"Women who weighed 8.8 pounds or more at birth have a 12% increase in breast cancer risk compared to women who weighed 6.6 to 7.69 pounds," says Isabel dos Santos Silva, MD, PhD, professor of epidemiology at the London School of Hygiene and Tropical Medicine, the study's lead author.
Women who were 20 inches in length at birth had a 17% increased risk compared to women who were 19.29 inches.
Head circumferences of 13.7 inches or more boosted risk 11%, compared to those whose head circumference was 12.9 inches, she also found.
Women who were bigger babies shouldn't be distressed by this news, she hastens to add. "This is a modest association," she tells WebMD. "At the moment, we don't know what it means."
The study appears in PLoS Medicine.
Eventually, the finding may help doctors predict who is more likely to have a higher risk of breast cancer risk, Silva says.
Birth Weight and Length
They found that higher birth weight was linked with a higher breast cancer risk in those whose birth data was recorded at birth or based on parental recall when the participants were children, but not in cases where birth weights were self-reported or recalled by parents when the participants were adults.
Length was the strongest predictor of the three, Silva says.
The link remained even when Silva's team took into account other established risk factors.
Overall, Silva says, about 5% of all breast cancers in developed countries could be due to high birth size.
Exactly why high birth size modestly raises breast cancer risk isn't known, Silva tells WebMD. Since 1990, scientists have discussed the possibility that prenatal exposure to high levels of estrogens could affect later breast cancer risk.
Among the explanations, according to Silva, is that the maternal or fetal hormonal environment associated with big birth size may somehow change the programming of the breast, making it more vulnerable to get cancer.
In an editorial accompanying the study, Harvard researchers (including a scientist who published a paper in 1990 about breast cancer possibly originating in the womb) call the study findings "the strongest evidence yet that birth size is a critical determinant of breast cancer risk in adult life."
Women should not be concerned, whatever their birth size, says Debbie Saslow, PhD, director of breast and gynecologic cancer at the American Cancer Society. While the study has research interest to scientists, she says, it does not trigger any new clinical advice for women.
"We aren't going to tell people to do anything differently, not going to label these women as high risk, or tell them to get screened at an earlier age," she says.
"It's just one more piece of the puzzle that someday will help the research community better understand the multiple, interplaying causes of breast cancer."