Grand Rounds: Prolonged pregnancy: How long do you wait?

June 1, 2005

Postterm birth seems to get a lot less attention than preterm birth when clinicians are discussing the risks of complications and death. But, unlike babies born too soon, timely delivery can almost entirely prevent the risks—including stillbirth—linked with prolonged pregnancy.

What distinguishes a good perinatal outcome from a tragic one like stillbirth? It has a lot to do with the timely onset of labor and delivery. In fact, both preterm birth (delivery before 37 weeks of gestation) and postterm birth have been linked to elevated neonatal morbidity and mortality. For prolonged births, for example, the risk of stillbirth plus early neonatal deaths doubles by 42 weeks of gestation and rises fourfold by 43 weeks (compared to the risk at term). And this is by conservative estimates.

Despite this link and the fact that up to 10% of pregnancies are suspected of being postterm, the management of prolonged low-risk pregnancies has received far less attention than preterm birth. This despite the fact that-unlike preterm labor and birth-you can easily avoid the risks associated with postterm pregnancy simply by inducing labor.1 Our goal is to review in detail the risks (including the often overlooked maternal risks) of continuing pregnancy beyond the due date in low-risk pregnancies, the option of labor induction, and finally, to offer our suggested algorithm for managing low-risk postterm pregnancies.

Defining the problem Only relatively recently has postterm pregnancy been considered abnormal. In ancient Rome, for example, Hadrian laid down the law that a child born 11 months after the death of the father could still be regarded as legitimate.2 And early modern obstetrics providers stopped short of defining an upper limit of normal for length of gestation and were reluctant to induce labor at any point in gestation.2,3 In part, this was due to the difficulty in establishing an accurate estimation of gestational age. It was only in the 1950s that ob/gyns first began to appreciate the risks associated with prolonged pregnancy.

Unfortunately, you can't rely on a patient's recollection of the first day of her LMP. That-coupled with the natural variation in the timing of ovulation vis-a-vis menstruation-results in a high rate of inaccurate dating. For this reason, clinicians usually turn to other historical and physical data (Table 1) to confirm GA dating. With the possible exception of assisted reproductive technology (ART), ultrasound examination in the first and early midtrimester is the most accurate technique of dating a pregnancy. Routine use of early U/S to date pregnancies can reduce the rate of false-positive diagnoses and thereby the overall rate of postterm pregnancy from 7% to 10% to roughly 1% to 3%.4-7