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