The precise mechanisms underlying these associations are as yet unknown. An early age at menarche and a late age at menopause would result in a longer lifetime exposure to higher estrogen levels and thus a greater breast cancer risk. Late age at menopause is associated positively with this disease, presumably due to the longer period of exposure to high premenopausal estrogen levels. As with breast cancer, parity appears to lower the risk of endometrial cancer. Only a handful of studies have evaluated the relationship between age at first birth and endometrial cancer, and their findings have been inconsistent and generally do not support any substantial association.
Most ovarian cancers arise from epithelial cells that make up the external surface of the ovary.
This theory suggests that factors, such as parity, that decrease the number of ovulations would decrease ovarian cancer risk. High levels of gonadotropins, hormones secreted by the pituitary gland, also have been hypothesized to increase ovarian cancer risk. Since pregnancy decreases gonadotropin levels and a late age at menopause postpones exposure to high postmenopausal gonadotropin levels, each of these factors would be expected to reduce ovarian cancer risk according to this hypothesis. A trend of increasing protection from this cancer with increasing parity has been observed in most studies. Generally, women who have had three or more children have a 30 to 50 percent lower risk of ovarian cancer relative to nulliparous women.