> Childbearing (Age at First Birth and Number of Children)
The link between having children and the risk of breast cancer is complex. Both your age when you give birth to your first child and the number of children you give birth to affect your breast cancer risk.
Age at first birth
A first pregnancy has two effects on breast cancer risk. First, it increases short-term risk and then, it lowers long-term risk. How these effects interact depend on a woman's age [46,86-89].
Women who have their first child at age 35 or younger tend to get a protective benefit from pregnancy. Though breast cancer risk is increased for about 10 years after a first birth, it then drops below the risk of women who don't have children . The younger you are when you have your first child, the sooner you get the protective effect of pregnancy [86-89].
Women who have their first child at later ages are 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 get breast cancer than women who have their first child before age 20 . For these women, the increase in risk from a first pregnancy never gets fully offset by its long-term protective benefits .
Women who are over age 35 when they have their first child also have a slight increase in lifetime risk of breast cancer compared to those who don't have children .
Why does age matter?
There are a few possible reasons for the different effects of age at first childbirth. One reason relates to breast cells. During pregnancy, breast cells grow rapidly. If there is any genetic damage in the breast cells, it is copied as the cells grow. This increased genetic damage in the cells 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 women who have their first child at a younger age.
Number of children (number of births)
In general, the more children a woman has given birth to, the lower her risk of breast cancer . After the first child, each childbirth lowers risk. The exact reasons for this are unclear at this time.
Women who never have children have a slightly higher risk of breast cancer compared to women who have had more than one child . However, women who are over age 35 and give birth only once have a slight increase in lifetime risk of breast cancer compared to women who never have children .
Whether having children protects equally against estrogen receptor-positive and estrogen receptor-negative (including triple negative) breast cancers is under study .