Associations between Passive Maternal Smoking during Pregnancy and Preterm Birth: Evidence from a Meta-Analysis of Observational Studies

Previous studies investigating the relationship between passive maternal smoking and preterm birth reveal inconsistent results. We conducted the current meta-analysis of observational studies to evaluate the relationship between passive maternal smoking and preterm birth. We identified relevant studies by searching PubMed, EMBASE, and ISI Web of Science databases. We used random-effects models to estimate summary odds ratios (SORs) and 95% confidence intervals (CIs) for aforementioned association. For the analysis, we included 24 studies that involved a total of 5607 women who experienced preterm birth. Overall, the SORs of preterm birth for women who were ever exposed to passive smoking versus women who had never been exposed to passive smoking at any place and at home were 1.20 (95% CI = 1.07-1.34, I-2 = 36.1%) and 1.16 (95% CI = 1.04-1.30, I-2 = 4.4%), respectively. When we conducted a stratified analysis according to study design, the risk estimate was slightly weaker in cohort studies (SOR = 1.10, 95% CI = 1.00-1.21, n = 16) than in cross-sectional studies (SOR = 1.47, 95% CI = 1.23-1.74, n = 5). Additionally, the associations between passive maternal smoking and preterm birth were statistically significant for studies conducted in Asia (SOR = 1.26, 95% CI = 1.05-1.52), for studies including more than 100 cases of preterm birth (SOR = 1.22, 95% CI = 1.05-1.41), and for studies adjusted for maternal age (SOR = 1.27,95% CI = 1.09-1.47), socioeconomic status and/or education (SOR = 1.28, 95% CI = 1.10-1.49), body mass index (SOR = 1.33, 95% CI = 1.04-1.71), and parity (SOR = 1.27, 95% CI = 1.13-1.43). Our findings demonstrate that passive maternal smoking is associated with an increased risk of preterm birth. Future prospective cohort studies are warranted to provide more detailed results stratified by passive maternal smoking during different trimesters of pregnancy and by different types and causes of preterm birth.