The adjectives postterm, prolonged, postdates, and postmature are often loosely used interchangeably to describe pregnancies that have exceeded a duration considered to be the upper limit of normal. We do not use the term postdates because the real issue in many postterm pregnancies is “post-what dates?” Postmature is reserved for the relatively uncommon specific clinical fetal syndrome in which the infant has recognizable clinical features indicating a pathologically prolonged pregnancy. Therefore, postterm or prolonged pregnancy is our preferred expression for an extended pregnancy.
The international definition of prolonged pregnancy, endorsed by the American College of Obstetricians and Gynecologists (2004), is 42 completed weeks (294 days) or more from the first day of the last menstrual period. It is important to emphasize the phrase “42 completed weeks.” Pregnancies between 41 weeks 1 day and 41 weeks 6 days, although in the 42nd week, do not complete 42 weeks until the seventh day has elapsed. Thus, technically speaking, prolonged pregnancy could begin either on day 294 or on day 295 following the onset of the last menses. Which is it? Day 294 or 295? We cannot resolve this question, and emphasize this dilemma only to ensure that litigators and others understand that some imprecision is inevitable when there is biological variation such as with prolonged pregnancy. Amersi and Grimes (1998) have cautioned against use of ordinal numbers such as “42nd week” because of imprecision. For example, “42nd week” refers to 41 weeks and 1 through 6 days, whereas the cardinal number “42 weeks” refers to precisely 42 completed weeks.
The definition of postterm pregnancy as one that persists for 42 weeks or more from the onset of a menstrual period assumes that the last menses was followed by ovulation 2 weeks later. This said, some pregnancies may not actually be postterm, but rather are the result of an error in gestational age estimation because of faulty recall of menstrual dates or delayed ovulation. Thus, there are two categories of pregnancies that reach 42 completed weeks: (1) those truly 40 weeks past conception, and (2) those of less advanced gestation but with inaccurately estimated gestational age.
Even with precisely recalled menstrual dates, there is still not precision. Specifically, Munster and associates (1992) reported that large variations in menstrual cycle lengths are common in normal women. Boyce and associates (1976) studied 317 French women with periconceptional basal body temperature profiles. They found that 70 percent who completed 42 postmenstrual weeks had a less advanced gestation based on ovulation dates. These variations in menstrual cycle may partially explain why a relatively small proportion of fetuses delivered postterm have evidence of postmaturity. Even so, because there is no accurate method to identify the truly prolonged pregnancy, all those judged to be 42 completed weeks should be managed as if abnormally prolonged.
Sonographic evaluation of gestational age during pregnancy has been used to add precision. Blondel and colleagues (2002) studied 44,623 women delivered at the Royal Victoria Hospital in Montreal. They analyzed postterm pregnancy rates according to six algorithms for gestational age estimates based on either the last menstrual period, sonographic evaluation at 16 to 18 weeks, or both. The proportion of births at 42 weeks or longer was 6.4 percent when based on the last menstrual period alone, but was 1.9 percent when based on sonographic measurements alone. Sonographic pregnancy dating at 12 weeks or less resulted in a 2.7-percent incidence of postterm gestation compared with 3.7 percent in a group assessed at 13 to 24 weeks (Caughey and co-workers, 2008). These findings suggest that menstrual dates are frequently inaccurate in predicting postterm pregnancy. Subsequent clinical studies have confirmed these observations (Bennett, 2004; Joseph, 2007; Wingate, 2007, and all their colleagues).
From their review, Divon and Feldman-Leidner (2008) report that the incidence of postterm pregnancy ranges from 4 to 19 percent. Using criteria that likely overestimate the incidence, approximately 6 percent of 4 million infants born in the United States during 2006 were estimated to have been delivered at 42 weeks or more (Martin and colleagues, 2009). The trend toward fewer births at 42 weeks suggests earlier intervention. Specifically, in 2000, 7.2 percent of births in this country were 42 weeks or beyond, compared with 5.6 percent in 2006.
There are contradictory findings concerning the significance of maternal demographic factors such as parity, prior postterm birth, socioeconomic class, and age. Olesen and colleagues (2006) analyzed a variety of risk factors in 3392 participants in the 1998 to 2001 Danish Birth Cohort. They reported that only prepregnancy body mass index (BMI) ≥ 25 and nulliparity were significantly associated with prolonged pregnancy. Denison (2008) and Caughey (2009) and their co-workers also reported similar associations.
The tendency for some mothers to have repeated postterm births suggests that some prolonged pregnancies are biologically determined. In 27,677 births in Norway, Bakketeig and Bergsjø (1991) reported that the incidence of a subsequent postterm birth increased from 10 to 27 percent if the first birth was postterm. This was increased to 39 percent if there had been two previous, successive postterm deliveries. Similar results were reported from Missouri by Kistka and colleagues (2007). And Mogren and colleagues (1999) reported that prolonged pregnancy recurred across generations in Swedish women. When mother and daughter had a prolonged pregnancy, the risk for the daughter to have a subsequent postterm pregnancy was increased two- to threefold. In another Swedish study, Laursen and associates (2004) found that maternal, but not paternal, genes influenced prolonged pregnancy. Rare fetal–placental factors that have been reported as predisposing to postterm pregnancy include anencephaly, adrenal hypoplasia, and X-linked placental sulfatase deficiency (MacDonald and Siiteri, 1965; Naeye, 1978; Rabe and colleagues, 1983).