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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.

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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.

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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.

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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.

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Sonographic evaluation of gestational age during pregnancy has been ...

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