The total duration of exposure to female reproductive hormone correlates well with a woman's chance of developing breast cancer. Onset of menstrual periods at early adolescence, delayed onset of menopause, and not having any pregnancies are associated with ncreased time duration of exposure to estrogen hormone, which in turn could be associated with increased risk of breast cancer. Overweight and obesity are linked to higher levels of estrogen hormone in the body. Overweight and post-menopausal hormone replacement therapy are linked to increased risk of breast cancer development in women who are postmenopausal. The risk of development of breast malignancy in those women who had menopause at the age of 55 is just 50 percent only, in comparison to women who had natural menopause at the age of forty-five. Those women who had surgical removal of both ovaries have significantly lower risk of breast cancer if they are not initiated on a hormone replacement treatment plan. For every year of delay in the onset of menstrual periods in a woman there may be twenty percent lowering of breast cancer risk.

The chance of breast cancer development is also related the age at which a woman completes the first full term pregnancy. If a woman delays her first pregnancy to age of 30, she may have approximately 2 to 5 fold increase in the risk of breast cancer in comparison to a woman who had completed her first full term pregnancy before age eighteen. Women who had no pregnancies have approximately 1.4 times increased risk of development of breast cancer in comparison to those women who had carried one or more pregnancies to full term. Some studies have shown that lengthier periods of breast-feeding may be linked to decreased risk of breast cancer. Does abortion have any adverse influence on the breast cancer risk? The answer is not entirely clear. Research findings have indicated that abortion and miscarriage nullifies any benefits the pregnancy may have conferred to the woman. Research has shown that postmenopausal hormone replacement is linked to increased risk of breast cancer, but currently there is no clear evidence to prove that use of oral contraceptive pills increase the risk of breast cancer.

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