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MS WordPerinatologist Corner - C.E.U/C.M.E. Modules
Post-term Pregnancy and Induction of Labor
Sponsored by The Indian Health Service Clinical Support Center
2 Background and Risks
Post-term pregnancy is defined as gestation lasting longer than 42.0 weeks (294 days) from the onset of the last menstrual period. About 10% (3-12%) of all pregnancies continue beyond 42 weeks.
The risks in pregnancies longer than 42 weeks include increases in perinatal morbidity and mortality. The most significant risk is perinatal death (stillbirths plus neonatal deaths) and is variously reported as 4 to 7 per 1000 deliveries (versus 2-3 per 1000 deliveries at 40 weeks). These risk are distinctly higher if the risk of stillbirth is expressed as a function of ongoing pregnancies (impending stillbirths divided by the total number of undelivered fetuses), rather than as a function of total births.
Other complications of post-term pregnancy include:
- Fetal macrosomia (>4500 g) that is often accompanied by labor dystocia, an increase in cesarean birth, shoulder dystocia, and birth injury.
- The opposite situation, fetal growth restriction (the classic small, wrinkled baby with peeling skin and long nails) may occur in the 2% of post-term pregnancy as a result of the development of severe uteroplacental insufficiency.
- Meconium passage occurs in 37% of such pregnancies (versus 17% at 40 weeks) and, while probably usually representing maturation of fetal colonic innervation, may be associated with fetal intolerance of labor, meconium aspiration, and neonatal asphyxia.
- Oligohydramnios is also more common and most likely reflects relative placental insufficiency as well. It is probably a later sign, reflecting shunting of blood to the central nervous system, with relative fetal lower body (and thus renal) hypoperfusion, and resulting in decreased fetal urine production. Thick meconium almost always represents a decreased volume of amniotic fluid diluent (oligohydramnios), and will commonly be accompanied by signs of cord compression in labor for this same reason.
- Other complications of the newborn include a short-term increase in the incidence of hypoglycemia and seizures, as well as long-term neurologic symptoms.
