CHAPTER 46: Postterm Pregnancy

The adjectives postterm, prolonged, postdates, and postmature are used interchangeably to describe pregnancies that have exceeded a duration considered to be the upper limit of normal. We eschew use of the term postdates because the real issue in many postterm pregnancies is uncertainty in the estimated date of delivery (EDD). Postmature is reserved for the uncommon clinical fetal syndrome in which the newborn has recognizable features indicating a pathologically prolonged pregnancy. Thus, postterm or prolonged pregnancy is our preferred term for an extended pregnancy.

The definition of postterm pregnancy is one that exceeds 42^0/7 weeks or is ≥294 days from the first day of the last menstrual period (LMP). Importantly, this is 42 “completed weeks.” Pregnancies between 41^1/7 and 41^6/7 weeks, although in the 42nd week, do not complete 42 weeks until the seventh day has elapsed. These are considered late term (American College of Obstetricians and Gynecologists, 2019e, 2020b).

ESTIMATED GESTATIONAL AGE

The current definition of postterm pregnancy assumes that ovulation occurs 2 weeks after the LMP. Thus, some pregnancies may not actually be postterm because of error in menstrual date recall or delayed ovulation. Even with exactly recalled menstrual dates, there still is imprecision, and first-trimester sonography is the most accurate method to establish or confirm gestational age (American College of Obstetricians and Gynecologists, 2019b, 2020b). Several studies support this (Bennett, 2004; Joseph, 2007). If available, gestational ages calculated from the LMP and from the first accurate ultrasound are reconciled as shown in Table 14-1 (p. 248), and the EDD is recorded (American College of Obstetricians and Gynecologists, 2019c).

INCIDENCE

Of the 3.75 million neonates born in the United States during 2019, 0.3 percent were delivered at ≥42 weeks (Martin, 2021). This rate has declined because of improved pregnancy dating accuracy and earlier intervention. To identify predisposing factors for postterm pregnancy, one analysis of the Danish Birth Cohort found only prepregnancy body mass index ≥25 and nulliparity to be significantly associated (Olesen, 2006). Others reported similar associations (Arrowsmith, 2011; Mission, 2015). Nulliparas with a long midpregnancy cervical length (third or fourth quartile) are twice as likely to deliver after 42 weeks (van der Ven, 2016). Last, the risk for adverse pregnancy outcomes in postterm pregnancies increases with advancing maternal age (Kortekaas, 2020).

The tendency for some mothers to have repeated postterm births suggests that some prolonged pregnancies are biologically determined. In one study, if a mother and daughter had a prolonged pregnancy, the risk for the daughter to have a subsequent postterm pregnancy was significantly increased (Oberg, 2013). Maternal, not paternal, genes influence prolonged pregnancy (Laursen, 2004). For example, genes from one locus of chromosome 2q13 are associated with gestational duration ...