Background Maternal smoking during pregnancy is associated with fetal growth restriction, but also with increased risk for overweight in childhood. If the mother stops smoking in early pregnancy fetal growth is not restricted, but whether the risk for later overweight persists is unclear.

Aim To study if four year old children of mothers who stopped smoking in early pregnancy have higher mean body mass index (BMI) and/or increased odds of being overweight compared with children of non-smokers.

Study design Prospective population based study on Norwegian mothers and children.

Subjects Among 711 children available for analysis, 540 were children of never smoking mothers, 114 of mothers who stopped smoking in early pregnancy, and 57 of mothers who continued to smoke throughout pregnancy.

Outcome measures BMI and overweight defined by international criteria at age four.

Results Compared with children of never smoking mothers, children of smoking mothers had higher mean BMI (mean difference: 0.47; 95% CI: 0.10 to 0.84 kg/m²), whereas mean BMI was not higher among children of mothers who stopped smoking (mean difference: 0.02; 95% CI: −0.24 to 0.28 kg/m²). Similarly, children of smoking mothers had increased odds for overweight (adjusted OR: 2.83; 95% CI: 1.13 to 7.10), whereas children of mothers who stopped smoking did not have increased odds (adjusted OR: 1.29; 95% CI: 0.62 to 2.68) compared with children of never smoking mothers.

Conclusions In this study, the association between smoking exposure and childhood overweight did not persist in children of mothers who stopped smoking early in pregnancy.