Fetal control of maternal prolactin production and bioactivity in utero.

Amniotic fluid prolactin is a product of maternal decidualized endometrium that is derived by translocation of the hormone across the reflected fetal membranes. Amniotic fluids from 26 second-trimester (14 to 23 weeks) and 75 third-trimester (29 to 40 weeks) normal singleton pregnancies were evaluated for prolactin content by radioimmunoassay and bioassay with the Nb2 rat lymphoma cell line. The relative bioactivity was calculated as the ratio of bioassay to radioimmunoassay for each fluid. Data segregated by gestational age and fetal genetic sex identified a highly significant difference (p = 0.0004) in amniotic fluid prolactin radioimmunoassay concentrations (mean +/- SEM) that surround male (682 +/- 49, n = 42) versus female (440 +/- 39, n = 33) fetuses of third-trimester age. Paired bioassay values were significantly lower (p = 0.002) than radioimmunoassay values among males (626 +/- 52) but equivalent (p = 0.1066) among females (464 +/- 44). The bioassay/radioimmunoassay ratios of third-trimester fetal female-associated amniotic fluid prolactin were significantly higher (p = 0.0004) than those of third-trimester males and second-trimester males and females. The results suggest a fetal gender-related factor is associated with both the production and the biologic activity of the maternally derived hormone. Thus the fetus appears to have some control over the dynamics of uterine prolactin production.