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Birth Size Linked To Women's Breast Cancer Risk

Medical researchers have suspected for decades that some human cancers might be related to things that happen to us before we are born. Specifically, they hypothesized that women exposed to high levels of hormones when their mothers were pregnant might have a higher risk for breast cancer as adults. A number of scientists have attempted to prove this idea, but their results were variable. Some found a link. Others didn't. But a new review of historical medical data suggests that there may, indeed, be a connection.

Epidemiologist Isabel dos Santos Silva says it's a difficult hypothesis to study. "In an ideal world, what we would like is to be able to measure hormonal levels from pregnancy, then measure the baby when the baby is born and have measures of the birth size of the baby and then wait 50 or 60 years until the women reach the ages at which incidence of breast cancer is high," she says.

Since that's impractical, dos Santos Silva says the next best way to test this idea is to get historical data from many, many women. She and colleagues from the London School of Tropical Medicine and Hygiene pooled the data from many studies of older women with breast cancer and looked at the women's birth records. She had information from close to 600,000 women. More than 22,000 of them had developed breast cancer.

"We showed that women who were heavy at birth, or who were long, or who had a large head circumference, had an increased risk of breast cancer," she says. "So, for instance, women who were 51 centimeters long or more had about a 70 percent increased risk compared with women who were less than 49 centimeters when they were born."

Dos Santos Silva says she's not sure why heavier and longer women grew up to have a greater risk of breast cancer. One hypothesis is that size reflects something in the uterine environment, perhaps that larger babies are exposed to more of the female hormone estrogen.

"But we don't know. We need more biological studies to look at underlying mechanisms behind these associations," she says. "Another possible explanation's that somehow the babies who are bigger, who have a larger number of stem cells - you know the stem cells are kind of the mother of all cells - and maybe they have them in a larger number in the breast gland.

"And if there are a larger number of such cells, it makes them more likely to suffer a malignant transformation later in life. It just increases the likelihood of that happening."

Dos Santos Silva says this additional risk is not a reason for women to become frightened if they were big babies. She says the additional risk influences about one case of breast cancer out of every 20. She says she and her colleagues will continue to study this phenomenon to pinpoint the extent of the risk and to understand better what women can do to reduce their risk for breast cancer.

Dos Santos Silva's research was published in the online journal PLoS Medicine.