, both of which are potentially associated with increased maternal and fetal morbidity. Other less common causes of postterm pregnancy are listed in Table 18.1.
Whatever the cause, there is a tendency for recurrence of postterm pregnancy. Approximately 50% of patients who have one postterm pregnancy will experience prolonged pregnancy with the next gestation.Other important risk factors include maternal obesity, nulliparity, and postterm delivery of the mother. Based on twin studies, there also appears to be a genetic influence.
EFFECTS
Compared with term pregnancies, the morbidity and mortality rates for both mother and fetus increase severalfold with post-term pregnancy. Risks of maternal vaginal trauma, labor dysfunction, and cesarean delivery increase. Cesarean delivery carries increased risks of infection, bleeding, thromboembolic phenomenon, and visceral injury. Stillbirth and neonatal mortality rates increase steadily after 37 weeks, approaching 1 in 300 at 42 weeks and increasing severalfold as the 44th week approaches. Postterm gestations are associated with several conditions that present diagnostic and management challenges: macrosomia, shoulder dystocia, meconium aspiration syndrome (MAS), dysmaturity syndrome, and oligohydramnios.
Macrosomia
Macrosomia is defined as an abnormally large infant size, specifically, an infant weighing 4,500 g or greater. It occurs in approximately 2.5% to 10% of postterm pregnancies. Maternal obesity, diabetes mellitus, or a previous macrosomic infant further raises the risk. Macrosomia is associated with an increased incidence of birth trauma, particularly if the infant is delivered vaginally. Such trauma includes shoulder dystocia; fracture of the clavicle; and associated brachial plexus injury, specifically, Erb-Duchenne palsy.
Shoulder dystocia is an obstetric emergency caused by impaction of the anterior fetal shoulder behind the symphysis pubis during the process of vaginal delivery. A series of particular maneuvers (see Chapter 9) can be accomplished to release this impaction. Brachial plexus injury is reported in approximately 0.85 to 1.89 per 1,000 term deliveries, but increases 18- to 21-fold in macrosomic infants delivered vaginally; it can also occur during cesarean deliveries. In Erb-Duchenne palsy, paralysis, stretch, or tear injury to the upper roots of the brachial plexus, at C5 and C6, results in paralysis of the deltoid and infraspinatus muscles and flexor muscles of the forearm, causing the limb to hang limply close to the side, with the forearm extended and internally rotated. Finger function is usually retained. Less frequently, damage is limited to the lower nerves of the brachial plexus, C8 and T1, causing Klumpke paralysis, or paralysis of the hand. Because most brachial injuries are mild, treatment is expectant with splints and physical therapy in anticipation of complete or nearly complete recovery in 3 to 6 months. Around 80% to 90% of brachial plexus injuries completely resolve by age 1 year. Maternal risks with fetal macrosomia include perineal and vaginal lacerations if the fetus is delivered vaginally and a twofold increase in the rate of cesarean delivery, with its associated operative risks and maternal trauma.
Meconium Aspiration Syndrome
Another special concern in postterm pregnancies is meconium passage and MAS. MAS can lead to severe respiratory distress from mechanical obstruction of both small and large airways, as well as to meconium chemical pneumonitis. Meconium passage is not limited to postterm pregnancies, although prolonged pregnancy, particularly in the setting of oligohydramnios, is a significant risk factor. Meconium passage occurs in 12% to 22% of women in labor, with aspiration occurring in up to 10% of these infants. The incidence of meconium passage increases as pregnancy becomes prolonged, as does the incidence of MAS.
Dysmaturity Syndrome
Dysmaturity syndrome, which refers to infants with characteristics resembling chronic growth restriction, affects up to 20% of postterm pregnancies. This syndrome may be associated with an aging placenta that is unable to provide adequate nutrition and/or oxygen diffusion for the fetus. These pregnancies are at increased risk of meconium aspiration, umbilical cord compression due to oligohydramnios, as well as short-term neonatal complications such as hypoglycemia, seizures, and respiratory insufficiency. In postterm pregnancies, there is an increased incidence of nonreassuring fetal testing, both antepartum and intrapartum.
Oligohydramnios
Oligohydramnios is defined as decreased amniotic fluid for gestational age and is generally quantified as an amniotic fluid index less than 5 cm. This is measured by dividing the gravid abdomen into quadrants and totaling the measurements of the largest vertical pockets of fluid in each of those quadrants. In some facilities, a maximum vertical pocket less than 2 cm is used to define oligohydramnios. Amniotic fluid is a reflection of fetal swallowing, fetal breathing, fluid transfer across the amniotic sac, and, especially, fetal urination. The amniotic fluid reaches its maximum volume at approximately 34 to 36 weeks and stays constant or slightly decreases from then through the remainder of the pregnancy. Any alterations in the above processes can cause changes in amniotic fluid volume. Oligohydramnios is associated with poor outcomes due to umbilical cord compression, uteroplacental insufficiency, and meconium aspiration. Because of these risks, after 40 weeks of gestation, close antepartum surveillance is warranted if the pregnancy is allowed to continue. At term, oligohydramnios is an indication for delivery.
DIAGNOSIS
The diagnosis of postterm pregnancy rests on establishment of the correct gestational age. The first step in management of a patient with suspected postterm pregnancy is a careful review of the criteria used to establish the gestational age. The most common information used to determine gestational age includes the patient’s reported LMP and the first-trimester ultrasound. Ultrasound is most accurate for determining dating for gestational age when it is performed from 6 to 12 weeks of gestation. If the patient’s LMP predicts an EDD that is within 10 days of an EDD determined by an ultrasound performed between 12 and 20 weeks of gestation, then the gestational age is considered fairly accurate. Once the EDD is determined, it should not be changed unless more accurate information is subsequently available.
With improved access to prenatal care and greater importance placed on accurate gestational age assessment, the percentage of patients in whom postterm pregnancy is suspected has diminished. Nonetheless, a substantial number of patients do not seek prenatal care early in pregnancy or do not have an accurate gestational age determination. The prevalence of postterm pregnancy varies regionally, depending on the use of first-trimester ultrasound for gestational dating and routine labor induction.
MANAGEMENT
Once the gestational age is believed to be firmly established and the patient approaches 41 weeks of gestation, the two management options are 1) induction of labor and 2) antepartum fetal surveillance, which continues either until spontaneous labor occurs or until approximately 42 weeks. In the United States, very few pregnancies are allowed to progress beyond 42 weeks and virtually none beyond 43 weeks. Factors that influence management include the patient’s concerns, the assessment of fetal well-being, and the status of the patient’s cervix. Induction of labor is appropriate if the cervix is favorable and if the patient prefers such management. The risk of failed induction is low with a favorable cervix, and most authorities believe it is low enough to recommend delivery in light of the risk of increased fetal morbidity in the postterm period.
The data on preventing postterm pregnancy are controversial. Some studies show that sweeping the membranes may decrease postterm pregnancy; other studies differ. Sweeping the membranes is a procedure by which the amniotic sac is gently detached from the uterine wall at the level of the cervix and/or lower uterine segment. This procedure is thought to release prostaglandins, which can increase cervical dilation, making the cervix more favorable and sometimes leading to the onset of labor. Sweeping the membranes should not be performed unless the gestational age is verified and the maturity of the fetus ensured.
If the gestational age is not well-established and the menstrual history and early ultrasound findings are not available, there is little additional information that can be used to determine the best estimate of gestational age.Amniocentesis is not especially helpful, because fetal lung maturity is rarely a question in the postterm evaluation. Once the best date is selected, a management plan similar to that for a postterm pregnancy with well-established gestational age is used.
Fetal Assessment
If the cervix is not favorable for induction, fetal well-being is monitored intermittently until either spontaneous labor occurs or the cervix “ripens,” thereby making induction more feasible. Fetal evaluation has not been shown to decrease mortality in postterm pregnancy; however, it is also not associated with any negative outcomes. Although a variety of management schemes have been devised to monitor fetal well-being, none has been shown to be superior. Thus, it is common practice to assess fetal well-being using several methods. Weekly monitoring of amniotic fluid volume is commonly used, insofar as oligohydramnios at term is a sufficient indication for delivery. Nonstress tests (fetal heart rate monitoring), biophysical profiles (ultrasound evaluation of fetal fluid, movement, tone, and breathing), or oxytocin challenge tests may be used once or twice a week. Another option is the combination of amniotic fluid assessment and nonstress test, known as the modified biophysical profile. Doppler flow studies of the umbilical artery are not considered useful. Daily fetal movement counting is included in most management plans, with decreased perceived fetal movement being an indication for immediate evaluation of fetal well-being. Results of these tests are most useful when considered within the context of other conditions affecting the mother and the fetus. If fetal test results are nonreassuring, delivery is indicated.
Labor Induction
The patient with an unfavorable cervix should be counseled about risks of induction of labor and the risks of continuing pregnancy with fetal evaluation to assist in clinical decision making. Both management plans—inducing labor and continued fetal surveillance—are associated with low rates of maternal and fetal morbidity in the low-risk patient. Although there is no absolute time by which labor must be induced, most physicians believe that delivery should occur before 42 completed weeks. Compared with expectant management, several studies have demonstrated that routine induction at 41 weeks, using cervical ripening agents, is associated with lower cesarean delivery rates, lower perinatal mortality, decreased length of hospital stay, decreased hospital cost, and higher patient satisfaction. Several different agents are now available for cervical ripening, including intracervical or intravaginal preparation of prostaglandin, intracervical Foley bulb placement, and misoprostol. Oxytocin should ideally be initiated after the cervix is ripened. Induction at 41 weeks is becoming the preferred management of postterm pregnancy.
Because of the risk of macrosomia-associated birth trauma, ultrasonographic estimation of fetal weight should be obtained before induction of labor in a postterm pregnancy when macrosomia is suspected. If the estimated fetal weight is more than 5,000 g in a woman who does not have diabetes or 4,500 g in a woman with diabetes, elective cesarean delivery may be considered. It should be noted that there is no accurate way of estimating fetal weight at term; ultrasonographic estimates have a calculation error up to 500 g late in pregnancy. Clinically determined estimated fetal weights by palpation of the patient’s abdomen and Leopold maneuvers are similarly inaccurate.
For patients who are postterm, special precautions are taken at the time of delivery to provide prompt evaluation of the infant in the event of meconium passage. In a depressed infant, aggressive suctioning of the fetus with a laryngoscope decreases, but does not eliminate, the likelihood of MAS. In a vigorous infant with meconium passage, laryngoscopy and aggressive suctioning have not been shown to decrease the risk of MAS and are no longer recommended.
Similarly, routine amnioinfusion is not recommended during labor when meconium passage has been noted.
Clinical Follow-Up
Without a firm due date, and with an advanced pregnancy at the time of presentation, the concern for this patient’s pregnancy is not prematurity, but the risk of a postterm delivery. Weekly fetal evaluation with testing such as a biophysical profile can be reassuring that the fetus is not in jeopardy. If testing is no longer reassuring prior to the spontaneous onset of labor, induction of labor would be appropriate.
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