Impact of maternal smoking and secondhand smoke exposure during singleton pregnancy on placental abruption: Analysis of a prospective cohort study (the Japan Environment and Children's Study)

Objectives This study aimed to investigate the association and population-attributable fraction (PAF) of maternal smoking and secondhand smoke (SHS) exposure during pregnancy with placental abruption. Design Prospective cohort study. Setting 15 regional centres in Japan. Participants Pregnant women registered between January 2011 and March 2014 from the Japan Environment and Children's Study. Outcome measures Data were transcribed from medical records and two self-administered questionnaires. The outcome measure was the incidence of placental abruption. Maternal smoking exposure during pregnancy was categorised based on the number of cigarettes smoked (≤10 or ≥11 cigarettes per day). SHS exposure during pregnancy was evaluated by frequency and duration (almost never or never, 1-3 days/week and/or <1 hour/day and 4-7 days/week and ≥1 hour/day). A modified Poisson regression model, adjusted for known placental abruption risk factors, calculated the risk ratio (RR) and PAF for placental abruption with a 95% Cl. Results Of the 81 974 eligible pregnant women, pregnant women smoking ≥11 cigarettes/day during pregnancy had a significantly higher risk of placental abruption. The adjusted RR (aRR) was 2.21 (95% CI 1.21 to 4.06), and the adjusted PAF (aPAF) was 1.90% (95% CI 0.09 to 3.71%). Pregnant women among never-smokers with SHS exposure of 4-7 days/week and ≥1 hour/day had a significantly higher risk (aRR: 2.34, 95% CI 1.29 to 4.28), and the aPAF was 1.89% (95% CI -0.05 to 3.83). Additionally, pregnant women among those who smoked during pregnancy with similar SHS exposure had a significantly higher risk (aRR: 2.21, 95% CI 1.30 to 3.76), with the aPAF of 2.29% (95% CI 0.11 to 4.48). Conclusions Maternal smoking and SHS exposure during pregnancy significantly contribute to the risk of placental abruption in Japan. Therefore, preventive interventions and measures to reduce exposure are required to prevent placental abruption.