Obstetrics and Gynecology

Postterm pregnancy

Postterm Pregnancy

1. What every clinician should know


Postterm pregnancy is currently defined as pregnancies at or beyond 42 weeks' gestation. However, pregnancies that reach 41 weeks' gestation are considered long enough to intervene upon as well and have been called prolonged pregnancies. Pregnancies that reach 41 and 42 weeks and beyond have also been inappropriately called post date pregnancies, which is not a preferred term.

Clinical features and incidence

The prevalence of postterm pregnancy depends on the particular patient population, including such factors as the percentage of primigravid women, the prevalence of obesity, proportion of women with pregnancy complications, the prevalence of ultrasound assessment of gestational age and the frequency of spontaneous preterm birth. Local practice patterns, such as the rates of scheduled cesarean delivery and routine labor induction, also will affect the overall prevalence of postterm birth, particularly as induction prior to 42 weeks of gestation has increased in recent years. In the United States in 2005, 14% of all pregnancies progressed beyond 41 weeks of gestation and just under 6% progressed beyond 42 weeks of gestation. This is lower than the approximately 18% of pregnancies beyond 41 weeks and 10% beyond 42 weeks in 1998.

Risk factors

There are a number of risk factors associated with postterm pregnancy that may or may not have biologic causal association. First among these is nulliparity, with a greater proportion of nulliparas reaching 40, 41, or 42 weeks of gestation and the median duration of pregnancy being 2 days longer in nulliparas as compared with multiparas. Recent data have also shown an association with male fetuses.

Additionally, it has been described that African-American women have higher rates of preterm delivery, raising the possibility that race/ethnicity may be associated with overall gestational age and prolonged pregnancy in particular. One recent study found a decreased risk of postterm pregnancy among African Americans, Asians, and Latinas as compared to Caucasians. Further, some effects of race/ethnicity have been described to vary between obese and non-obese patients.

Obesity has been found to be associated with postterm pregnancy in several studies. The association may have actual causality. First, studies have demonstrated this finding consistently and secondly there is a dose-response effect with a greater Incidence among women who are obese than overweight women.

The theoretical mechanisms for the association between obesity and postterm pregnancy remain unclear. Since adipose tissue is hormonally active, and since obese women may have an altered metabolic status, it is possible that endocrine factors involved in the initiation of labor are altered in obese women. The long-noted associations between lower pre-pregnancy BMI and increased spontaneous preterm birth are consistent with our findings and may be explained by a common as yet unknown mechanism regarding parturition, potentially related to circulating levels of estrogen or progesterone.

Since, evolutionarily, as a species we have evolved to face the environmental pressures of food scarcity, it is likely that the outcomes of a postterm pregnancy rarely served as an evolutionary pressure related to obesity 10,000 years ago when we existed primarily as nomadic tribes with a lower median BMI than today. Thus, there is likely little benefit to the fetus for the pregnancy to proceed beyond 42 weeks of gestation, and such postterm pregnancies may be a product of current intrinsic and environmental factors.

2. Diagnosis and differential diagnosis

Accurate pregnancy dating is critical to the diagnosis of postterm pregnancy. In one recent study, the routine use of ultrasound to confirm pregnancy dating decreased the overall incidence of postterm pregnancy from 12% to 3%. This is because women are far more likely to be oligo-ovulatory and have delayed ovulation than polyovulatory with early ovulation in any given menstrual cycle, which would place them at an earlier gestational age than that predicted by the first day of their last menstrual period.

Moreover, early ultrasound for pregnancy dating may be superior to mid-trimester ultrasound in this regard. In a small prospective randomized trial, Bennett et al demonstrated that routine first trimester ultrasound for pregnancy dating reduced the incidence of postterm pregnancy from 13% to 5% when compared with second trimester ultrasound dating.

In another recent study of this issue, it was demonstrated that not only did first trimester ultrasound dating lead to lower rates of postterm pregnancy and pregnancy beyond 41 weeks of gestation being diagnosed, it also reveals a greater difference in the rate of perinatal complications between term and postterm pregnancies. This is due to the misclassification bias that usually occurs with misdating.

Misclassification of women who are term as postterm and women who are postterm as term leads to a smaller difference in the rate of complications between term and postterm pregnancies. Thus, older studies of women whose pregnancies did not have dating confirmation by ultrasound underestimate the rates of complications seen in postterm pregnancies.


As noted above, a number of women with a diagnosis of postterm or prolonged pregnancy are misdiagnosed. The leading reason for misdiagnosis is that adequate pregnancy dating is not established early in pregnancy. This underscores the importance of early pregnancy dating including the use of first trimester ultrasound to establish the gestational age of the pregnancy. From the study by Bennett et al., early pregnancy dating reduced the risk of postterm pregnancy from 13% to 5%.

4. Complications

Recent studies have shown that the risks to the fetus and to the mother of continuing the pregnancy beyond the estimated date of delivery is greater than originally appreciated. Risks have traditionally been underestimated for two reasons. First, earlier studies were published before the routine use of obstetric ultrasound and, as a result, likely included many pregnancies that were not truly postterm. As noted above, such a misclassification bias would artificially lower the complication rates of pregnancies designated postterm and increase the complication rates in those designated term, resulting in a diminution in the difference between term and postterm pregnancies.

The second issue relates to the definition of stillbirth rates. Traditionally, stillbirth rates were calculated using all pregnancies delivered at a given gestational age as the denominator. However, once a fetus is delivered, it is no longer at risk of intrauterine fetal demise, and use of this denominator has traditionally underestimated the risk of stillbirth. The appropriate denominator is not all deliveries at a given gestational age but ongoing (undelivered) pregnancies. In one retrospective study of over 170,000 singleton births, for example, Hilder et al demonstrated that the stillbirth rate increased six-fold (from 0.35 to 2.12 per 1,000 pregnancies) when the denominator was changed from all deliveries to ongoing (undelivered) pregnancies.

5. Prognosis and outcome

Maternal and fetal/neonatal outcomes

The overall prognosis is excellent, but to be clear, there are risks of a number of outcomes including cesarean delivery, induction of labor, stillbirth, and meconium/meconium aspiration syndrome. We also see increased risk of postpartum hemorrhage, fetal macrosomia, and chorioamnionitis as well. These complications are noted above in section 4.

Impact on long-term health

Long-term, postterm pregnancy may impact a woman specifically through the complications of pregnancy - increased risk of cesarean delivery, induction of labor, and fetal complications such as stillbirth and meconium aspiration syndrome. Additionally, there is evidence that women with a previous postterm pregnancy have increased risk in future pregnancies of going postterm.

6. What is the evidence for specific management and treatment recommendations

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