An analysis of 52,198 pregnancies was undertaken to determine if male and female fetuses had differing environmental effects on their mothers. Early gestational blood pressure and weight gain were significantly higher in preeclamptic women with male than with female fetuses. Preeclamptic women had lower third trimester hemoglobin values and less frequent proteinuria with male than with female fetuses. Excessive syncytial knots, a characteristic placental consequence of low uteroplacental blood flow, were seen less frequently with male than with female fetuses. These findings are hypothesized to be due to a greater maternal blood plasma volume expansion with male than with female fetuses in preeclamptic pregnancies.