However, researchers say the overall risk remains low and the odds even out after 20 years.
Women who have recently given birth may have an increased risk of developing breast cancer.
Past research has suggested giving birth provides a protective effect against breast cancer, but new research published in the Annals of Internal Medicine has concluded that the preventive impact may take two decades to develop.
Researchers analyzed data from 15 studies to examine breast cancer risk in relation to recent childbirth.
They said they found that compared with women of the same age who had never had children, women who had given birth were at an increased risk for breast cancer.
This risk peaked five years after giving birth and continued for 24 years after childbirth.
Breastfeeding wasn’t found to change the pattern of risk.
But the study authors say women shouldn’t be alarmed by the findings of the study.
“The overall risk of breast cancer is low during reproductive years and, even with this increase, the risk is still low among women who have had children recently,” Hazel Nichols, PhD, study author and member of the University of North Carolina Lineberger Comprehensive Cancer Center, told Healthline.
“Breast cancer risk is not determined by one factor alone. We need tools to help women and providers put information from a woman’s complete history: her age, her family structure, whether others in her family have had breast cancer, her lifestyle, to help make decisions around when to start participating in mammography screening programs,” she said.
Breast cancer is more common in older women, with 80 percent of breast cancer diagnoses in the United States being in women age 50 or older. The average age of diagnosis is 62.
There have been other studies that show an increased risk of breast cancer for young women following childbirth. However, these studies didn’t have access to further information on factors that may impact that risk, such as a family history of breast cancer or whether a woman breastfed.
In the recent study, researchers were able to take data from multiple studies involving almost 900,000 women.
As well as examining breast cancer risk after childbirth, researchers also looked at whether breastfeeding or having a family history of breast cancer had an impact.
Increased risk of breast cancer after childbirth was found to be higher in women who had a greater number of births, had a family history of breast cancer, and who were older the first time they gave birth.
This pattern was the same for women who breastfed and those who didn’t.
“Understanding these risk factors will be important to clinicians and women in developing a surveillance plan for each individual woman. Pregnant women with these risk factors, for example [those who are] older or [have] family cancer history, might benefit from closer surveillance with more frequent clinical breast exams, and a lower threshold for imaging or tissue sampling if a breast mass is found. Further studies should be done to develop and evaluate protocols for breast surveillance in higher-risk pregnant women,” Dr. Allison Kurian, director of the Women’s Clinical Cancer Genetics Program at the Stanford University School of Medicine in California, told Healthline.
Although breast cancer risk increased for women after pregnancy, the overall risk of breast cancer in this group still remained low.
The researchers found for women aged 41 to 45, there were 41 more cases of breast cancer for every 100,000 women who gave birth in the previous three to seven years, compared with women who hadn’t had children.
For women 50 and older, there were 247 more diagnoses of breast cancer for every 100,000 in women who had recently given birth.
The findings weren’t consistent for all young women.
Risk of breast cancer was higher in women who had given birth to their first child after age 35. Women who had their first child before age 25 saw no increased risk of breast cancer after a recent birth.
“We’re now starting to understand that risk factors, like childbirth, can have different associations with breast cancer risk across the course of a woman’s life. This means that it’s important for us to consider different life stages and specific types of breast cancer when we investigate changes in breast cancer risk over time,” Nichols said.
Nichols argued the findings of the study could be useful in developing prediction models for breast cancer risk as well helping clinicians make better decisions for screening patients.
Dr. Deanna Attai, assistant clinical professor at the University of California Los Angeles David Geffen School of Medicine, said women shouldn’t be concerned by the findings of the study, but they should discuss any changes to their breasts with their doctor.
“I don’t think this study should discourage women from starting a family if they want to have children and the authors have not recommended enhanced breast cancer screening for women who have been pregnant. Any new breast finding or change should be evaluated, whether a woman has had children or not,” she said.
Kurian said it’s a good reminder of the importance of regular breast self-exams.
“The magnitude of risk increase for an individual woman is small, so I don’t think women need to be very worried. However, I do think breast self-exam once a month is a good idea for all women. When pregnant and postpartum women visit their clinicians for routine checkups, it is appropriate that a clinical breast exam be done. And if a woman ever notices a mass in her breast or axilla, she should seek medical attention promptly,” she said.