Definition

The American College of Obstetricians and Gynecologists (ACOG), International Federation of Gynecology and Obstetrics (FIGO), and the World Health Organization (WHO) have defined a postterm pregnancy as a gestation that has completed or gone beyond 42, weeks or 294 days, from the first day of the last menstrual period (LMP). This gestational age cutoff has been used for several decades and was first suggested based on early studies that reported an increased risk of fetal death at 42 weeks and beyond. However, in view of more recent perinatal mortality data derived from accurately dated pregnancies, it would be reasonable to conclude that the gestational age that warrants clinical concern should be 41 weeks.

Many terms have been used in the literature, including postmature, postdates, prolonged, and postterm. These terms have been used with varying definitions, which has led to some confusion regarding proper terminology. Recently ACOG and the Society for Maternal-Fetal Medicine (SMFM) has endorsed the use of new terminology recommended by the Defining Term Pregnancy Workgroup to decrease confusion among physicians, patients, and researchers and to designate gestational ages at higher risk. Pregnancies are now designated to be “early term” if they are 37 ^0/7 weeks through 38 ^6/7 weeks. Full term is defined at 39 ^0/7 weeks through 40 ^6/7 weeks. Pregnancies are to be designated as “late term” if they are 41 ^0/7 weeks through 41 ^6/7 weeks. Postterm will continue to be defined as 42 ^0/7 weeks and beyond.

Incidence

According to the vital statistics reported by the Centers for Disease Control and Prevention (CDC), the overall incidence of postterm pregnancies was 5.6% in 2012 and has not significantly changed compared with previous years. Other published studies have shown varying frequency of postterm pregnancies depending on the population studied. The incidence of prolonged pregnancies in European countries also varies widely, with rates as low as 0.4% in Austria and as high as 7% in Denmark and Sweden. These differences are most likely explained by different approaches for managing pregnancies beyond the estimated date of delivery (EDD) and different criteria for gestational age dating.

Etiology

The etiology of the majority of pregnancies that are late term or postterm is unknown, but some pregnancies may be defined as late term or postterm as the result of an error in dating. It is common practice to assign an EDD based on the LMP. This practice has been proven by several studies to be unreliable and may have led to the incorrect classification of a pregnancy as late term or postterm.

Understanding the events that lead to parturition in human gestation may help to provide clues to the pathophysiology in prolonged pregnancies. Parturition is the result of a complex interplay among the mother, fetus, and placenta. The mechanism in human gestation is unknown but may be similar to that of other mammals. In sheep, the hypothalamic-pituitary-adrenal (HPA) axis is important in the timing of birth. The release of corticotropin-releasing hormone (CRH) from the fetal brain results in the secretion of adrenocorticotropic hormone (ACTH) from the pituitary gland and cortisol from the adrenal gland. The increase in cortisol parallels an increase in the secretion of prostaglandin and estrogens and a fall in progesterone. Decreases in progesterone and increases in prostaglandins are known triggers of uterine myometrium. Further support for the role of the HPA axis in the initiation of labor is seen in studies with hypophysectomized sheep; disruption of the HPA axis results in prolonged pregnancy. More recent studies have proposed a similar involvement of the HPA axis in human gestation, and its dysregulation may play a role in prolonged pregnancies.

Early studies likened anencephaly to the hypophysectomized sheep. It is hypothesized that the absence of the fetal brain in the anencephalic fetus may result in a similar dysfunction of the HPA axis and may lead to prolonged gestation. Epidemiologic studies of anencephalic pregnancies have observed prolongation of pregnancy. These findings support current thinking that the interaction between the fetal brain and placenta plays an important role in triggering labor.

Pregnancies complicated by placental sulfatase deficiency, an X-linked recessive disorder characterized by the absence of the enzyme steroid sulfatase, are marked by abnormally low estriol levels and, in general, fail to go into spontaneous labor. This is an example of a genetic etiology for prolonged pregnancy and lends further support to the important role of the placenta in the initiation of labor.

A number of observational studies have identified risk factors for postterm pregnancy including primigravidity, prior postterm pregnancy, male fetus, obesity, and a genetic predisposition. A 10-year cohort study of births in Norway failed to find a strong association of risk factors with postterm pregnancy but may have had a bias toward nondetection. Intergenerational studies suggest a genetic predisposition for postterm pregnancy. Mothers who themselves were postterm also have an increased risk of prolonged pregnancy. Twin studies have found higher rates of concordance for postterm pregnancy among female twins, compared with male twins, implicating a maternal influence on the risk for prolonged pregnancy.

Diagnosis

The diagnosis of truly late term and postterm pregnancy is based on accurate gestational dating. The three most commonly used methods to determine the EDD are (1) knowledge of the date of the LMP, (2) knowledge of the timing of intercourse, and (3) early ultrasound assessment. Other methods have been described but are rarely used in contemporary practice, including the determination of uterine size, quickening, ability to detect fetal heart tones by Doppler auscultation, and fundal height measurement. In most cases, the date of conception is rarely known and therefore is infrequently used to determine gestational age. The EDD is most commonly assigned based on the first day of the LMP, but this assumes that conception occurs on the fourteenth day of the menstrual cycle. This method can be very inaccurate because the timing of ovulation is variable among an individual’s menstrual cycles and between individuals. Basing gestational age solely on the LMP generally results in an overestimation of gestational age and may result in a higher frequency of induction of labor for presumed postterm pregnancy.

The use of ultrasound to determine the accuracy of gestational dating based on the LMP is superior to the use of the LMP alone. The EDD is most accurately determined if the crown-rump length is measured in the first trimester with an error of ± 5 to 7 days. Boyd and colleagues showed that the incidence of patients whose pregnancy exceeded 293 days was 7.5% based on menstrual dating and declined to 2.6% when dates were determined by early sonographic examination. A similar conclusion was reached by Gardosi and associates, who evaluated 24,675 spontaneous, normal singleton deliveries and showed a decline in the postterm (>294 days) pregnancy rate from 9.5% when pregnancies were dated by LMP to 1.5% when ultrasound dating was used. These authors also reported that about 72% of routine labor inductions at 42 weeks’ gestation were not indicated because they were performed before the patients reached 42 weeks based on ultrasound assessment of gestational age. Similarly, Nguyen and coworkers evaluated 14,805 spontaneous deliveries with a reliable LMP and showed that ultrasound dating reduced the proportion of deliveries beyond 294 days of gestation by 39% (from 7.9% to 5.2%). Bennett and colleagues confirmed these findings in a prospective, randomized study of 218 women and found fewer postterm inductions of labor in women dated by a first-trimester sonogram when compared with women whose dates were established by second-trimester sonography.

Perinatal Morbidity and Mortality

Numerous studies have evaluated the risk to the fetus in late-term and postterm pregnancies. Early descriptive studies found that pregnancies that continued past their EDD had an increased risk of fetal death. In 1963, McClure found a twofold increase in “fetal distress” at 42 weeks with an increase in operative deliveries and surmised that 42 weeks constituted a significant risk to the fetus and proposed intervening with induction of labor or cesarean delivery to avoid the risk of fetal death. Early studies were likely fraught with inaccurate dating and inconsistent definitions of postterm pregnancy. Lastly, it is important to note that these studies included pregnancies complicated by fetal anomalies, intrauterine growth restriction (IUGR), and mothers with coexisting medical conditions, all of which increase the risk of fetal demise.

More recent observational studies that have evaluated the risk of perinatal mortality at each gestational week show an increased risk as gestational age advances beyond the EDD. Divon and associates evaluated fetal and neonatal mortality rates in 181,524 accurately dated full-term, late-term, and postterm pregnancies. A significant increase in fetal mortality was detected from 41 weeks’ gestation onward (odds ratios [ORs] of 1.5, 1.8, and 2.9 at 41, 42, and 43 weeks, respectively). Campbell and colleagues performed a multivariate analysis of factors associated with perinatal death among 65,796 singleton postterm births (≥294 days). Three variables were identified as independent predictors of perinatal mortality: (1) birthweight lower than the 10th percentile for gestational age had a relative risk (RR) of 5.7 and a 95% confidence interval (CI) of 4.4 to 7.4; (2) maternal age 35 years or greater had an RR of 1.88 and a 95% CI of 1.2 to 2.9; and (3) birthweight at the 90th percentile for gestational age or above was associated with a modest protective effect for perinatal death (RR, 0.51; 95% CI, 0.26 to 1.0).

Many of these studies have used perinatal mortality rate (PMR), which has been suggested by Smith and others to be an inappropriate assessment of risk to the fetus. The denominator in the calculation of the PMR is the number of deliveries. As stated by Smith, “Estimating the probability of an event requires that the number of events (numerator) be divided by the number of subjects at risk for that event (denominator).” Therefore it seems logical to calculate fetal mortality as fetal deaths per 1000 ongoing pregnancies, rather than per 1000 deliveries. When Hilder and colleagues used ongoing pregnancies in a large retrospective study that included 171,527 births, higher rates of stillbirth were found. A nadir was seen at 41 weeks, but compared with 37 weeks’ gestation, an eightfold increase in stillbirths at 43 weeks was reported ( Fig. 36-1 ). Using the Scottish birth registry, Smith also found a significant increase in the risk of stillbirth from 37 weeks (0.4/1000) to 43 weeks (11.5/1000).

The summed mortality at each gestation for the rate of stillbirth ( red ), neonatal death ( blue ), and postneonatal death ( green ) expressed per 1000 ongoing pregnancies.

(Modified from Hilder L, Costeloe K, Thilaganathan B. Prolonged pregnancy: evaluating gestation specific risks of fetal and infant mortality. BJOG. 1998;105:169.)

Several studies have examined the association of perinatal morbidity with postterm pregnancy. Clausson and colleagues evaluated a large Swedish database of term and postterm (defined as ≥294 days) singleton, normal neonates and showed that postterm pregnancies were associated with an increased frequency of neonatal convulsions, meconium aspiration syndrome, and Apgar scores of less than 4 at 5 minutes ( Table 36-1 ). Tunon and associates compared neonatal intensive care unit (NICU) admission rates among 10,048 term pregnancies and 246 postterm pregnancies (≥296 days by both scan and LMP dates). Postterm pregnancy was associated with a significant increase in NICU admissions (OR, 2.05; 95% CI, 1.35 to 3.12).

Guidetti and colleagues reported an increased incidence of perinatal morbidity at 41 weeks’ gestation or greater. Maternal and fetal complications were evaluated in a large ( n = 45,673) retrospective cohort study by Caughey and Musci. These authors documented a significant increase in the rate of intrauterine fetal death (IUFD) beyond 41 weeks. They concluded that risks to both the mother and the infant increase as pregnancy progresses beyond 40 weeks’ gestation.