REPRODUCTIVE FACTORS AND FAMILY HISTORY OF BREAST-CANCER IN RELATION TO PLASMA ESTROGEN AND PROLACTIN LEVELS IN POSTMENOPAUSAL WOMEN IN THENURSES HEALTH STUDY (UNITED-STATES)

Parity, age at first birth, age at menarche, and a family history of b reast cancer have each been associated consistently with breast cancer risk Whether this increase in risk is mediated, at least in part, thr ough changes in endogenous hormone levels is unclear. We conducted a c ross-sectional study of the relationships between these factors and pl asma hormone levels in 216 healthy postmenopausal women in the Nurses' Health Study (United States). The hormones evaluated were estradiol, percent and total free estradiol, percent and total bioavailable estra diol, estrone, estrone sulfate, and prolactin. After controlling for a ge, body mass index (weight/height(2)), and alcohol use, we observed i nverse associations between estrone sulfate and parity (r = -0.15, P = 0.03) and between percent bioavailable estradiol and age at first bir th (r = -0.17, P = 0.02). Although women with a family history of brea st cancer tended to have higher estrogen levels compared with women wi thout such history, the differences were not statistically significant , Age at menarche was not related significantly to any of the hormones . These data provide some additional evidence that the inverse relatio nship observed between parity and breast cancer risk may be mediated, at least in part, through decreased estrogen levels. Our data do not s upport a substantial influence of either family history of breast canc er or age at menarche on postmenopausal estrogen or prolactin levels.