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POSTTERM GESTATION

The average length of human pregnancy, calculated from the first day of the last menstrual period, is 280 days (ie, 40 weeks). By current definition, a postterm pregnancy is a gestation lasting 42 weeks or more postterm gestation, late pregnancy, postdate, overdue (ie, 295 days from the first.day of the last menstrual period). The incidence ranges from 4% to 14%, with an average of 10%. Although the true frequency is unknown because many cases result from the inability to time conception accurately, some pregnancies clearly proceed beyond 42 weeks. In 1993 approximately 9% of pregnancies were delivered after 42 weeks or more.

The expected date of delivery is most reliably determined early in pregnancy. The date of the last menstrual period, time of quickening (first fetal movement), and early uterine size determination are all helpful historic and physical data that help to determine the expected date of delivery.

Most of the complications of a postterm gestation relate to the fetus. Specifically, most studies show antepartum, intrapartum, and neonatal deaths are increased at 42 weeks. At 43 weeks of gestation, the perinatal mortality rate is doubled; it is increased four- to sixfold by 44 weeks of gestation.

Antepartum interventions are indicated in the management of postterm gestation. If the cervix is considered ripe for induction at 41-42 weeks of gestation, it seems reasonable to do so. Considerable controversy exists as to the most judicious mode of management in the presence of an unripe cervix. At present, the use of techniques to assess fetal well-being are recommended, including the BPP, NST, and CST. There are no data from randomized trials that demonstrate the superiority of the BPP over weekly or twice-weekly FHR.

During labor, umbilical cord compression may be present as a result of oligohydramnios; meconium may also be noted. If macrosomia exists, careful attention should be paid to evaluating the risk of shoulder dystocia, which is increased twofold in postterm pregnancies. Finally, at the time of delivery, the infant's airway should be suctioned with a mechanical device after delivery of the head and before delivery of the thorax. This action, combined with immediate intubation of the infant and suction of the trachea.