Effect of suckling on serum prolactin, luteinizing hormone, follicle-stimulating hormone, and estradiol during prolonged lactation.

Eight nursing subjects had serum prolactin (PRL), luteinizing hormone, follicle-stimulating hormone, and estradiol levels studied during the first six months postpartum. Each subject had serum samples obtained just before the initiation of suckling and during the next 120 minutes. Baseline PRL levels were high at ten days postpartum (90.1 ng/mL), then slowly declined but remained elevated at 180 days postpartum (44.3 ng/mL), with the stimulus of suckling being able to double the baseline PRL value throughout the study period. Mean estradiol levels were low at ten days postpartum (7.2 pg/mL), then gradually rose to a mean level of 47.3 pg/mL at 180 days postpartum; however, in four subjects who were amenorrheic during the study period, the mean estradiol levels remained low (4.25 pg/mL), while baseline PRL levels remained high (63.6 ng/mL).