Estrogen and progesterone treatment mimicking pregnancy for protection from breast cancer.

Early age at full-term pregnancy lowers the risk of breast cancer in women; lactation seems to be of marginal importance and aborted pregnancy is not associated with reduced risk. Although early full-term pregnancy provides protection against breast cancer, first full-term pregnancy in older women appears to increase the risk. The protective effect of pregnancy has also been observed in rats and mice; in these animals, lactation has an additive effect and interrupted pregnancy provides partial but significant protection. Pregnancy at a young age (< or = 3 months) is highly effective, but pregnancy in older animals (> or = 4 months) is less effective. Parity-induced protection against mammary cancer in rodents can be reproduced by short-term treatment (approximately equivalent to gestational period of rodent or shorter) with the pregnancy hormones, estrogen and progesterone. Administration of pregnancy hormones to nulliparous women may be a useful strategy for protection against breast cancer. However, estrogen and progesterone are thought to play major roles in promotion of the proliferation of breast epithelial cells. Thus, the duration of such treatment and the age at which it is administered are essential factors that require further study. Experimental data suggest that short-term treatment of older rats (aged 6 months) with estrogen and progesterone accelerates mammary carcinogenesis and that long-term (>20 weeks) treatment abolishes the cancer-suppressing effect or even accelerates mammary carcinogenesis. Thus, the available evidence suggests that age and duration of estrogen and progesterone treatment are particularly important factors for protection from breast cancer.