An early first pregnancy is known to protect against subsequent breast cancer. We speculated that this effect may be mediated by a long-term depression of prolactin secretion after pregnancy. We therefore measured basal and post-stimulation serum levels of prolactin, luteinizing hormone (LH), and follicle-stimulating hormone (FSH) in two groups--15 women 18 to 23 years of age and 9 women 29 to 40--before and after a first full-term pregnancy, and in 40 appropriate nulliparous controls. We observed no significant change in basal levels of serum LH or FSH or in the levels stimulated by gonadotropin-releasing hormone in any group. A significant decrease was seen, however, in basal and perphenazine-stimulated levels of prolactin after pregnancy in both the younger and older first-pregnancy groups but not in the controls. In a separate cross-sectional study, we compared basal serum prolactin levels in 29 parous and 19 nulliparous women of similar age. The serum prolactin levels were significantly lower in the parous group but were not related to the number of pregnancies (one to three) or the time elapsed (12 to 150 months) since the last delivery. We conclude that a first pregnancy leads to a long-term decrease in serum prolactin secretion, lasting at least 12 to 13 years.
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