Excerpt from Postterm Pregnancy

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Postterm pregnancies define pregnancies extending up to or after 42 weeks. The reported frequency is approximately 3-12%. The most frequent cause of postterm pregnancy is inaccurate dating criteria. Additional risk factors include primiparity, prior postterm pregnancy, male gender of the fetus, and genetic factors. Laursen et al studied monozygotic and dizygotic twins and their subsequent development of prolonged pregnancies. They found that maternal but not paternal genetic factors influenced the rate of postterm pregnancies and accounted for the etiology in as many as 30% of these pregnancies.

Although the last menstrual period (LMP) has been traditionally used to calculate the estimated due date (EDD), many inaccuracies exist using this method in women who have irregular cycles, have been on recent hormonal birth control, or who have first trimester bleeding. Ultrasonographic dating early in pregnancy can improve the reliability of the EDD; however, it is necessary to understand the margin of error reported at various times during each trimester. A calculated gestational age by composite biometry from a sonogram must be considered an estimate and must take into account the range of possibilities.

For example, crown-rump length (CRL) is 3-5 days, ultrasonography performed at 12-20 weeks of gestation is 1 week, at 20-30 weeks is 2 weeks, and after 30 weeks is 3 weeks. Thus, a pregnancy that is 35 weeks by a 31-week ultrasound could actually be anywhere from 32 weeks to 38 weeks (35 wk +/- 3 wk). If the calculated ultrasonographic gestational age varies from the LMP more than the respective range of error, it is used instead to establish the final EDD. The importance of determining by what method a pregnancy is dated cannot be overemphasized because this may have significant consequences if the physician delivers a so-called term pregnancy that is not or observes a so-called term pregnancy that is very postterm.

When determining a management plan for an impending postterm pregnancy (>40 wk of gestation but <42 wk), the first decision is whether to deliver a patient and, if so, when and by what route. If the physician decides not to deliver, the decision whether to institute antepartum fetal surveillance and what method(s) of surveillance to use must be discussed with the patient.

Note that if the pregnancy is at risk for an adverse outcome from an underlying condition, either maternal or fetal, inducing labor may proceed without documented lung maturity. Also, an elective induction of labor may proceed at or after 39 weeks of gestation in the absence of documented lung maturity provided that 36 weeks have elapsed since documentation of a positive human chorionic gonadotropin (+hCG) test finding, 20 weeks of fetal heart tones have been established by a fetoscope or 30 weeks by a Doppler examination, or 39 weeks' gestation have been established by a CRL or by an ultrasound performed before 20 weeks of gestation consistent with dates by the patient's LMP.

Please click here to view the full topic text: Postterm Pregnancy