Research in to the role of menstrual, reproductive and hormonal factors as the risk factor for breast cancer has indicated that the risk of development of breast cancer is probably related to the number of years of exposure to the female hormone estrogen. The fact that early onset of menstruation before the age of 12 years is associated with about 2-fold increase in the risk of breast cancer support this assumption. Women who have premature menopause will have a reduction of
risk of development of breast cancer as expected. These women typically may have a 50 percent reduction in the risk of breast cancer. A full term pregnancy prior to age 30 is associated with decrease risk of breast cancer where as late first pregnancy or not having any pregnancy is associated with increased risk of breast cancer. The role of protective effect of lactation on breast cancer is not very clear, but there is some evidence to suggest that this may be the case.