When a woman becomes pregnant, it is very important for her to lead a healthy life: To eat plenty of nourishing food, get plenty of rest, and exercise regularly. It is also vital that she avoid anything that might harm her or her baby-to-be. It is especially important to give up alcohol, cigarettes, and drugs.

For a pregnant woman, drug abuse is doubly dangerous. First, drugs may harm her own health, interfering with her ability to support the pregnancy. Second, some drugs can directly impair prenatal development.

Virtually all illegal drugs, such as heroin and cocaine, pose dangers to a pregnant woman. Legal substances, such as alcohol and tobacco, are also dangerous, and even medical drugs, both prescription and over-the-counter, can be harmful. For her own health and the health of her baby-to-be, a woman should avoid all of them as much as possible, from the time she first plans to become pregnant or learns that she is pregnant.

Some drugs can be harmful when used at any time during pregnancy; others, however, are particularly damaging at specific stages.

Most of the body organs and systems of the baby-to-be are formed within the first ten weeks or so of pregnancy (calculated from the date of the last menstrual period). During this stage, some drugs, and alcohol in particular, can cause malformations of such parts of the developing fetus as the heart, the limbs, and the facial features.

After about the tenth week, the fetus should grow rapidly in weight and size. At this stage, certain drugs may damage organs that are still developing, such as the eyes, as well as the nervous system. Continuing drug use also increases the risk of miscarriage and premature delivery. But the greatest danger drugs pose at this stage is their potential to interfere with normal growth. Intrauterine growth retardation (IGR) is likely to result in a low-birthweight baby a baby born too early, too small, or both. Low-birthweight babies require special care and run a much higher risk of severe problems or even death.

Some drugs can be especially harmful at the end of pregnancy. They may make delivery more difficult or dangerous, or they may create health problems for the newborn baby.

Alcohol is one of the most dangerous drugs for pregnant women, especially in the early weeks. In the mother's body, alcohol breaks down into a cell-damaging compound that is readily absorbed by the fetus. Heavy drinking during early pregnancy greatly increases the risk of a cluster of birth defects known as fetal alcohol syndrome. This cluster includes a small skull (microcephaly), abnormal facial features, and heart defects, often accompanied by impeded growth and mental retardation. Heavy drinking in later pregnancy may also impede growth.

It is not known whether light to moderate drinking can produce these effects. However, even if the risk is low, the stakes are very high. Medical experts agree that a woman should avoid alcohol entirely when she decides to become pregnant, or at least when the first signs of pregnancy appear. Even such mild beverages as beer and wine coolers should be off limits.

Smoking during pregnancy appears to raise the risk of miscarriage or premature labor. But the primary danger is hindered fetal growth. Nicotine depresses the appetite at a time when a woman should be gaining weight, and smoking reduces the ability of the lungs to absorb oxygen. The fetus, deprived of sufficient nourishment and oxygen, may not grow as fast or as much as it should.

Cocaine (including crack) and methamphetamines (speed, or ice) are powerful stimulants of the central nervous system. They suppress the mother's appetite and exert other drastic forces on her body, causing the blood vessels to constrict, the heart to beat faster, and the blood pressure to soar. The growth of the fetus may be hindered, and there are higher risks of miscarriage, premature labor, and a condition known as abruptio placentae (the partial separation of the placenta from the uterus wall, causing bleeding).

If these drugs are taken late in pregnancy, the baby may be born drug dependent and suffer withdrawal symptoms such as tremors, sleeplessness, muscle spasms, and sucking difficulties. Some experts believe learning difficulties may later develop.

Heavy narcotics use increases the danger of premature birth with such accompanying problems for the infant as low birthweight, breathing difficulties, low blood sugar (hypoglycemia), and bleeding within the head (intracranial hemorrhage).

The babies of narcotics-dependent mothers are often born dependent themselves and suffer withdrawal symptoms, such as irritability, vomiting and diarrhea, and joint stiffness.

Women who inject narcotics may become infected with the HIV virus from dirty needles and may subsequently develop AIDS. HIV-infected women obviously run a high risk of passing the virus on to their babies.

At least one inhaled substance has been clearly connected with birth defects. The organic solvent Toluene, widely used in paints and glues, appears to cause malformations like those produced by alcohol (which is itself an organic solvent). It is possible that all organic solvents cause birth defects.

PCP (phencyclildine or angel dust) taken late in pregnancy can cause newborns to have withdrawal symptoms, such as lethargy alternating with tremors.

Studies of marijuana use by pregnant women are inconclusive, because marijuana is often used with other drugs, such as tobacco and alcohol. Like them, it is associated with premature birth and low-birthweight babies.

Many medications have side effects that are potentially harmful during pregnancy, but their benefits may outweigh their risks. A woman should consult her doctor or midwife before taking any drug, even one sold over the counter. Below are a few examples of medical drugs that must be used with extreme caution or avoided altogether.