Sex differences in fetal lung maturation.

It has been recognized that male infants are at greater risk of respiratory distress syndrome than are female infants. Amniotic fluid was obtained from 73 male and 76 female fetuses between 28 and 40 wk of gestation. To assess fetal pulmonary maturity, we determined the lecithin-sphingomyelin ratio and concentrations of saturated phosphatidyl choline and cortisol in all of these fluids. Analysis of covariance showed that female infants had higher indexes of pulmonary maturity than did male infants. The difference in degree of fetal pulmonary maturity was 1.2 to 2.5 wk, on the basis of these measurements, with females ahead of males. We conclude that there is a biochemical basis for the increased risk of respiratory distress syndrome in male infants.