Chapter 35: Postterm Pregnancy

INTRODUCTION

Definition

Postterm refers to a pregnancy that has reached or exceeded 42 weeks’ gestation or 294 days from the first day of the last menstrual period (LMP). Confusion often arises when patients are referred to as “postterm” at 41 weeks’ gestation. This appears to have evolved as a result of current recommendations for postterm surveillance, which generally begins at 41 weeks’ gestation. Maternity care providers should be clear about gestational age when using the phrase “postterm,” which in this chapter refers to pregnancies at or beyond 42 weeks unless otherwise specified.

Prevalence

In general, postterm pregnancy occurs in approximately 7 percent of gestations, with up to 1.4 percent of pregnancies reaching at least 301 days (43 weeks). When first- or second-trimester ultrasound is used for pregnancy dating, rates of postterm pregnancy are decreased. In one study, the incidence fell from 12.1 percent using LMP data to 3.4 percent using an ultrasound estimate. Over time, the number of deliveries occurring at 42 weeks has decreased (from 7.1% in 1980 to 2.9% in 1995). This pattern reflects the decision of many women to undergo delivery at 41 weeks of gestation based on recommendations from national organizations.

Risk Factors

There are several recognized risk factors for postterm pregnancy, including primiparity, maternal age >30 years, and history of previous postterm pregnancy (27% recurrence risk with one previous postterm pregnancy and 39% with two prior postterm pregnancies). Genetic predisposition appears to also play a role, as do excess maternal weight and male fetal sex. Women who were themselves born postterm are more likely to go postterm, and the risk of postterm pregnancy is increased with a paternal history of postterm pregnancy. Rarer associations include fetal anencephaly, fetal adrenal insufficiency, and placental sulfatase deficiency. In the absence of ultrasound dating, postterm pregnancy correlates with predictors of inaccurate recall of LMP, including young maternal age, those of nonoptimal prepregnancy weight, and smoking. Menstrual dating is also more inaccurate in people with long or irregular menstrual cycles in which there is substantial variation in timing of ovulation.

Complications of Postterm Pregnancy

The potential for complications in postterm pregnancy has long been recognized. Much effort has been put into identifying and quantifying these risks. Table 35-1 highlights the generally accepted maternal and fetal neonatal risks associated with postterm pregnancy.

Maternal risks are largely related to the fetal overgrowth that frequently accompanies postterm pregnancy. Most postterm ...