In Western cohort studies, 75-80% of breast cancers occur after menopause and these cases can be well prevented, compared to 20-25% of premenopausal breast cancers, and prevention in these patients must begin early in life. The risk of breast cancer can accumulate during childhood and adolescence, especially during the first menstrual period and first pregnancy, when breast cells are undifferentiated and sensitive to carcinogenic factors. Excessive growth in childhood and excessive alcohol and tobacco use in early adulthood increase the risk of cancer, while soy intake during this period may reduce the risk of developing the disease. Weight gain during the pre- and postmenopausal periods increases the risk of postmenopausal (but not premenopausal) breast cancer. Therefore, preventive interventions for breast cancer should focus on preventing premenopausal weight gain. Obesity in childhood or early adulthood does not increase the risk of breast cancer and may sometimes place this group of women at low risk. This paradox of weight on breast cancer risk is unclear, but it likely reflects the fact that young overweight women do not gain significant weight in adulthood, and it is weight gain in adulthood that places women at high risk for cancer. In addition, serum progesterone concentrations tend to be lower in young overweight women.