Later Birth Order May Decrease Breast-Cancer Risk among Women Breast-Fed as Infants
June 10, 2008

MADISON–Does having older siblings help protect a woman from breast cancer? It might, if she were breast-fed as an infant.

A team of researchers from the University of Wisconsin Paul P. Carbone Comprehensive Cancer Center looked at early-life risk factors to see if the age of the mother, or birth order, affected a woman’s chances of developing breast cancer later in life. In a study published in the most recent (May 2008) issue of Epidemiology, they found an association with birth order, but not one linked to maternal age.

The study by the cancer epidemiology group compared 2,016 Wisconsin women with invasive breast cancer against a control group of 1,960 Wisconsin women who were selected from drivers’ license lists.

Cancer researchers have studied early-life risk factors before, said lead author Hazel B. Nichols, but “this is the first time that the interaction between factors was looked at.”

In general, being breast-fed as an infant seems to protect women against cancer in later life, with those who were breastfed having 17 percent lower risk overall of developing breast cancer later.

But researchers wondered whether infant girls breast-fed by older mothers would be at increased relative risk because those mothers would have had a longer time to accumulate toxins that might be passed along in breast milk. Another question was whether first-, second- and third-born children would be exposed to higher levels of toxins in the breast milk than those children born later.

Nichols, now a doctoral candidate at the Johns Hopkins Bloomberg School of Public Health, said, “We didn’t see what we expected; there was no association with maternal age among breast-fed women.”

They did see a cancer-risk association with birth order. Women who were fourth- born or later had a 43 percent decrease in breast cancer risk compared to women who were first born. These results were not seen in women who were bottle-fed as infants.

“This study does agree with many others that factors very early in life may play a role in risk of cancer in adulthood,’’ said Amy Trentham-Dietz, associate professor of population health sciences at UW School of Medicine and Public Health. “The results do not suggest ways women can make choices later in life to avoid breast cancer. However, additional research building on these results may help us to better understand how breast cancer develops.”

Other Wisconsin co-authors are cancer researchers Brian Sprague, John Hampton, and Polly Newcomb.