Led by Isabel dos Santos Silva (Professor of Epidemiology), the researchers re-analysed data from published and unpublished studies to get more precise estimates of the extent to which birth size affects the risk of breast cancer later in life.
They also wanted to investigate whether associations with other risk factors could explain the estimates.
After analysing 32 studies, comprising 22,058 cases of breast cancer among a total of more than 600,000 women mainly from developed countries, the researchers found that birth weight was positively associated with breast cancer risk in studies where information on birth size was based on birth records (although not in those based on adult self-reports, which tend to be less accurate).
The study of data from birth records showed that a 0.5 kg increment in birth weight was associated with an estimated 7 percent increase in the risk of breast cancer.
When studies with data from birth records were analysed, the researchers found that birth length and head circumference were positively linked with breast cancer risk. Out of the three birth size measures examined, birth length appeared to be the strongest independent predictor of risk.
However, when the effects of established breast cancer risk factors were accounted for, the estimated magnitude of the birth size association with breast cancer risk was not affected.
"Our study indicates that birth size is a marker of susceptibility to breast cancer in adulthood, at least in developed countries. The birth size - breast cancer association appeared to be largely independent of known risk factors. Little is known on how the pre-natal environment may affect breast cancer risk later in life. Further research is needed to unravel the biological mechanisms underlying the birth size - breast cancer association," said Isabel dos Santos Silva.
The study was published in PLoS Medicine.