Meconium does not guarantee fetal lung maturity

In utero passage of meconium may represent a response to hypoxic stress or a normal maturational event. When found during the third trimester, one may be tempted to use its presence as prima facie evidence of fetal lung maturity. The purpose of our study was to determine the frequency of meconium-stained fluid in the third trimester and the incidence of biochemical and physiologic lung immaturity in these fetuses.

Amniotic fluid specimens obtained at our institution from 1991 through 1993 (n = 2,377) were analyzed for maturity and visually inspected for meconium. Perinatal outcome was obtained for intramural deliveries occurring within 3 days of amniotic fluid collection (n = 905). Gestational age was defined as the best obstetric estimate based on menstrual dates, clinical examination, and ultrasound results.

Meconium staining was present in 2.7% (n = 64) of specimens. Although meconium-stained specimens were more likely to have mature lecithin-sphingomyelin (L:S) ratios (OR 2.1, 95% confidence interval [CI] = 1.2-3.6) and phosphatidylglycerol (PG) concentrations (OR 3.8, CI 2.2-6.7), 17.2% were immature for both L:S and PG (n = 11, CI = 9.9-28.2%). When analysis was limited to fetuses delivering intramurally within 3 days of amniotic fluid collection, respiratory distress syndrome occurred in 3.0% (CI = 0.5-15%) with meconium-stained fluid.

The presence of meconium in amniotic fluid does not guarantee lung maturity. The same consideration of the risks of prematurity must be given to the fetus with meconium-stained fluid as given to the fetus with clear fluid.