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It must be admitted that the duration of pregnancy not infrequently exceeds 280 days from the last menstrual period, and that when it lasts much longer large children are developed, which are frequently delivered only after great difficulty. Thus, whenever the menstrual history of the patient indicates that she has passed much beyond the tenth and is approaching the eleventh lunar month, we should consider the propriety of the induction of labour, provided that examination shows the child is larger than usual.

—J. Whitridge Williams (1903)


The above passage from Williams shows that pregnancies exceeding the expected normal length were problematic more than 100 years ago. These postterm pregnancies remain so today.

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?” Postmature is reserved for the relatively uncommon specific clinical fetal syndrome in which the newborn has recognizable 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 (2016b,d) is one that exceeds 420/7 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, 420/7 through 426/7 weeks.


The current definition of postterm pregnancy assumes that the last menses was followed by ovulation 2 weeks later. That said, some pregnancies may not actually be postterm. Instead, the calculation may reflect an error in gestational age estimation 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, 2017b) 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 (Martin, 2017). In the past, the proportion was much higher. This trend suggests earlier intervention, however, the added accuracy from ...

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