Lifestyle behaviors such as cigarette smoking, weight gain during pregnancy, and use of other drugs play an important role in determining fetal growth. The relationship between lifestyle risk factors and low birth weight is complex and is affected by psychosocial, economic, and biological factors. Cigarette smoking is the largest known risk factor for low birth weight. Approximately 20% of all low birth weight could be avoided if women did not smoke during pregnancy. Reducing heavy use of alcohol and other drugs during pregnancy could also reduce the rate of low birth weight births. Pregnancy and the prospect of pregnancy provide an important window of opportunity to improve women's health and the health of children. The adoption before or during pregnancy of more healthful lifestyle behaviors, such as ceasing to smoke, eating an adequate diet and gaining enough weight during pregnancy, and ceasing heavy drug use, can positively affect the long-term health of women and the health of their infants. Detrimental lifestyles can be modified, but successful modification will require large-scale societal changes. In the United States, these societal changes should include a focus on preventive health, family-centered workplace policies, and changes in social norms.