Water and salt conservation in the human fetus and newborn. I. Evidence for a role of fetal prolactin.

A study was performed on 94 women delivering at 34-44 weeks gestation, whose pregnancies were uncomplicated to determine the role of PRL in human fetal and neonatal salt and water conservation. Ultrasonic estimation of amniotic fluid (AF) volume and sampling of maternal blood, AF, cord blood, and 2-h neonatal blood were performed to analyze PRL, osmolality, sodium ion concentration ([Na]), and blood solids [hematocrit (Hct), total serum solids (tss), and total protein concentration]. In this report, which addresses the role of fetal PRL, Pearson correlations showed the following significant relationships: 1) approximation of cord serum osmolality and [Na] in cord and maternal serum, as well as parallel changes in cord blood Hct, tss, and total protein; 2) reduced estimated AF volume and increased AF osmolality in the face of elevated cord serum osmolality and [Na]; 3) a shift toward normal in cord Hct and tss over the first 2 h of neonatal life after an initially increased or decreased cord serum osmolality, [Na], Hct, or tss; and 4) relationship between fetal pituitary PRL levels of 230 micrograms/L or less and cord serum osmolality, [Na], and Hct. The entire range of cord serum PRL levels correlated with changes in AF osmolality and [Na] as well as with neonatal changes in Hct and tss. These findings support the hypotheses that osmotic equilibrium exists between maternal and fetal circulations; that disturbances in this balance lead to changes in fetal and neonatal water excretion; and that fetal PRL, stimulated by increases in cord serum osmolality and [Na], acts as an antidiuretic, leading to restoration of the offspring's extracellular fluid volume.