Written by Adam Marcus

Updated on July 18, 2002

THURSDAY, July 18, 2002 (HealthDayNews) -- The longer women breast-feed and the more children they have, the less likely they are to develop breast cancer, a new study has found.

Nursing is associated with a slew of benefits for infants, from higher intelligence to fewer early infections, but the benefits for women have been less certain.

The new study shows women can cut their risk of breast tumors by more than 4 percent for every year of nursing. Even more protective -- women, make sure you're sitting down -- is having six or seven children.

The study, reported this week in The Lancet, helps explain why breast cancer is about twice as common in developed countries as it is in the Third World, says Gillian Reeves, a co-author of the paper.

"This big difference can be accounted for by differences in family size and breast-feeding," says Reeves, a staff scientist at Cancer Research UK in London.

Western women now breast-feed an average of about two to three months, and have between two and three children each. However, women in the developing world have six to seven kids, whom they nurse an average of two years each, Reeves says.

Of course, the typical American woman isn't likely to triple her offspring output. And while the American Academy of Pediatrics recommends exclusive breast-feeding for the first six months of a baby's life, most women in this country don't do so even that long.

Still, if women here could be encouraged to nurse for a year, it could cut the number of breast cancer cases by 5 percent, or about 10,000, Reeves says: "Even a small change could actually prevent a sizable number of cancers."

Reeves and her colleagues analyzed 47 previous studies that included data on breast-feeding and childbearing in 30 countries. In all, they covered 50,000 women with breast cancer and almost 97,000 without the disease.

Women who had breast cancer tended to have fewer children than the other women. Those with the disease who did have children were roughly 10 percent less likely to have breast-fed than unaffected mothers -- 71 percent versus 79 percent. They also nursed for shorter periods than cancer-free women, doing so 9.8 months versus 15.6 months, on average.

Every year a woman nursed reduced her risk of breast cancer by 4.3 percent, while every child she bore reduced her risk by 7 percent.

Dr. Ruth Lawrence, a pediatrician at the University of Rochester and a noted breast-feeding expert, calls the latest findings "reassuring."

"It makes sense, and in smaller studies that has been the conclusion," Lawrence says.

Scientists aren't sure why breast-feeding is protective; one probable mechanism is that nursing suppresses the secretion of estrogen, which can stimulate abnormal cell growth in the breasts. In fact, nursing keeps a woman's hormonal environment similar to that of when she's pregnant, reducing her lifetime exposure to estrogen, Lawrence explains.

What To Do

For more on nursing, try Breastfeeding.com or La Leche League.

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