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 [6-11].
Women who give birth to their first child at age 35 or younger tend to get a protective benefit from pregnancy. Breast cancer risk is increased for about 10 years after a first birth, and then it 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 [9-11].
Women who give birth to 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 is never fully offset by its long-term protective benefits .
Women who are over age 35 when they give birth to their first child also have a small increase in lifetime risk of breast cancer compared to those who don't give birth .
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 childbirths
In general, the more children a woman has given birth to, the lower her risk of breast cancer [6-11]. After the first child, each childbirth lowers risk. The exact reasons for this are unclear at this time.
Women who never give birth have a slightly higher risk of breast cancer compared to women who have had more than one birth . 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 give birth .
Whether giving birth protects equally against estrogen receptor-positive and estrogen receptor-negative (including triple negative) breast cancers is under study .
*Please note, the information provided within Komen Perspectives articles is only current as of the date of posting. Therefore, some information may be out of date at this time.