Twenty-four hour patterns of prolactin secretion during lactation and the relationship to suckling and the resumption of fertility in breast-feeding women.

In breast-feeding women prolactin released in response to suckling is essential for the maintenance of lactation. This physiological hyperprolactinaemia is also associated with lactational infertility. However, it is not clear whether there is any direct relationship between changes in prolactin per se and the duration of infertility. To address this question, our study determined the pattern of prolactin secretion in relation to suckling and the return of ovarian activity in the same cohort of breast-feeding women. Blood samples were withdrawn at 10 min intervals for 24 h from 09:00 to 09:00 h at either 4 (n = 9) or 8 weeks (n = 11) post-partum when the women had completely suppressed ovarian activity, at the time of the introduction of supplements to the baby (n = 17), a time associated with reduction of suckling activity, at first menses while still breast-feeding (n = 13) and in the follicular phase (n = 9) of the first menstrual cycle after weaning. During sampling, mothers and babies continued their normal pattern of suckling activity. The pattern of prolactin release was very variable at each stage of lactation, depending on the pattern of suckling. Frequent suckling was associated with elevated prolactin concentrations during the 24 h period throughout lactation. When suckling was less frequent, prolactin concentrations fell to baseline values between breast-feeds, but prolactin was released in response to all suckling episodes. An increase in prolactin concentrations at night, independent of suckling, was only evident once breast-feeding had ceased. The prolactin response to suckling declined significantly only after the return of menses at 33.6 +/- 3.5 weeks post-partum. There was no relationship between the duration of amenorrhoea and the plasma concentrations of prolactin over 24 h, or day or night separately, throughout lactation. However, there was a strong correlation (r = 0.843; P < 0.01) between the timing of the introduction of dietary supplements to the baby and the duration of amenorrhoea. These results suggest that there may be no precise link between the release of prolactin during lactation and the duration of lactational infertility in breast-feeding women.