(CNN) - Compared with women who have never had children, women who have given birth may have an increased breast cancer risk that continues for up to 23 years after their most recent birth, according to a new study published in the journal Annals of Internal Medicine on Monday.
"What we saw was this pattern where risk was highest about five years after birth, and then it gradually declined as time went on," said Hazel Nichols, an assistant professor of epidemiology at the University of North Carolina at Chapel Hill's Gillings School of Global Public Health, who was first author of the study.
Yet beyond those 23 years, the risk seems to change so that childbirth then appears to be protective against breast cancer, according to the study.
Still, experts warn that women should not worry and that the risk is small.
The study was conducted in women younger than 55, and "breast cancer risk is low in women of these ages in general," Nichols said.
"So even though risk is highest five years after birth, that still translates to a very small increase in the absolute number of women who are being diagnosed with breast cancer," she said.
"If we took the women in our study and started following them around age 40, for example, and over the next 10 years by about age 50, we would have had about 2.2% of women develop breast cancer in the group that had had a recent child, and we would have had about 1.9% in the women who had not had any children," she said. "These are very small differences in the absolute measure, but they're still important because they are in the opposite direction of what we know about these risk factors in older women."
In general, pregnancy-related factors tied to a lower risk of developing breast cancer later in life include having your first full-term pregnancy at an early age, such as younger than 20; having more than one birth; having a history of preeclampsia; and breastfeeding over a longer period of time, according to the National Cancer Institute.
Factors associated with a higher breast cancer risk include having your first full-term pregnancy at an older age, such as older than 30; having given birth recently, within about 10 years; and having taken diethylstilbestrol, a synthetic form of estrogen, during pregnancy.
Globally, breast cancer causes the greatest number of cancer-related deaths among women, according to the World Health Organization.
This year alone, it is estimated that 627,000 women died from breast cancer worldwide, which is about 15% of all cancer deaths among women, according to WHO. In the United States, breast cancer is the most common cancer in women.
About 41,000 women and 450 men die each year from breast cancer, according to the US Centers for Disease Control and Prevention.
Relationship between childbirth and breast cancer risk
The new study included data on 889,944 women younger than 55. The data came from 15 international studies previously conducted in a research project called the Premenopausal Breast Cancer Collaborative Group.
None of the women had breast cancer when they were enrolled in the studies, which involved data from several European countries, the United States and Australia.
The data showed that, compared with women who had not given birth, women with children had an elevated breast cancer risk that peaked around five years after childbirth and continued for about 20 years -- regardless of whether women breastfed or not.
The researchers then noticed that this difference in risk among women who had had children and women who hadn't shifted about 23.6 years after childbirth.
"That's actually the point at which we started to not see much of a difference between the women who had a child and women who had not had children," Nichols said. "As you went further out from there, then the benefits of having children started to emerge."
The researchers also found that, even though breastfeeding did not appear to influence the findings, other factors did, including the age at which a woman first gave birth, how many children she had and whether she had a family history of breast cancer.
"One of the things that we want our study to be able to do is to contribute to new tools that will help predict breast cancer risk more effectively for young women -- so women who might be trying to decide with their providers when to start having mammogram screenings," Nichols said.
"In order to have a good tool to do that prediction, we need to know how risk factors operate in young women, and it's becoming increasingly clear that we can't take the information or the patterns we see in older women and apply them to younger women and assume they'll be the same," she said.
The study had some limitations, including that any breast cancer diagnoses during pregnancy in the data were not uniformly distinguishable from diagnoses made during early postpartum.
Also, the study was unable to explore the biological mechanisms behind why an association between breast cancer risk and childbirth even exists, but Nichols offered some ideas.
"What could be happening is that pregnancy is a time when there is rapid development of the breast tissue, and the cells in the breast are rapidly dividing, and we know that when tissue is expanding quickly that there's more potential for an error to occur within a cell or an error that's already there in the cell to have multiple copies made of it," Nichols said.
"So we don't think it's actually anything about pregnancy that's causing breast cancer. We think that it's possible that this rapid expansion of the breast tissue might make it easier for cancer cells to either grow or be started," she said, but more research is needed.
'This data should not be used to influence decisions about childbearing'
Though there is no clear explanation for this increase in breast cancer risk, young women should be aware of the small risk -- and that it can last for more than 20 years, especially if the woman is older during her first birth, has had several pregnancies or has a family history of breast cancer, said Dr. Francisco Esteva, medical oncologist at NYU Langone Health's Perlmutter Cancer Center in New York, who was not involved in the study.
"At the same time, screening mammograms are not generally recommended in women younger than 40 years of age," he said. "Therefore, if these results are confirmed, we would need novel approaches to identify breast cancer early in younger women, especially after childbirth."
Still, women also should know that the study findings should not influence their decision-making regarding having children, said Dr. Erica Mayer, a medical oncologist and clinical investigator in the Breast Oncology Center at the Dana-Farber Cancer Institute, who was not involved in the study.
"Deciding when to get pregnant, or whether or not to get pregnant, can be an intensely personal decision. This data should not be used to influence decisions about childbearing, as the cancer risks observed are overall fairly modest, as well as on a large population level, and it's difficult to extrapolate these risks to an individual person," Mayer said.
"However, evaluating personal cancer risk in a premenopausal woman is important," she said. "If a woman has a history that may influence risk -- such as a family history of breast cancer -- then she should discuss with her doctor the timing of initiation of mammographic screening, as well as whether referral to a genetic counselor would be helpful to evaluate for a genetic risk of breast cancer."
Overall, the study found what has been seen in previous research, Mayer said.
"It's known that pregnancy at a younger age may confer some long-term protective benefits against breast cancer, however it's also been recognized that in the immediate years following a pregnancy, there seems to be an increased risk of breast cancer that slowly resolves with time," she said. "The findings in this paper support and confirm the prior observations."
Dr. Katrina Armstrong, chair of the Department of Medicine and physician-in-chief at Massachusetts General Hospital in Boston, wrote an editorial that accompanied the new study in Annals of Internal Medicine.
"Nichols and colleagues' results add to a large body of evidence demonstrating a transient increase in breast cancer risk after childbirth. Given the consistency of this finding across studies, should women who have recently had a child make different decisions about breast cancer diagnosis, screening, or prevention? The answer, in general, is no," Armstrong wrote.
"Women with breast abnormalities should have a diagnostic evaluation regardless of time since childbirth," she wrote. "All women should pursue the healthy lifestyle behaviors, such as exercise and limiting alcohol intake, that reduce risk for breast cancer -- again, regardless of childbirth history."
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