Does Having Kids Lower Your Risk of Breast Cancer?

Pregnancy has long been known to offer some protection against breast cancer. But newer research has challenged the notion of a simple link between the two.

Scientists have found key differences between the short-term and long-term effects of pregnancy on breast cancer risk. They’ve also discovered that these effects depend on other factors tied to having children, too, like age at first childbirth and total number of births.

“A pregnancy, especially under the age of 25, is very protective against breast cancer in the future,” says Nimmi Kapoor, MD, a breast surgical oncologist at UCLA Health. Shortly after childbirth, however, there’s an increased risk of breast cancer, particularly among people who give birth at an older age, she explained.

Exactly how long the window of risk lasts after a pregnancy isn’t clear. Research suggests the risk peaks about five years after giving birth but may stay elevated for several years longer. Cancers that emerge within five to 10 years of childbirth are considered postpartum breast cancer. The crossover from risk to benefit happens about 20 years after a childbirth: At this point, a past full-term pregnancy reduces risk compared to having never given birth at all. 

It’s worth noting that the overall risk of breast cancer remains low in young women, even just after childbirth. A large study on the topic showed that the temporary rise in breast cancer risk after a full-term pregnancy amounted to about 2.2%, compared to 1.9% in those who hadn’t given birth.

Why pregnancy raises short-term risk

Researchers think it has to do with what happens in the breast tissue after childbirth. Many new breast cells are formed during pregnancy to create and deliver milk to a newborn. “After a pregnancy — when a woman finishes breastfeeding, or sooner, if she doesn’t breastfeed — all these extra [cells] go away, which is called involution,” says Susan Neuhausen, PhD, the Morris & Horowitz Families Professor in Cancer Etiology & Outcomes Research at City of Hope. As a result, the breast tissue goes through an inflammatory process that’s similar to healing from a wound, Neuhausen says. This environment may allow one abnormal cell to grow into cancer.

It’s thought that breastfeeding and then weaning — versus not breastfeeding at all — can make this transition in the breast tissue more gradual. “The microenvironment [of the breast] is still disrupted, but it’s at a lower level,” Neuhausen says. This may reduce the slight increase in breast cancer risk in the years just following childbirth but does not seem to erase it entirely.

Why pregnancy lowers long-term risk

After the postpartum window closes, by about 10 years after childbirth, people who have given birth are at less risk for developing breast cancer than those who didn’t have kids. This is because the breast changes of pregnancy make this tissue more mature and stable in the long run.

Some of those changes may improve the immune activity within breast cells, says Judy Garber, MD, MPH, chief of the Division of Cancer Genetics and Prevention at Dana-Farber Cancer Institute and a professor of medicine at Harvard Medical School. She pointed to research showing that pregnancy and lactation is linked to a greater presence in the breasts of certain immune cells called T cells that target and destroy cells that have become infected as well as tumor cells. 

Another reason pregnancy can reduce lifetime breast cancer risk is because it gives your body a break from the cycling hormones of menstruation, including estrogen and progesterone. These hormones can contribute to the growth of certain kinds of breast cancer. As a result, having fewer total menstrual cycles is linked to lower overall breast cancer risk.

The short-term risk of breast cancer after pregnancy is highest in people who have their first child later in life. After age 30, the risk of a first birth starts to outweigh the longer-term protection from having kids. “After 35, having your first pregnancy seems to increase your breast cancer risk [more] than having no pregnancies,” Garber says. 

The reason why “is really unknown right now and is an extremely important area of active research,” Kapoor says. There’s a theory that younger breast tissue may be better at healing and recovery than slightly older breast tissue, she adds. If that’s the case, younger breasts might be better equipped to handle the inflammatory process of involution.

Another theory relates to how pregnancy helps breast cells mature and grow. Older, first-time mothers spend a longer period of their life with immature, unstable breast tissue. That means their breast cells have more opportunity to become damaged or pre-cancerous before a pregnancy. If this happens, then going through the hormone changes of pregnancy and the inflammatory period of involution might allow those abnormal cells to grow into cancer.

“Each additional birth will decrease long-term risk [of breast cancer],” says Kapoor, so long as the first birth happens before age 35. (Past this point in life, having multiple kids may raise breast cancer risk higher than not having kids at all due to the risks mentioned above.)

The protective effect of multiple childbirths seems to be strongest for hormone-receptor positive breast cancers (or, breast cancers that grow when exposed to hormones like estrogen or progesterone). This is because additional childbirths can reduce risk by lowering your total lifetime number of menstrual cycles. Fewer cycles means less exposure to fluctuating estrogen and progesterone levels, which can contribute to hormone-receptor positive cancers.

Pregnancy mainly lowers long-term breast cancer risk in women who have kids young, before age 25 or 30 — “but this can be a hard pill to swallow,” Kapoor says. There are plenty of valid reasons why you may not want to have kids or want to by this age. Many people also struggle with infertility, and treatments can push childbirth later in life, too.

Regardless of whether or when you have children, experts recommend that people get a breast cancer risk assessment by the age of 25. If you learn that you’re high-risk, you may qualify for screening before age 40, which can include yearly mammograms as well as additional imaging (for instance, breast MRI or ultrasound) at a more frequent cadence. These tests improve your ability to identify suspicious areas before they become cancer or catch cancer at an earlier stage. During and after a pregnancy, it’s especially important to flag any lumps or unusual breast changes for your doctor, Kapoor adds.

Ultimately, experts say the data on breast cancer risk is just one piece of the puzzle when it comes to family planning. “There’s a small rise [in risk] for a short period of time [after childbirth] that affects a relatively small number of people,” Kapoor says.