When is an amniocentesis for fetal maturity unnecessary in nondiabetic pregnancies at risk?

For several years standard obstetric practice has been to perform an amniocentesis for evaluation of fetal maturity. In order to provide a more definitive answer as to which pregnancies need an amniocentesis, a group of 294 nondiabetic pregnant women in whom an amniocentesis for the evaluation of fetal maturity had been performed for clinical indications were evaluated. Three predictors of fetal maturity--obstetric estimate of gestational age, fetal biparietal diameter, and ultrasonic determination of placental maturation--were evaluated for their ability to predict three outcomes of fetal maturity, including positive amniotic fluid phosphatidylglycerol, pediatric estimate of gestational age greater than or equal to 38 weeks, and absence of hyaline membrane disease. A fetal biparietal diameter of greater than or equal to 90 mm was present in 36% of the total population and was associated with 97% term delivery, 87% positive amniotic fluid phosphatidylglycerol, and 0% hyaline membrane disease. The results associated with an obstetric estimate of gestational age of greater than or equal to 38 weeks were similar. In the present data set over one third of clinically indicated amniocenteses could potentially be avoided without losing any predictive capability for fetal maturity.