The link between having children and the risk of breast cancer is complex and is related to:
A first pregnancy has two effects on breast cancer risk. It increases short-term risk and it lowers long-term risk. How these effects interact depend on a woman's age [5-10].
Women who give birth to their first child at age 35 or younger tend to get a protective benefit from pregnancy [5-10].
Breast cancer risk is increased for about 10 years after a first birth . Then, it drops below the risk of women who don't have children .
The younger you are when you have your first child, the younger you are when you get the protective effect of pregnancy [7-9].
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 [5-10].
For example, women who give birth for the first time after age 35 are about 40 percent more likely to get breast cancer than women who have their first child before age 20 . For these women, the increased risk from a first pregnancy is never fully offset by its long-term protective effect .
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 never give birth .
One possible reason for the different effects of age at first childbirth relates to breast cells.
During pregnancy, breast cells grow rapidly. If there is any genetic damage in the breast cells, it's copied as the cells grow. This increased genetic damage in the cells can lead to breast cancer.
The chance of having such genetic damage goes up with age. This may help explain why women who have their first child at a later age have a higher risk of breast cancer than women who have their first child at a younger age.
In general, the more childbirths a woman has had the lower her risk of breast cancer [5-10]. After the first child, each childbirth lowers risk.
Women whose childbirths are spaced close together may get more benefit than women whose childbirths are spaced far apart . The exact reasons for this are unclear, but may be related to changes in breast cells that occur during pregnancy.
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 over age 35 who give birth only once have a slightly higher 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.
Young Women and Breast Cancer
Research Fast Facts: Prevention