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Postterm Pregnancy

by Julie S. Moldenhauer, MD

Postterm pregnancy refers to gestation that lasts 42 wk. A full or modified biophysical profile, including nonstress testing, and often routine delivery are recommended at 41 wk.

Accurate gestational age estimation is essential in making a diagnosis of postterm pregnancy. In women with regular, normal menstrual cycles, gestational age can be estimated based on the first day of the last normal menstrual period. If dating is uncertain or inconsistent with menstrual dating, ultrasonography early in gestation (up to 20 wk) is the most accurate with accepted variation of +/ 7 days. Later in gestation, the variation increases to +/ 14 days at 20 to 30 wk gestation and +/ 21 days after 30 wk.

Postterm pregnancy increases risks for the woman and fetus. Risks include

  • Abnormal fetal growth (macrosomia and dysmaturity syndrome)

  • Oligohydramnios

  • Meconium-stained amniotic fluid

  • Nonreassuring fetal test results

  • Fetal and neonatal death

  • Dystocia (abnormal or difficult labor)

  • Cesarean delivery

Most experts recommend that antenatal surveillance be initiated at 41 wk; it involves a modified biophysical profile (nonstress testing and assessment of amniotic fluid volume) or a full biophysical profile (assessment of amniotic fluid volume and fetal movement, tone, breathing, and heart rate). If there is evidence of fetal compromise or oligohydramnios, delivery is required. For many obstetricians, the trend is to induce labor (see Induction of Labor) in all pregnancies > 41 wk, particularly if the cervix is favorable.

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