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.
In the US and the United Kingdom, about 12% of women will develop breast cancer at some point in their lives, according to the National Cancer Institute and the organization Cancer Research UK.
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 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.