They weigh an average of a half-pound less and are more likely to be born early and need special care after birth, according to the Centers for Disease Control and Prevention (CDC). Smoking nearly doubles a woman's risk of having a low-birth-weight baby.

Recent studies also suggest that smoking increases the risk of preterm delivery by about 30 percent, the CDC says.

Children of mothers who smoke may lag behind in school and may be smaller than children whose mothers don't smoke, according to the American Lung Association.

According to the U.S. Surgeon General, estimates range from 12 to 22 percent. If all expectant mothers in the United States stopped smoking, the U.S. Public Health Service estimates the number of infant deaths would decrease by 10 percent.

SIDS is the leading cause of death in infants after 1 month of age, according to the National Institute of Child Health and Human Development (NICHD).

Studies suggest that regular exposure to secondhand smoke may reduce fetal growth, increasing a woman's chances of having a low-birth-weight baby, the CDC says.

One study found fetuses with a certain gene were at increased risk of developing cleft lip and/or cleft palate if their mothers smoked during the first three months of pregnancy, the CDC says.

Smoking increases a woman's risk of an ectopic pregnancy, in which the embryo becomes implanted in a fallopian tube or other abnormal site instead of the uterus.

Nicotine can be transmitted to infants through breast milk, the NICHD says.