Because many adverse outcomes associated with smoking are rare (such as SIDS, abruptio placenta, and perinatal death), birth weight is frequently used as an outcome in studies of smoking in pregnancy. Most studies published to date report an average decline in birth weight among smokers of 200-300 grams. Smoking may be responsible for up to 30 percent of all cases of low birth weight.

Despite the risks, most women are unable to stop smoking during pregnancy. Only one-in-four quits successfully. However, one-in-three reduces the amount smoked. The effect of reducing the amount smoked (without quitting) on birth outcomes is unknown.