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.
PIP: On the theory that early pregnancy may protect women against breast cancer by a long-term depression of prolactin secretion, basal and perphenazine-stimulated release of prolactin, as well as basal and GnRH- stimulated release of LH and FSH were assayed in women before and after their 1st full term pregnancy, and in groups of parous and nulliparous women. The study groups were 15 women aged 18-23 and 9 women aged 29- 40. All hormone samples were taken at 0800 in the early follicular phase on women who had never taken oral contraceptives, or in the cross section survey, women who had not been exposed for at least 6 months. There were no significant differences in LH or FSH basal or stimulated levels for 100 minutes after GnRH. In contrast after term pregnancy both basal and stimulated prolactin levels were significantly lower than comparable levels in nulliparous controls. Parous women returned for their second prolactin assay from 5-11 months after delivery. The cross-section basal prolactin levels were done from 12-150 months after delivery, with no evidence of an effect of age, parity or elapsed time. These results are appropriate for a protective factor against breast cancer, and prolactin is known to stimulate breast neoplasm in rodents.