The link between having children and the risk of breast cancer is fairly complex. This is because a first pregnancy has two effects on breast cancer risk--one that typically raises risk over the short term and another that lowers risk over the long term. Exactly how these effects interact depends on a woman's age.
Women who have their first child at age 35 or younger tend to get an overall protective benefit from pregnancy. While the risk of breast cancer goes up for about 10 years following a first birth, after that it drops below that of women who don't have children. The younger women are when they have their first child, the sooner the effect of pregnancy becomes protective. The older women are when they have their first child, the longer it takes to see the benefit of pregnancy on risk [55].
Women who are over age 35 when they have their first child have a slight increase in lifetime risk of breast cancer compared to women who don't have children. For these women, the initial increase in risk that goes along with a first pregnancy never gets fully offset by its long-term protective benefits [55].
Similarly, among women who have children, women who have their first child at later ages are also at increased risk of breast cancer compared to women who have their first child at younger ages. For example, women who give birth for the first time after age 35 are 40 percent more likely to develop breast cancer than women who have their first child before age 20 [56].
Why this differing effect with age? We don't know for sure, but there are several possible reasons. One reason relates to breast cells. During pregnancy, breast cells grow rapidly. And if there is any genetic damage in the breast cells, it is copied as the cells grow. This increases genetic damage in the cells, which can lead to breast cancer. Because the chance of having such genetic damage goes up with age, women who have their first child at a later age may have a higher risk of breast cancer than younger women.
Updated 08/14/08
