Merck Manual

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Postterm Pregnancy

By

Julie S. Moldenhauer

, MD, Children's Hospital of Philadelphia

Medically Reviewed Jul 2021 | Modified Sep 2022
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Postterm pregnancy refers to gestation that lasts 42 (> 41 6/7) weeks. Late-term pregnancy is defined as 41 0/7 to 41 6/7 weeks. Antenatal surveillance should be considered at 41 weeks. Induction of labor should be considered after 41 weeks and is recommended after 42 weeks.

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 weeks) is the most accurate with accepted variation of +/ 7 days. Later in gestation, the variation increases to +/ 14 days at 20 to 30 weeks gestation and +/ 21 days after 30 weeks.

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

Postmaturity Postterm Infants A postterm infant is an infant born after 42 weeks gestation. The cause of postmaturity is generally unknown, but previous postterm delivery increases the risk 2- to 3-fold. Postmaturity may... read more refers to the condition of the fetus that results when the placenta can no longer maintain a healthy environment for growth and development, usually because the pregnancy has lasted too long. The fetus may have dry, peeling skin, overgrown nails, a large amount of scalp hair, marked creases on the palms and soles, lack of fat deposition, and skin that is stained green or yellow by meconium. Meconium aspiration syndrome Meconium Aspiration Syndrome Intrapartum meconium aspiration can cause inflammatory pneumonitis and mechanical bronchial obstruction, causing a syndrome of respiratory distress. Findings include tachypnea, rales and rhonchi... read more is a risk.

Antenatal surveillance should be considered at 41 weeks; it involves one of the following:

  • Nonstress testing

  • Modified biophysical profile (nonstress testing and assessment of amniotic fluid volume)

  • A full biophysical profile (assessment of amniotic fluid volume and fetal movement, tone, breathing, and heart rate)

Treatment

  • Induction of labor and delivery

  • Sometimes cesarean delivery

If there is evidence of fetal compromise or oligohydramnios, delivery is required. Induction of labor Induction of Labor Induction of labor is stimulation of uterine contractions before spontaneous labor to achieve vaginal delivery. Induction of labor can be Medically indicated (eg, for preeclampsia or fetal compromise)... read more can be considered at 41 to 42 weeks, particularly if the cervix is favorable, and is recommended after 42 weeks.

Sometimes cesarean delivery is required.

Key Points

  • Accurate gestational age estimation is essential in making a diagnosis of postterm pregnancy; ultrasonography early in gestation (up to 20 weeks) is the most accurate method.

  • Consider antenatal surveillance (eg, nonstress testing, biophysical profile) at 41 weeks.

  • If there is evidence of fetal compromise or oligohydramnios, delivery is required.

  • Consider inducing labor at 41 to 42 weeks; it is recommended after 42 weeks.

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