Each year in the United States approximately 50,000 miscarriages are attributed to smoking during pregnancy. On average, babies born to mothers who smoke weigh less than those born to nonsmokers, and low birth weight is correlated with many developmental problems. Pregnant women who stop smoking in the first three or four months of their pregnancies give birth to higher-birth-weight babies than do woman who smoke throughout their pregnancies. Infant mortality rates are also higher among babies born to smokers.