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
Abnormal fetal growth (macrosomia and dysmaturity syndrome)
Meconium-stained amniotic fluid
Nonreassuring fetal test results
Fetal and neonatal death
Need for neonatal intensive care
Dystocia (abnormal or difficult labor)
Perineal lacerations
Postmaturity 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 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 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.
