Postterm refers to a pregnancy that has reached or exceeded 42 weeks of 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 of gestation. This appears to have evolved as a result of current recommendations for postterm surveillance, which generally begin 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.
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).3 This pattern reflects the decision of many women to undergo delivery at 41 weeks of gestation based on recommendations from national organizations.
There are several recognized risk factors for postterm pregnancy, including primiparity and history of previous postterm pregnancy. Genetic predisposition appears to also play a role, as do excess maternal weight and male fetal sex. Rare associations include fetal anencephaly and placental sulfatase deficiency. In the absence of ultrasound dating, postterm pregnancy correlates with predictors of inaccurate recall of LMP, including young mothers, those of non-optimal prepregnancy weight, and smokers.
Complications of Postterm Pregnancy
The potential for complications in the 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 and neonatal risks associated with postterm pregnancy.
TABLE 35-1:RISKS ASSOCIATED WITH POSTTERM PREGNANCY ||Download (.pdf) TABLE 35-1:RISKS ASSOCIATED WITH POSTTERM PREGNANCY
|Maternal Risks ||Fetal and Neonatal Risks |
|Increased rate of labor induction ||Perinatal death |
|Dysfunctional labor ||Meconium aspiration syndrome |
|Macrosomia-related birth trauma ||Macrosomia and related birth trauma |
|Postpartum hemorrhage || |
The Cochrane review and meta-analysis of induction to improve birth outcomes found the data concerning cesarean section difficult to interpret because of heterogeneity of trial methodology. They also identified significant sources of confounding, including cervical ripeness at the time of induction. However, they recently concluded that a policy of induction of labour is associated with a significant reduction in abdominal delivery.
In contrast, data on postterm pregnancies between 2000 and 2003 from the British Columbia Perinatal Database Registry showed consistently higher rates of cesarean section among nulliparous women who were induced ...