Fetal gender effects on maternal serum prolactin levels.

Circulating maternal prolactin (PRL) levels have been reported to be higher in term pregnancies yielding male infants. The mechanism for this gender difference is unknown, but we theorized that it was mediated through the fetal adrenal cortex. To test this theory we measured circulating PRL and estriol (E3) concentrations with radioimmunoassay in 37 pregnant women at 34 and 36 weeks' gestation. We then separated the groups by newborn gender. Maternal serum PRL levels were significantly higher in the women bearing male fetuses. There was no significant difference by gender in E3 concentrations, and there was no PRL surge corresponding to the E3 surge at 34-36 weeks' gestational age. There was no correlation between E3 and PRL levels. Transmission of the fetal gender effect on maternal PRL does not appear to be mediated through the fetal adrenal as measured by the fetoplacental production of E3. The effect probably is mediated by the fetal gonad.