NEW YORK: Adult women who were breast-fed as infants may have a lower risk of developing breast cancer than those who were not breast-fed, unless they were first-born, study findings suggest.

As a general group, women who reported they had been breast-fed in infancy had a 17 per cent decrease in breast cancer risk, Hazel B Nichols, who was involved in the study, told Reuters Health.

However, we did not observe this reduction when we looked specifically among first-born women, said Nichols, of the University of Wisconsin, in Madison. A woman's age at childbirth helps predict the levels of environmental contaminants in her breast milk, and studies have suggested a possible link between increased breast cancer risk and the accumulation of these contaminants, Nichols and colleagues note in the medical journal Epidemiology.

To analyze whether an adult woman's birth order, mother's age at the time of her birth, and whether or not she was breast-fed alters her risk for breast cancer, the investigators interviewed 2,016 women, aged 20 to 69 years, with breast cancer, and 1,960 women of similar age without breast cancer.

As noted, women breast-fed during infancy generally had reduced breast cancer risk. However, in analyses restricted to breast-fed women, those with 3 or more older siblings had a lesser risk for breast cancer than first born women, the researchers found. But breast-fed women showed no altered breast cancer risk according to their mothers age at childbirth.

Among women who were not breast-fed, reduced adult breast cancer risk was linked with their mothers older age at childbirth, but the investigators identified no association between breast cancer risk and birth order in this group.

While the current results hint that breast cancer risk may differ according to whether or not women were breast-fed during infancy, additional studies are needed to determine if these associations vary with duration of breast-feeding or according to measured levels of environmental contaminants present in breast milk, Nichols said.