Ninety four lactating women and 119 lactating women with menstruation, aged between 21 and 38 years, volunteered for this study.

The pituitary was relatively insensitive to LH-RH during the first 3 weeks following delivery. The recovery of FSH responsiveness to LH-RH occurred earlier than that of LH. Normal FSH response resumed in the 2nd week while the LH response, although not normal, started at the 3rd week postpartum. Pituitary responsiveness after the 5th week postpartum was similar to that occurring in normally menstruating women, except that FSH response was exagerated. Serum prolactin levels were elevated above 160 ng/ml until the 5th week postpartum and decreased to 84.2 ng/ml in the 6th week postpartum.

It appears that at least one reason for anovulation during the first four weeks following delivery is the relative insensitivity of the pituitary to hypothalamic stimulation. Prolactin does not seem to modulate pituitary responsiveness to LH-RH.

In order to clarify hormonal profiles during the lactational amenorrhea beyond the 5th week puerperium, serum levels of LH, FSH, prolactin, estradiol and progesterone were determined during different postpartum periods. Serum FSH and LH levels during 1-10 months postpartum were similar to basal levels seen during the normal menstrual cycle. Serum estradiol concentrations throughout 1-10 months postpartum, however, were significantly decreased as compared with the levels during the follicular phase of the normal menstrual cycle. Serum prolactin levels were elevated throughout 1-10 months postpartum in lactating amenorrhic women but decreased as the postpartum period lengthened. As compared with lactating amenorrhic women, lactating women with resumed menstruation showed a decrease in prolactin levels from 89.20 ng/ml to 51.39 ng/ml at 1-3 months, from 75.08 ng/ml to 49.99 ng/ml at 4-6 months, and from 54.73 ng/ml to 28.74ng/ml at 7-10 months postpartum.

These results suggest that the apparent anovulation seen beyond 5th week postpartum during lactation was not due to pituitary insensitivity to LH-RH. Rather, prolactindependent mechanism interfering with cyclic activity may be operative during long term lactation.