Later Birth Order May Decrease Breast-Cancer Risk
MADISON - Does having older siblings help protect a woman from breast cancer? It might, if she were breastfed as an infant.
A team of researchers from the University of Wisconsin Carbone 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 recently published in Epidemiology, they found a cancer link with birth order, but not to maternal age.
The study compared 2,016 Wisconsin women who had 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, of the Johns Hopkins Bloomberg School of Public Health, 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. Researchers found that those who were breast fed had a 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. They hypothesized that there may be an increase 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 was surprised by the results, "We didn't see what we expected; there was no association with maternal age among breast-fed women."
But they did see a cancer-risk association with birth order. Women who were fourthborn 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 the 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."
Date Published: 11/06/2009