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 eschew use of the term postdates because the real issue in many postterm pregnancies is "post-what dates?" Post mature is reserved for the relatively uncommon speciic clinical fetal syndrome in which the newborn has recognizable features indicating a pathologically prolonged pregnancy. herefore, 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 (2016b,d) is one that exceeds 42°/7 weeks, namely, 294 days or more from the first day of the last menstrual period. Importantly, this is 42 "completed weeks," as 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. The method that we use widely in this book is to divide the 42nd week into 7 days, that is, 42°/7 through 426' weeks.
The current deinition of postterm pregnancy assumes that the last menses was followed by ovulation 2 weeks later. hat said, some pregnancies may not actually be postterm. Instead, the because of faulty menstrual date recall or delayed ovulation. Thus, the two categories of pregnancies that reach 42 completed weeks are those truly 40 weeks past conception and those of less-advanced gestation but with inaccurately estimated gestational age. Even with exactly recalled menstrual dates, there still is imprecision, and the American College of Obstetricians and Gynecologists (2016d, 20 17b) considers first-trimester sonography to be the most accurate method to establish or confirm gestational age. Several clinical studies support this practice (Bennett, 2004; Blondel, 2002; Joseph, 2007).
Of the 3.93 million neonates born in the United States during 2015, 0.4 percent were delivered at 42 weeks or later (11artin, 2017). In the past, the proportion was much higher. This trend suggests earlier intervention, however, the added accuracy from earlier sonographic dating ofgestational age is another factor.
To identiy potential predisposing factors for postterm pregnancy, Olesen and associates (2006) analyzed various characteristics in the Danish Birth Cohort. Only prepregnancy body mass index (BMI) :25 and nulliparity were signiicantly associated with prolonged pregnancy. Mission (2015) and Arrowsmith (2011) and their coworkers also reported similar associations. In nulliparas, those whose cervical length at midpregnancy is longer, that is, in the third or fourth quartile, are twice as likely to deliver after 42 weeks (van der Ven, 2016).
The tendency for some mothers to have repeated postterm births suggests that some prolonged pregnancies are biologically determined. Oberg and colleagues (2013) reported that when mother and daughter had a prolonged pregnancy, the risk for the daughter to have a subsequent postterm pregnancy was signiicantly increased. Laursen and associates (2004) found that maternal, but not paternal, genes inluenced prolonged pregnancy. As discussed in Chapter 5 (p. 105), rare fetal-placental factors that predispose to postterm pregnancy include anencephaly, adrenal hypoplasia, and X-linked placental sulfatase deiciency (Ayyavoo, 2014; MacDonald, 1965).