Women who were bigger and longer babies may be more likely to develop breast cancer, researchers reported on Tuesday.
The study adds to evidence that, at least in some cases, something that happens in the womb may cause cancer later in life.
Previous research into links between birth size and breast cancer have proved inconsistent, but the findings published in the Public Library of Science journal PLoS Medicine are strong evidence that the two may be related.
"These findings provide strong evidence that birth size — in particular birth length — is a marker of a woman's breast cancer risk in adulthood, although the mechanisms underlying this association are unclear," Isabel dos Santos Silva of the London School of Hygiene and Tropical Medicine and colleagues wrote.
Breast cancer is the leading cause of cancer deaths among women worldwide, according to the American Cancer Society. The group estimates about 465,000 women died of breast cancer globally in 2007, and 1.3 million new cases were diagnosed.
Declining death rates from breast cancer in developed countries have been attributed to early detection through mammography screening and to improved treatment.
Dos Santos Silva and colleagues examined 32 studies comprising 600,000 women, mainly in developed countries. Their analysis included more than 22,000 women who had breast cancer.
After considering established risk factors such as age and late menopause, the researchers found a strong association with birth size, birth length and head circumference. Of the three, birth length showed the strongest link.
"The amount by which birth size affected breast cancer risk was not affected by allowing for other established risk factors," the researchers wrote.
For example, women with recorded birth weights of 4 kilograms or more had a 12 percent higher chance of breast cancer than babies weighing 3 to 3.5 kilograms at birth, the study found.
The link between breast cancer and birth size appeared smaller when compared with other risk factors. The researchers estimated that birth size may be responsible for up to 5 percent of all breast cancers in women up to the age of 80.
Some research has linked hormones such as estrogen and human growth hormone with cancer.
The researchers noted age of puberty and adult height are also associated with breast cancer risk, and growth as a fetus can predict a girl's growth, so there could be a link there.
"The maternal and/or fetal hormonal environment associated with large birth size may alter programming of the breast, making it more susceptible to cancer," the researchers wrote.