To our knowledge, no previous study has been able to examine the effect of smoking cessation during the first trimester. We found that among children of women who stopped smoking before 16 weeks of gestation the risk of stillbirth and infant death was comparable with that in children of women who had been nonsmokers from the beginning of pregnancy. This result indicates that smoking exerts its influence on mortality at some point after the first trimester and supports the causal role of fetal exposure to tobacco smoke in late pregnancy. The result also emphasizes the importance of providing smoking cessation programs for pregnant women who indicate at the first antenatal visit that they smoke. In our population, very few women gave up smoking after the first trimester, and therefore, we could not study the effect of smoking cessation after that period.