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Researchers Link Birth Weight to Risk of Pre-Menopausal Breast Cancer (dateline June 7, 2003)

As researchers continue to search for risk factors that may increase the chances of breast cancer, a new study shows that birth weight may play a role. In the study, scientists found that heavier babies were more likely than lighter babies to develop pre-menopausal breast cancer. The researchers attribute the increased breast cancer risk to hormonal factors exerted on babies in the womb. Further research on the pre-natal environment may provide more answers about future cancer risk.

To investigate whether the size of a baby at birth and the rate of growth influence breast cancer risk later in life, Valerie McCormack, a research fellow at the London School of Hygiene and Tropical Medicine, and colleagues studied 5,358 Swedish females born between 1915 and 1929. They found that 359 of the women developed breast cancer.

McCormack and her team found that the women who weighed 8 pounds, 13 ounces (4,000 grams) or more at birth were 3.5 times more likely to develop pre-menopausal breast cancer compared to the women who weighed 6 pounds, 10 ounces (3,000 grams) or less at birth. However, birth weight only affected the risk of developing breast cancer before menopause; weight did not play a factor in post-menopausal breast cancer risk (post-menopausal breast cancer is significantly more common than pre-menopausal breast cancer).

In fact, the researchers say that the breast cancer risk they found in their study was less associated with actual birth weight. Rather, it was linked to a baby’s length and head size. Women who were longer babies and those born with bigger heads were more likely to develop pre-menopausal breast cancer.

McCormack and her colleagues say that hormonal influences in the womb are a likely explanation for the association between birth characteristics and the risk of pre-menopausal breast cancer. Those further research is needed to investigate this link, the researchers suggest that larger babies may be exposed to different levels of growth hormones in the womb that increase the chances for pre-menopausal breast cancer. Factors that can influence hormones in the womb include a mother’s nutritional habits, variations in natural hormones, and whether or not a mother smokes during pregnancy.

This study highlights ongoing research on causes of breast cancer. While several factors have already been identified as contributing to breast cancer risk (including advanced age, early onset of menstruation, late menopause, not having children, long-term use of hormone replacement therapy, etc.), approximately 80% of women who develop breast cancer have no known risk factors. Research is also focused on genes and gene-environment interactions that may increase breast cancer risk. 

Additional Resources and References

  • The report, "Fetal Growth and Subsequent Risk of Breast Cancer: Results from Long Term Follow Up o Swedish Cohort," is published in the February 1, 2003 issue of the British Medical Journal,
  • The January 31, 2003 Reuters Health report, "Larger Babies, Higher Future Risk of Breast Cancer," by Alison McCook is available to paid subscribers at
  • To learn more about risk factors for breast cancer, please visit