Background. Maternal smoking during pregnancy is associated with increased childhood body mass index (BMI) but the relationship may be due to confounding by maternal factors. This study tested the hypothesis that siblings born to mothers who begin to smoke between pregnancies will have higher BMI than older unexposed siblings.Methods. Maternal details from the Aberdeen Maternity and Neonatal Databank were linked to the Study of Trends in Obesity in North East Scotland which holds offspring BMI at five years of age.Change in maternal smoking status between pregnancies was linked to offspring BMI and also to the difference in BMI between siblings.Results. Maternal smoking status in successive pregnancies was linked to child BMI at age 5 years in 6,581 mother-child pairs of whom 718 included sibling pars. Children whose mothers had quit,started smoking or smoked in consecutive pregnancies had higher BMI compared to those not exposed to maternal smoking. Siblings born after onset of maternal smoking had higher mean BMI z score or (0.20 95% confidence interval CI [(0.03, 0.38])) compared to unexposed older siblings. Mean BMI z score was also higher by mean of 0.10 [(95% CI0.00, 0.19]) in younger sibling compared to older siblings born to mothers who smoked in both pregnancies. BMI z score was not significantly different between siblings whose mothers quit between pregnancies.Conclusions. In utero exposure to maternal smoking during pregnancy may increase the likelihood of increased BMI in childhood.