Women who waited 15 years after their first period to have children had 60 percent less chance of developing an aggressive form of breast cancer.

PROBLEM: There have been some indications that maternity might be a protective factor against breast cancer. The most common type of breast cancer, ER positive, occurs less among women who've carried a child to term and who breast-feed. Breast-feeding, additionally, has also been shown to decrease the risk of one the most aggressive breast cancers: "triple-negative." But as yet, not enough is known about the relationship between reproductive history and one's risk of developing any types of breast cancer.

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METHODOLOGY: In this observational study, researchers from the Fred Hutchinson Cancer Research Center looked at data from almost 2,000 women between the ages of 20 and 44. About half had a history of breast cancer; their reproductive histories were compared, by sub-type, with the women who had never had the disease.

RESULTS: The longer women waited before having their first child after their first period, the lower their risk of triple-negative breast cancer. At the peak of the findings, a 15-year interval was associated with a 60 percent reduction in risk.

Breast-feeding, too, was associated with an inverse risk of the same sub-type of breast cancer. But no associations between childbirth or breast-feeding were found for ER positive or HER2-overexpressing breast cancers.

IMPLICATIONS: A rare but aggressive form of breast cancer occurs most often in African American women who, according to lead author Christopher Li, are more likely to start having children at a younger age and are less likely to breast-feed. The association he found between delayed childbirth and breast cancer risk could possibly explain why African American women appear to be more susceptible to the disease, and indicates a possible way of reducing their risk.

However, correlation in this case does not even begin to approach causation, and the researchers are breaking a lot of new ground here -- this is one of the first studies to look at breast cancer in premenopausal women. While there is now enough evidence, they say, to suggest that breast-feeding is a well-established protective factor against triple-negative breast cancer, the rest of their findings remain to be verified.

The full study, "Reproductive factors and risk of estrogen receptor positive, triple-negative, and HER2-neu overexpressing breast cancer among women 20-44 years of age," was published in the journal Breast Cancer Research and Treatment.