Fetal sex-related differences in maternal serum alpha-fetoprotein during the second trimester of pregnancy.

In an analysis of data gathered from routine maternal serum alpha-fetoprotein screening, we determined that the sex of the fetus is significantly correlated with differences in maternal serum alpha-fetoprotein concentration during the second trimester of pregnancy. Between the fifteenth and nineteenth gestational weeks, the mean maternal serum alpha-fetoprotein level is significantly higher for the male fetus than for the female fetus. There was no difference, however, in the rate of increase in maternal serum alpha-fetoprotein concentration with time between male and female fetuses during the weeks of gestation that were studied. No apparent cause for these observations has been demonstrated.