Association between environmental tobacco smoke before and during pregnancy and the risk of adverse birth outcomes: a birth cohort study in Wuhan, China.

Associations between environmental tobacco smoke (ETS) and the risk of adverse birth outcomes may be confounded. We aimed to clarify the associations of ETS before and during pregnancy with the risk of adverse birth outcomes and determine whether the associations were modified by key potential confounders. A total of 7147 non-smoking mothers from the Tongji Maternal and Child Health Cohort (TMCHC) in Wuhan, China, were included in the final analyses. ETS status was self-reported by pregnant women at their first antenatal care visit before 16 gestational weeks. Information on birth outcomes was extracted from delivery records. Poisson regression was used to identify the association between ETS before and during pregnancy and the risk of adverse birth outcomes. Exposures to ETS before and during pregnancy were reported by 645 (9.0%) and 464 (6.5%) mothers, respectively. Compared with no ETS, continued ETS during pregnancy was associated with an increased risk of preterm birth (PTB) (RR: 1.55, 95% CI: 1.07, 2.25) after adjustment for potential confounders. The association remained consistent in medically indicated PTB and late PTB. The risk of PTB associated with continued ETS during pregnancy was significantly increased in mothers more educated (P for interaction < 0.05). However, the increased risk of low birth weight (LBW) or small for gestational age (SGA) births by ETS during pregnancy was not observed. Exposure to ETS during pregnancy was associated with a higher risk of PTB, but not LBW or SGA births in a Chinese birth cohort.