Background Birth outcomes are relevant for future children's heath. Capitalising on a natural experimental design in Switzerland, we evaluated how regional smoking bans introduced at different time points affected birth outcomes, including preterm and early-term births.

Methodology We used birth registry data of all singleton neonates born in Switzerland (2007–2012). We developed canton-specific interrupted time-series followed by random meta-analysis to evaluate the benefits of smoking bans on preterm (<37 gestational weeks) and early-term (37–38 gestational weeks) births. Heterogeneity across type of ban and contextual characteristics was explored through metaregression. A time-to-event approach was used for evaluating duration of pregnancy under the smoking bans and effects, taking into account individual maternal factors.

Results We observed a decrease in the risk of preterm birth of 3.6% (95% CI, −9.3% to 2.5%), and early-term birth of 5.0% (95% CI −7.5% to −2.5%). Results showed a clear dose–response relationship. Greater risk reductions were obtained for preterm births in areas with more comprehensive bans (−6.8%; 95% CI −12.1% to 0.1%), and for pregnancies with the longest gestational time under smoking bans (HR, 0.991; 95% CI 0.984 to 0.997 per 10% increase in duration). Benefits were unequal across outcomes and characteristics of cantons and mothers.

Conclusion Smoking bans resulted in improved birth outcomes in Switzerland with cantons that adopted more comprehensive smoking bans achieving greater benefits. Early-term births constitute a previously ignored though important group.