Objectives: Maternal smoking during pregnancy is associated with low birth weight (LBW). Reduction of cigarette consumption does not seem to improve birth weight but it is not known whether implementation of periods of smoking abstinence improves it. We assessed whether the number of 7-day periods of smoking abstinence during pregnancy may help reduce the number of newborns with LBW. Design and setting: Secondary analysis of a randomised, controlled, multicentre, smoking cessation trial among pregnant smokers. Participants: Pregnant women were included at <18 weeks of gestational age and assessed at face-to-face, monthly visits. Data of 407 singleton live births were analysed. Primary outcome measure: Newborns with low birth weight. Results: 40 and 367 newborns were born with and without LBW, respectively. Adjusted for all available confounders, 3 or more periods of at least 7 days' smoking abstinence during pregnancy was associated with reduced likelihood of LBW compared with no abstinence periods (OR = 0.124, 95% CI 0.03 to 0.53, p = 0.005). Reduction of smoking intensity by at least 50% was not associated with birth weight. Conclusion: Aiming for several periods of smoking abstinence among pregnant smokers unable to remain continuously abstinent from smoking may be a better strategy to improve birth weight than reducing cigarette consumption.