Prolactin in human milk: the influence of nursing and the duration of postpartum lactation.

The concentrations of prolactin in the milk of nine postpartum lactating mothers were determined by radioimmunoassay between days 3 to 280 of the puerperium (n = 324 samples). In addition, prolactin in milk was also determined at the beginning and the end of suckling in three of the same women, who provided 21 paired samples of foremilk and hindmilk between days 7 and 88 of the puerperium. Prolactin content was highest at 43.1 +/- 4 ng/ml (mean +/- SEM) in the early transition milk immediately after the colostrum phase during the first postpartum week, decreasing to 11.0 +/- 1.4 ng/ml in the mature milk (p less than 0.01) when weaning occurred in those mothers who breastfed for up to 40 weeks post partum. During suckling, the foremilk contained significantly more prolactin as compared with the hindmilk (29.5 +/- 2.7 versus 21.0 +/- 3.2 ng/ml; p less than 0.01). These findings, taken together with the known biologic potency of prolactin in breast milk, the osmoregulatory influence of the hormone in mammary and intestinal function, and its absorption by the newborn experimental animal, suggest that the presence of prolactin in milk may play some role in both lactation and the intestinal absorptive function of the suckling newborn.