Zhongzheng Niu

Background Although many studies suggested the benefit of smoking cessation among pregnant women in reducing the risk of preterm birth (PTB), the timing of the effect of the cessation remains inconclusive. Objectives To examine the association of trimester‐specific smoking cessation behaviours with PTB risk. Methods We included 199,453 live births in Western New York between 2004 and 2018. Based on self‐reported cigarette smoking during preconception and in each trimester, we created six mutually exclusive groups: non‐smokers, quitters in each trimester, those who smoked throughout pregnancy, and inconsistent smokers. Risk ratios (RRs) and 95% confidence intervals (CIs) were estimated using Poisson regression to examine the association between smoking cessation and PTB. Effect modification by illegal drug use, maternal age, race and ethnicity and pre‐pregnancy body mass index (BMI) was investigated multiplicatively by ratio of relative risk and additively by relative excess risk due to interaction (RERI). Results Overall, 6.7% of women had a PTB; 14.1% smoked throughout pregnancy and 3.4%, 1.8% and 0.8% reported quitting smoking during the first, second and third trimesters, respectively. Compared to non‐smokers, third‐trimester cessation (RR 1.20, 95% CI 1.01, 1.43) and smoking throughout pregnancy (RR 1.27, 95% CI 1.21, 1.33) were associated with a higher PTB risk, while quitting smoking during the first or second trimester, or inconsistent smoking was not associated with PTB. A positive additive interaction was identified for maternal age and late smoking cessation or smoking throughout pregnancy on PTB risk (RERI 0.17, 95% CI 0.00, 0.36), and a negative interaction was observed for pre‐pregnancy BMI ≥30 kg/m 2 (ratio of relative risk 0.70, 95% CI 0.63, 0.78; RERI −0.42, 95% CI −0.56, −0.30). Conclusion Compared to non‐smokers, smoking throughout pregnancy and third‐trimester smoking cessation are associated with an increased risk of PTB, while quitting before the third trimester may not increase PTB risk.