Association between ambient air pollution and pregnancy complications: A systematic review and meta-analysis of cohort studies

Background: Pregnancy complications, such as gestational diabetes mellitus (GDM) and hypertensive disorders of pregnancy (HDP), have a great impact on public health. Exposure to ambient air pollution during pregnancy may cause pregnancy complications. The aim of our study is to explore the risk of trimester-specific maternal exposure to air pollutants on complications of pregnancy. Methods: PubMed, EMBASE, Web of Science, and Cochrane were systematically searched for cohort studies published before October 27, 2019 which reported the association between ambient air pollutants (PM2.5, PM10, CO, NO, NO2, NOx, O-3, and SO2) and pregnancy complications (GDM, HDP, preeclampsia, and gestational hypertension) during different exposure windows. A meta-analysis was applied to combine relative risks (RRs) and their confidence intervals (CIs) from eligible studies. Quality assessment was conducted and Egger test was used to evaluate the publication bias. All statistical analyses were performed by STATA software (Version 15, StataCorp, College Station, Texas, USA). Results: This meta-analysis consisted of 33 cohort studies conducted on 22,253,277 pregnant women. Meta-analyses showed during the first trimester, there were significant associations of PM10 with gestational hypertension (RR = 1.07, 95% CI: 1.02-1.12 per 10 mu g/m(3), I-2 = 0.0%), of SO2 with GDM (RR = 1.04, 95% CI: 1.00-1.08 per 1 ppb increment, I-2 = 54.1%), of PM2.5 with preeclampsia (RR = 0.97, 95% CI: 0.95-1.00 per 5 mu g/m(3), I-2 = 4.1%). During the entire pregnancy, PM2.5 significantly increased the risk of hypertensive disorders of pregnancy (RR = 1.18, 95% CI: 1.02-1.34 per 5 mu g/m(3), I-2 = 85.1%). Egger test indicated that wide-scale publication bias was unlikely. Conclusion: Maternal exposure to ambient air pollutants is associated with pregnancy complications especially during the first trimester. Further large multicenter cohort studies considering different constituents of pollutants, levels of disease severity, sensitive populations, and various exposure windows are warranted in the future research.