Baby girls who are of larger than average length and weight at birth grow up being at increased risk of breast cancer, analysis suggests.
The analysis of 32 studies involving more than 600,000 women provides the strongest evidence yet of such a link.
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It could point to some link between cancer and the environment of the baby before birth, PLoS Medicine reports.
One theory is that a larger baby may be exposed to more oestrogen and other hormones in the womb.
The data analysed by Professor Isabel dos Santos Silva showed birth weight, length and head circumference were all linked with breast cancer risk.
A 0.5kg (1.1lb) increment in birth weight was associated with an estimated 7% increase in the risk of breast cancer.
But birth length appeared to be the strongest independent predictor of risk.
The risk of developing breast cancer by the age of 80 was 11.5 in 100 for the women who measured more than 51cm (20in) in length at birth, compared with 10 in 100 for those who had measured less than 49cm (19.3in).
Hostile womb
This increase in risk, although modest, is of a similar magnitude to that of other more established risk factors for breast cancer, such as alcohol consumption, say the authors.
Professor dos Santos Silva said: "Little is known on how the pre-natal environment may affect breast cancer risk in later life. Further research is needed."
Dr Sarah Cant of Breakthrough Breast Cancer said: "Breast cancer is a complex disease that can be influenced by many factors throughout a woman's life.
"We don't yet know what all of these factors are, so it is very interesting that birth size may be one of them.
"This research could add to our increasing knowledge about the causes of breast cancer - possibly helping us to better predict breast cancer risk and potentially prevent the disease in the future."
She added that although there was nothing a woman could do about her size at birth, she could help reduce her risk of developing breast cancer with actions such as maintaining a healthy weight, eating a balanced diet and limiting her alcohol intake.
A spokeswoman for Breast Cancer Care said: "We would urge all women, particularly new mothers who may be worried by this study, to remember that gender and age are by far still the biggest known risk factors for developing breast cancer, with the majority of cases occurring in women aged over 50.
"Therefore, it is vital that all women remain breast aware throughout their lives and accept regular invitations to routine screening."
Around 46,000 cases of breast cancer are diagnosed every year in the UK. The disease kills an estimated 12,400 women annually.
(BBC)
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