Maternal tobacco use and extremely premature birth - a population-based cohort study

Objective To study the associations of maternal tobacco use (smoking or use of snuff) and risk of extremely preterm birth, and if tobacco cessation before antenatal booking influences this risk. To study the association between tobacco use and spontaneous or medically indicated onset of delivery. Design Population-based cohort study. Setting Sweden. Population All live singleton births, registered in the Swedish Medical Birth Register, 1999-2012. Methods Odds ratios (OR) with 95% confidence intervals (CI) were calculated using multiple logistic regression analysis. Main outcome measures Extremely preterm birth (<28 weeks of gestation), very preterm birth (28-31 weeks), moderately preterm birth (32-36 weeks). Results Maternal snuff use (OR 1.58; 95% CI: 1.14-2.21) and smoking (OR 1.61; 95% CI: 1.39-1.87 and OR 1.91; 95% CI: 1.53-2.39 for moderate and heavy smoking, respectively) were associated with an increased risk of extremely preterm birth. When cessation of tobacco use was obtained there was no increased risk of preterm birth. Snuff use was associated with a twofold risk increase of medically indicated extremely preterm birth, whereas smoking was associated with increased risks of both medically indicated and spontaneous extremely preterm birth. Conclusions Snuff use and smoking in pregnancy were associated with increased risks of extremely preterm birth. Women who stopped using tobacco before the antenatal booking had no increased risk. These findings indicate that nicotine, the common substance in cigarettes and snuff, is involved in the mechanisms behind preterm birth. The use of nicotine should be minimized in pregnancy.