TUESDAY, Sept. 30 (HealthDay News) -- Women who are heavier and longer at birth are at increased risk of developing breast cancer later in life, British researchers report.

In fact, as birth weight and length increases, so does the risk for breast cancer, according to the results of a study published in the Sept. 30 online edition of PLoS Medicine.

"These researchers have documented in unequivocal terms that larger birth size is associated with increased breast cancer risk several decades later," said Dr. Dimitrios Trichopoulos, the Vincent L. Gregory Professor of Cancer Prevention at Harvard University School of Public Health Department of Epidemiology and author of an accompanying journal editorial.

Birth size reflects, to a considerable extent, the effects of the intrauterine environment on the fetus, Trichopoulos noted. "To this day, they had not been sufficiently appreciated by the scientific community, because each individual study could not provide conclusive evidence. We are facing now a new reality: that breast cancer has its origins several decades before its clinical appearance," he said.

For the study, a research team led by Dr. Isabel dos Santos Silva, a professor of epidemiology at the London School of Hygiene and Tropical Medicine, collected data on more than 600,000 women, 22,058 of whom had breast cancer. The data came from 32 studies.

The researchers found that women who were heavier and longer at birth had increased risk for breast cancer as adults. An analysis of birth records, among these women, found that for every 17.6 ounces of birth weight, the risk for breast cancer increased 7 percent.

In addition, birth length and head circumference were also associated with an increased risk of breast cancer. The strongest association between size at birth and an increased risk for breast cancer was seen for birth length, the researchers reported.

"Recognition of early life influences are critical in the etiology of breast cancer and helps to explain why several adult life primary prevention practices -- as distinct to secondary prevention ones focusing on early detection -- have been of limited effectiveness," Trichopoulos said.

"Prevention of breast cancer needs to take into account the very long natural history of the disease," he added.

Expert reaction to the new research was fairly guarded.

"There's good evidence for these findings, but there is really no clinical relevance for them," said Debbie Saslow, director of breast and gynecologic cancer at the American Cancer Society.

"There is nothing that women should do differently to try to have smaller babies, or women who were born with a longer length or larger head circumference should do anything differently when they grow up or get screened differently, or consider themselves at high risk -- it's really just a research issue," Saslow said.

More information

For more on breast cancer, visit the American Cancer Society.