According to the 1989 report of the Public Health Service, the preconception visit may be the single most important health care visit when viewed in the context of its effect on pregnancy. Healthy birth outcomes depend heavily on a woman's health before pregnancy. Tobacco use and second hand smoke. Smoking lowers the amount of oxygen that the fetus receives through the placenta, and may also reduce available nutrients. For non-smoking mothers, passive smoke exposure may increase the risk of low birth weight.
Smoking is one of the most preventable determinants of low birth weight .
Alcohol crosses the placenta and can exert a direct toxic effect on the fetus. Heavy alcohol use may interfere with the absorption and utilization of nutrients. It also contributes to maternal malnutrition. Alcohol intake during the first trimester is associated with fetal malformation, during the second with fetal loss, and during the third with low birth weight. Cocaine is considered one of the most dangerous illicit drugs available today.