Lecithin:sphingomyelin ratio in the amniotic fluid of male and female fetuses.

The effect of fetal sex on the amniotic fluid lecithin:sphingomyelin (L:S) ratio was determined with a prospective, randomized, double-blind study designed to evaluate the effect of antenatal steroid administration on the incidence of neonatal respiratory distress syndrome. The immature L:S ratios of 183 single births to nondiabetic women were analyzed. L:S ratios increased with increasing gestational age. Regression lines of L:S versus gestational age suggested that male infants' L:S ratios were lower than females', although the differences were not statistically significant.