Relationship of trimester-specific smoking patterns and risk of preterm birth.

Background: In 2011, the US national rate of smoking early in pregnancy was 11.5%. Unfortunately, our home state of Ohio had a rate twice as high at 23%. Smoking in pregnancy remains one of the most important modifiable risk factors for pregnancy complications, specifically preterm birth.

Objective: The objective of the study was to quantify the preterm birth risk to various trimester-specific smoking behaviors.

Study Design: The study was a population-based, retrospective cohort study of singleton non-anomalous live births, using Ohio birth records 2006 to 2012. Preterm birth rates were compared between non-smokers and women who smoked in the preconception period only, those who quit smoking after the 1st and 2nd trimesters, and those who smoked throughout pregnancy. Multivariate logistic regression quantified the risk of smoking with cessation at various times in pregnancy and preterm birth risk, adjusted for maternal race, education, age, Medicaid use, marital status, and parity. A stratified analysis was performed on the basis of preterm birth subtype: spontaneous preterm birth versus indicated preterm birth. We also performed an additional analysis stratifying for maternal race using the 2 largest categories of race (non-Hispanic white and non-Hispanic black).

Results: Of the 913,757 birth records analyzed, nearly 25% of the women reported some smoking behavior on the birth certificate data. Of smokers, less than half quit during pregnancy (38.8% vs 61.2% smoked throughout pregnancy). Early quitters had a similar preterm birth rate compared with non-smokers. Women who smoked through the 1st trimester only did not have a significant increase in their overall preterm birth odds ratio <37 weeks; however, it did increase the odds of extreme preterm birth <28 weeks by 20% (adjusted odds ratio, 1.20; 95% confidence interval [CI], 1.02, 1.40). Quitting late in pregnancy resulted in the highest odds ratio increase: 70% for preterm birth <37 weeks (adjusted odds ratio 1.70; CI, 1.60, 1.80), even after adjustment for the confounding influences. Quitting smoking early in pregnancy after the 1st trimester did not increase the overall risk of spontaneous or indicated preterm birth <37 weeks significantly. However, quitting after the 1st trimester was associated with a significant increase in risk of extreme spontaneous preterm birth <28 weeks, an effect not seen with indicated preterm birth <28 weeks. Delaying cessation until late in pregnancy-after the 2nd trimester-was associated with the highest risk increases, 65% increased odds of spontaneous and 78% increase in odds of indicated preterm births. The rate of preterm births to non-Hispanic black mothers was increased in all categories over those of non-Hispanic white mothers. The relative influence of smoking cessation in pregnancy was similar in black compared with white mothers. The effect modification in the regression model was analyzed and revealed no significant interaction between race and smoking patterns on preterm birth risk.

Conclusion: Smoking throughout pregnancy is associated with an increased risk of preterm birth. However, quitting early in pregnancy negates this risk. Widespread programs aimed at smoking cessation early in pregnancy could have a significant impact on reducing the rate of preterm birth nationally.