Prolonged and Postterm Pregnancy

Key Points

Ultrasonography, preferably done in the first trimester, is the most accurate method with which to establish the EDD.
No gestational cutoff has been established by which to define a prolonged pregnancy in multiple gestations. The risk for stillbirth increases after 38 weeks in twins and after 35 weeks in triplets.
Late-term and postterm pregnancies are associated with an increased risk for perinatal morbidity and mortality, oligohydramnios, macrosomia, postmaturity, and maternal morbidity.
It seems prudent to initiate antenatal fetal surveillance at 41 weeks in a normal, uncomplicated pregnancy in the absence of intrauterine growth restriction.
Antenatal fetal surveillance at 41 weeks should include a modified biophysical profile at least once a week.
If the cervix is favorable at 41 weeks, induction of labor can be considered.
Delivery after 42 ^0/7 weeks and by 42 ^6/7 weeks is recommended based on the small but increased risk of perinatal morbidity and mortality.
Either prostaglandin preparation, PGE ₁ or PGE ₂ , can be used for induction of the postterm pregnancy.

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Postterm pregnancy is a gestation that has completed or gone beyond 42 weeks or 294 days, from the first day of the last menstrual period (LMP).
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Pregnancies are designated as “late term” at 41 ^0/7 weeks through 41 ^6/7 weeks.

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According to the vital statistics reported by the Centers for Disease Control and Prevention, the overall incidence of postterm pregnancies was 5.6% in 2012 and has not significantly changed compared with previous years.

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The etiology of the majority of pregnancies that are late term or postterm is unknown. Some pregnancies may be defined as late term or postterm as the result of an error in dating.
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A number of observational studies have identified risk factors for postterm pregnancy, including primigravidity, prior postterm pregnancy, male fetus, obesity, and a genetic predisposition.

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The diagnosis of truly late term and postterm pregnancy is based on accurate gestational dating.
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The use of ultrasound to determine the accuracy of gestational dating based on the LMP is superior to the use of the LMP alone. The estimated date of delivery (EDD) is most accurately determined if the crown-rump length is measured in the first trimester with an error of ± 5 to 7 days.

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