Although cigarette smoking harms both pregnant women and their fetus, only about 20% of women who smoke are able to quit during pregnancy. The most consistent effect of smoking on the fetus during pregnancy is a reduction in birth weight: The more a woman smokes during pregnancy, the less the baby is likely to weigh. The average birth weight of babies born to women who smoke during pregnancy is 6 ounces less than that of babies born to women who do not smoke.

Birth defects of the heart, brain, and face are more common among babies of smokers than among those of nonsmokers. Also, the risk of sudden infant death syndrome (SIDS) may be increased. A mislocated placenta (placenta previa), premature detachment of the placenta (abruptio placentae), premature rupture of the membranes (containing the fetus), preterm labor, uterine infections, miscarriages, stillbirths, and premature births are also more likely. In addition, children of women who smoke have slight but measurable deficiencies in physical growth and in intellectual and behavioral development. These effects are thought to be caused by carbon monoxide and nicotine. Carbon monoxide may reduce the oxygen supply to the body's tissues. Nicotine stimulates the release of hormones that constrict the vessels supplying blood to the uterus and placenta, so that less oxygen and fewer nutrients reach the fetus.

Because of the possible harmful effects of smoking during pregnancy, pregnant women should make every effort to not smoke during pregnancy, including discussing strategies with their doctor.

Pregnant women should avoid exposure to secondhand smoke because it may similarly harm the fetus.

Drinking alcohol during pregnancy is the leading known cause of birth defects. Because the amount of alcohol required to cause fetal alcohol syndrome is unknown, pregnant women are advised to abstain from drinking any alcohol regularly or on binges. Avoiding alcohol altogether is even safer.

The risk of miscarriage almost doubles for women who drink alcohol in any form during pregnancy, especially if they drink heavily. Often, the birth weight of babies born to women who drink regularly during pregnancy is substantially below normal. The average birth weight is about 4 pounds for babies exposed to large amounts of alcohol, compared with 7 pounds for all babies. Newborns of women who drank during pregnancy may not thrive and are more likely to die soon after birth.

Fetal alcohol syndrome is one of the most serious consequences of drinking during pregnancy. Binge drinking as few as three drinks a day can cause this syndrome. It occurs in about 2 of 1,000 live births. This syndrome includes inadequate growth before or after birth, facial defects, a small head (probably caused by inadequate growth of the brain), intellectual disability, and abnormal behavioral development. Less commonly, the position and function of the joints are abnormal and heart defects are present.

Babies or children of women who drank alcohol during pregnancy may have severe behavioral problems, such as antisocial behavior and attention deficit disorder. These problems can occur even when the baby has no obvious physical birth defects.

Whether consuming caffeine during pregnancy harms the fetus is unclear. Evidence seems to suggest that consuming caffeine in small amounts (for example, one cup of coffee a day) during pregnancy poses little or no risk to the fetus. Caffeine, which is contained in coffee, tea, some sodas, chocolate, and some drugs, is a stimulant that readily crosses the placenta to the fetus. Thus, it may stimulate the fetus, increasing the heart rate. Caffeine also may decrease blood flow across the placenta and decreases the absorption of iron (possibly increasing the risk of anemia (see Anemia During Pregnancy). Some evidence suggests that drinking more than seven cups of coffee a day may increase the risk of having a stillbirth, premature birth, low-birth-weight baby, or miscarriage. Some experts recommend limiting coffee consumption and drinking decaffeinated beverages when possible.

Aspartame, an artificial sweetener, appears to be safe during pregnancy when it is consumed in small amounts, such as in amounts used in normal portions of artificially sweetened foods and beverages. Pregnant women with phenylketonuria, an unusual disorder, should not consume any aspartame.

Opioids, such as heroin, methadone, and morphine, readily cross the placenta. Consequently, the fetus may become addicted to them and may have withdrawal symptoms 6 hours to 8 days after birth (see Opioids). However, use of opioids rarely results in birth defects. Use of opioids during pregnancy increases the risk of complications during pregnancy, such as miscarriage, abnormal presentation of the baby, and preterm delivery. Babies of heroin users are more likely to be small.

Use of amphetamines during pregnancy may result in birth defects, especially of the heart, and possibly inadequate growth before birth.

Whether use of marijuana during pregnancy can harm the fetus is unclear. The main component of marijuana, tetrahydrocannabinol, can cross the placenta and thus may affect the fetus. However, marijuana does not appear to increase the risk of birth defects or to slow the growth of the fetus. Marijuana does not cause behavioral problems in the newborn unless it is used heavily during pregnancy.

Bath salts refers to a group of designer drugs made from various substances that resemble amphetamine. More and more pregnant women are using these drugs.

The drugs may cause the blood vessels in the fetus to narrow, reducing the amount oxygen the fetus gets. Also, these drugs increase the risk of stillbirth, premature detachment of the placenta (abruptio placentae), and possibly birth defects.