You’ve probably heard that having children and breastfeeding them reduces a woman’s lifetime risk of breast cancer. But according to new research, in the years immediately following childbirth, women are actually at a small, heightened risk of the disease.

That’s the finding of a major new review of 15 studies from three continents on the relationship between breast cancer and childbirth, published in the Annals of Internal Medicine. It’s not the first paper to come to this conclusion, but it’s certainly the biggest — and one that clarifies a little-appreciated breast cancer risk factor.

Globally, breast cancer is the top cancer diagnosis in women of reproductive age. In America, it’s the most common cancer for women after skin cancer, according to the American Cancer Society. One in eight women in the US will develop the disease, and in 2018, approximately 266,000 new cases of invasive breast cancer were diagnosed.

We’ve known for a while that women who smoke as well as women who have the BRCA1 and BRCA2 gene mutations are at a heightened risk. Now women who have recently had a child are in that higher risk group as well, said the study’s lead author, University of North Carolina epidemiologist Hazel Nichols. That risk is highest about five years after childbirth, but it lasts some 20 years.

“This is one example of how risk factors can change over the course of a woman’s life,” she added. Which means that when doctors are trying to assess breast cancer risk, they should take into account when and whether she has had children, along with the other well-known breast cancer risk factors. Together, these risks might inform a woman’s decision to get screened earlier.

Why childbirth can both prevent and promote breast cancer

Researchers have long shown that a woman’s lifetime risk of breast cancer is reduced if she’s had children. Hormones produced during the ovulation cycle can stimulate cell growth, which can help cancerous cells multiply. But women who have been pregnant and breastfed have fewer periods, and therefore produce fewer cancer-inducing hormones. That may be why lifetime breast cancer risk generally declines with every child a woman has.

But those figures account for lifetime risk. This new study pooled the results of other studies, from 1976 to 2012, and looked at how breast cancer risk changes at every stage of life in both moms and women who haven’t had kids. What they found has complicated the thinking around breast cancer and motherhood.

By about 60, when breast cancer is most common, women who have had kids are at a lower risk of the disease. But in the years immediately after childbirth, moms are at a greater risk. That risk peaks five years after childbirth, and then gradually declines until childbearing becomes protective against breast cancer, about 20 years later. You can see that changing lifetime risk in this chart:

One important point: The heightened risk in the years following childbirth, while statistically significant, is not huge. Moms 41 to 50 years old had a 2.2 percent risk of developing cancer, while women that age who didn’t have children had a 1.9 percent risk.

“We’re not arguing with the idea that, overall, having children is beneficial for reducing breast cancer risk,” Nichols said. “But that information came from looking at the majority of breast cancer cases, and the majority are diagnosed later in life. By that time in your life, you are experiencing a benefit of your childbearing history.”

With the new study, she added, “We were able to look more closely at the time period after birth, when breast cancer is overall much less common. And that’s where we saw an elevated risk.”

She also acknowledged that, since it’s an observational study, there is the potential that unmeasured factors may be driving the trend. For example, maybe women who are pregnant are more exposed to the healthcare system and more likely to get diagnosed. But the pattern of breast cancer risk after childbirth was similar across several different countries, where mammography screening ages vary. And the pattern was similar whether they included or excluded early-stage breast cancer, the kind most likely to be detected by screening. That should provide “reassurance that the findings are unlikely to be due solely to differences in breast cancer detection,” Nichols said.

Researchers aren’t sure why a woman’s risk would be elevated, but suspect it might have to do with the composition of breasts changing during and after pregnancy. In this period, breast tissue develops and the cells within divide rapidly. “We know that whenever a tissue has rapid cell division, there’s going to be a possibility of being an error, or if there’s already an error in the cell or tissue, that might be an opportunity for multiple copies of the error to be made.”

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So what does this mean for women? “We’re not trying to highlight this increased risk because it’s scary or alarming or something to worry about.” But it’s another relevant piece of information for women when talking to health providers about possible screening for breast cancer.

Right now, health groups offer different advice about when women should start annual screening. The American College of Obstetricians and Gynecologists has some of the most aggressive guidance, recommending age 40 as the start time. Meanwhile, the US Preventive Services Task Force — an evidence-based group that governments and insurers usually follow — suggests screening every two years starting at age 50, but also that the decision to start screening sooner should be an individual one.

So whether to get a mammogram or not is left to women and their individual health histories, values, and preferences. When considering one’s risks, though, Nichols contends, a recent childbirth might be included in the mix.