Uncertainty continues about how best to manage pregnancies that exceed 40 weeks. Rather than routinely inducing labor a week or longer beyond the estimated date of delivery, monitoring fetal status could suffice if there is a test sensitive enough to detect fetuses at risk. This prospective, double-blind cohort study was planned to find whether a single ultrasound scan at or after 40 weeks gestation, done to detect the single deepest pool of amniotic fluid less than 2 cm in size and an amniotic fluid index (AFI) less than 5 cm, helps to predict adverse outcomes. The study population included 1584 women at 40 weeks or more gestation. A pool of amniotic fluid was defined as a fluid column at least 1 cm wide that contained no umbilical cord. The AFI was estimated by summing measurements of the deeper pool in each of the 4 abdominal quadrants. An AFI less than 5 cm was found in 7.9% of women and a single deepest pool less than 2 cm in 1.4%. Twenty women had both these findings. There were no perinatal deaths, but the major morbidity, meconium aspiration or asphyxia at birth, was present in 7 infants (0.4%). An AFI less than 5 cm correlated significantly with 1 of these outcomes. The relative risk of an adverse outcome, as defined, was 4.6. This finding was 28% sensitive and 91.9% specific for a poor outcome. In none of these pregnancies was there a single amniotic fluid pool less than 2 cm. Measuring the single deepest pool did not predict which infants would be admitted to the neonatal unit for intensive care. It was not sensitive for identifying those fetuses that developed distress at any stage of labor or, because of fetal distress, required cesarean section. An AFI less than 5 cm was significantly associated with operative delivery for fetal distress in labor (26% vs. 15.6%). A cord pH less than 7 correlated with an AFI less than 5 cm (5.1% vs. 1.3%). This finding was 26.7% sensitive and 91.9% specific for a pH less than 7. An AFI signifying oligohydramnios is associated with neonatal morbidity but is not a sensitive predictor of adverse outcomes. Its routine use probably would lead to more obstetric interventions without improving fetal outcomes.
Nuffield Department of Obstetrics and Gynaecology, John Radcliffe Hospital, Headington, Oxford, U.K.