Plasma hormones in parous, nulliparous and postmenopausal Japanese women.

Mechanisms associated with protection against breast cancer in parous women with an early pregnancy are unknown. Elevated or deficient steroid hormone levels or dysfunctional release of prolactin have been postulated. Since these effects have not been reported in low-risk Japanese women, we have compared the hormone levels in age-weight-risk factor matched premenopausal nulliparous and parous Japanese women with established menstrual regularity. Higher luteal phase plasma levels of estradiol and estrone occurred in parous versus nulliparous women, while plasma prolactin and androgen levels were unaltered. It is suggested that the higher estrogen and lower LH levels, as in short versus long menstrual cycles, results from failure of the opioid GnRH. LH axis to return to pre-pregnancy levels.