Patterns of quitting smoking during pregnancy and subtypes of preterm birth

Background: Although stopping smoking reduces the risk of being born preterm, the effect of smoking cessation in different trimesters of pregnancy on subtypes of preterm birth has not been well studied. Methods: We analyzed 2010 natality data from U.S. resident, singleton births from 24-41 weeks' gestation in 33 states and the District of Columbia, based on the 2003 revised birth certificate (n=2,815,243). Preterm births were categorized as spontaneous preterm, preterm premature rupture of membranes (PPROM), and medically indicated. Self-reported maternal smoking status was grouped as “continuous smoker”, “after 2nd trimester quitter”, “after 1st trimester quitter” and “non-smoker” (referent). We generated odds ratios for the relationship between smoking status and preterm birth, using multinomial logistic regression, adjusting for socio-demographic and clinical risk factors. Results: Of the pregnant smokers [7% (n=208,162)], 1.1% quit after the 1st trimester, 0.39% quit after the 2nd trimester, and 5.89% smoked continuously. Smokers had increased odds of all subtypes of preterm birth ranging from a 12% increase in medically indicated preterm births among the “after 1st trimester quitter” (aOR [95% confidence interval]= 1.12 [1.06-1.18]) to 67% increase in PPROM among the “continuous smoker” (aOR 1.67 [1.58-1.77]). The odds of PPROM and medically indicated preterm birth among smokers were lowest in “after 1st trimester quitter”, intermediate in “after 2nd trimester quitter” and highest among “continuous smoker”. Conclusion: Smoking cessation should be encouraged even after the 1st and 2nd trimesters because the odds of preterm birth of all clinical subtypes are decreased in quitters compared to continuous smokers.