A large international study finds that breast cancer is linked to the number of children a woman bears and how long she breastfeeds them. More children and longer nursing mean less cancer risk.

An analysis of childbearing, breastfeeding and breast cancer patterns in nearly 150,000 women around the world tells the story.

"The more children that a woman has, the lower her risk for breast cancer as she gets older," says American Cancer Society researcher Eugenia Calle, who was among dozens of investigators in 30 countries who participated in the study. "Where the study really has added a valuable piece is to clarify that independent of having children, breastfeeding children and breastfeeding them for longer amounts of time also offers protection to women in terms of lowering their risk of breast cancer."

The research, published in the journal Lancet, shows that women with breast cancer had fewer children on average than disease-free mothers, 2.2 children versus 2.6. Of mothers who had never breastfed, 29 percent developed cancer, compared to 21 percent who had breastfed.

Here's another statistic. No matter where the women lived or what their age, their relative risk of breast cancer dropped more than four percent for every year of breastfeeding and seven percent for each birth.

The World Health Organization recommends that women breastfeed their children for two years because of the nutritional benefits it provides the babies. But Eugenia Calle says few in Western nations follow that guideline, as their great grandmothers did in an era when large families and nursing were much more common. "What this paper is able to say is that these changes in childbearing and breastfeeding patterns could really account for a great deal of the increased risk that we have observed over the last 50 to 100 years in breast cancer rates, and could explain a fair amount of the difference in breast cancer rates between developed countries and developing countries," she says.

Ms. Calle and her colleagues estimate that the number of breast cancer cases in Western countries would be cut by more than half if women had as many children and nursed as long as women in developing nations.

But the researchers realize that this is unrealistic in industrial societies. So what should modern mothers do? "It is the case that they still do have two to three children in some countries and if they increase their breastfeeding to six months or a year per child, they would still enjoy some reduction in risk," she says.

The study estimates that just six extra months of nursing could reduce breast cancer in richer nations by 25,000 cases each year.