Abruptio placentae and placenta previa are responsible for at least half the excessive fetal and neonatal death associated with maternal smoking during pregnancy. Data from a large prospective study were analyzed to determine if stopping smoking reduced the frequency of these disorders. Mothers who stopped smoking had a 23% lower frequency of abruptio placentae and a 33% lower frequency of placenta previa than women who continued to smoke during pregnancy. The mothers who stopped smoking had only about half as many fetal and neonatal deaths due to abruptio placentae and one third fewer losses to placenta previa than did nothers who continued to smoke. These seeming benefits of stopping smoking were greatest in mothers 30 years of age and older.