Studies on human sexual development. IV. Fetal pituitary and serum, and amniotic fluid concentrations of prolactin.

Prolactin concentrations were measured in 161 amniotic fluid specimens from 8-40 weeks fetal age and the levels compared with those observed in 45 fetal and neonatal cord sera and in 42 fetal pituitary specimens. Amniotic fluid prolactin levels rose steeply between 12-16 weeks gestation, and then declined to term; the calculated total amniotic fluid content of prolactin showed a similar pattern, but the peak was later, at about 26 weeks gestation. Amniotic fluid concentrations consistently exceeded fetal serum prolactin levels, even during the last trimester, when fetal serum and pituitary levels were highest. The data are compatible with a fetal origin for amniotic fluid prolactin, but only if one assumes that flux of prolactin out of amniotic fluid compartment is negligible, that the fetal kidney in mid-pregnancy clears prolactin at a rate virtually equal to the glomerular filtration rate, and the fetal pituitary shows secretion characteristics quite different from those of the adult gland.