Women do not always have a choice about when -- or if -- they give birth and nurse babies. But there are other factors that can raise risk of breast cancer over which women do have control. According to Dr. Graham A. Colditz and Dr. A. Lindsay Frazier of Harvard Medical School, preventive efforts should be focused on girls because it is young breasts that are most vulnerable to molecular damage that can accumulate over years.

Two habits that often start in the teen-age years are especially dangerous: alcohol consumption and cigarette smoking. The Nurses' Health Study, based at Harvard, found that compared with nondrinkers, women who consumed more than one drink a day faced a 2 1/2-fold increase in breast cancer risk. Other studies have indicated that this risk is limited almost entirely to women who start drinking before age 25.

As for smoking, a large Danish study found a 60 percent increase in risk of breast cancer among women who had smoked cigarettes for more than 30 years. Smokers also tended to develop cancer at younger ages than nonsmokers. The Harvard researchers noted that among women who smoked more than 25 cigarettes a day, those who had started to smoke before they were 16 faced an 80 percent increase in breast cancer risk.

On the other hand, vigorous physical activity in adolescence and young adulthood is protective, perhaps because it can delay menarche and, like pregnancy, reduce the number of ovulatory menstrual cycles. But even after menopause, exercise is likely to be helpful because it reduces body fat, where estrogens are formed from other steroids. As you might guess, weight gain in adulthood increases the postmenopausal breast cancer risk.

As for diet, women would be wise to eat more fiber and less fat. Women on high-fat diets have higher levels of estrogen in their blood, which can spur the growth of breast cancer. However, in a new study of premenopausal women by Dr. David Rose of the American Health Foundation, wheat bran -- one cup or two servings of a whole-bran cereal daily -- diminished blood levels of estrogen. Studies at Tufts University have indicated that dietary fiber from vegetables and fruits, legumes and cereal brans can lower the risk of breast cancer.