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

In an analysis of data gathered from routine maternal serum α-fetoprotein screening, we determined that the sex of the fetus is significantly correlated with differences in maternal serum α-fetoprotein concentration during the second trimester of pregnancy. Between the fifteenth and nineteenth gestational weeks, the mean maternal serum α-fetoprotein level is significantly higher to the male fetus than for the female fetus. There was no difference, however, in the rate of increase in maternal serum α-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.