The efficacy of starting postterm antenatal testing at 41 weeks as compared with 42 weeks of gestational age.

Postterm antenatal fetal surveillance has traditionally begun at 42 completed weeks of gestation. However, recent data have shown that a significant percentage of cases of perinatal asphyxia occurs between 40 and 42 weeks of gestation. We compared the perinatal outcome of fetuses with antenatal surveillance beginning at 41 weeks to those starting at 42 weeks of gestation. The study groups consisted of 908 patients who began antenatal testing at 41 weeks and 352 who began testing at 42 weeks. Antenatal testing consisted of twice-weekly amniotic fluid assessments and nonstress tests (including evaluation for late and variable decelerations). Between 41 and 42 weeks, the group whose testing started at 41 weeks had an overall incidence of intrapartum fetal distress of 2.7%, no stillbirths, and no infants with major neonatal morbidity. Patients without antenatal testing who delivered between 41 to 42 weeks did not have a significantly increased incidence of fetal distress (3.3%; p = 0.07). However, this group had a significantly increased incidence of adverse outcomes (p less than 0.05), including three stillbirths and seven cases of major neonatal morbidity. Beyond 42 weeks, the group whose testing started at 41 weeks had a 2.3% overall incidence of fetal distress. This was significantly less (p less than 0.01) than the group whose testing started at 42 weeks (5.6%). Neither of the groups had any stillbirths or infants with major neonatal morbidity. These findings suggest that starting antenatal testing at 41 weeks of gestation may result in decreased postterm perinatal mortality and morbidity as well as a decreased incidence of intrapartum fetal distress.