This situation may occur in early adult life in a few women but is quite common after age 35 or so when anovulatory periods tend to occur. It also occurs after menopause when women are given estrogen supplements without progesterone. Pregnancy occurring before age 30 is known to have a protective effect.
Progesterone is the dominant hormone during pregnancy.
Only the first, full-term, early pregnancy conveys protection. Women having their first pregnancies before age 18 have approximately one-third the risk of women bearing the first child after age 35. Women without children are at a higher risk than those with one or more children. The protective effects of early oophorectomy are negated by the administration of estrogen. Recently, industrial pollutants having potent estrogenic effects, called xenoestrogens, are being recognized as a pervasive environmental threat, likely to be a contributing factor in the incidence of breast cancer.