The longer mothers breast feed and the younger they begin, the more they lower their risk of getting breast cancer before menopause, a large study has found.

It found that a woman who begins breast feeding in her teens and continues for at least six months cuts the risk almost in half.The study found no evidence that nursing prevents the disease from occurring after menopause. Breast cancer before menopause is relatively rare.

Except for those who began nursing in their teens, the reduction in risk was generally small, about 22 percent. The finding is still noteworthy because breast feeding represents one of the few voluntary things women might do to change their odds of this cancer.

Experts in the past have wondered about the possible effects of breast feeding, but the data have been murky. Some earlier studies have linked nursing with reduced cancer risk, while others have found no correlation.

The latest study was directed by Dr. Polly A. Newcomb of the University of Wisconsin.