Prenatal ambient air pollution exposure and the risk of stillbirth: systematic review and meta-analysis of the empirical evidence

Background Individual studies on the relations between ambient air pollution and the risk of stillbirth have provided contradictory results. We conducted a systematic review and meta-analysis to summarise the existing evidence. Methods We conducted a systematic search of three databases: PubMed, Scopus and Web of Science, from their time of inception to mid-April, 2015. Original studies of any epidemiological design were included. Data from eligible studies were extracted by two investigators. To calculate the summary effect estimates (EE), the random effects model was used with their corresponding 95% CI. Results 13 studies met the inclusion criteria. Although not reaching statistical significance, all the summary effect estimates for the risk of stillbirth were systematically elevated in relation to mean prenatal exposure to NO2 per 10ppb (EE=1.066, 95% CI 0.965 to 1.178, n=3), CO per 0.4ppm (EE=1.025, 95% CI 0.985 to 1.066, n=3), SO2 per 3ppb (EE=1.022, 95% CI 0.984 to 1.062, n=3,), PM2.5 per 4g/m(3) (EE=1.021, 95% CI 0.996 to 1.046, n=2) and PM10 per 10g/m(3) (EE=1.014, 95% CI 0.948 to 1.085, n=2). The effect estimates for SO2, CO, PM10 and O-3 were highest for the third trimester exposure. Two time series studies used a lag term of not more than 6days preceding stillbirth, and both found increased effect estimates for some pollutants. Conclusions The body of evidence suggests that exposure to ambient air pollution increases the risk of stillbirth. Further studies are needed to strengthen the evidence.