title: Prenatal ambient air pollution exposure and the risk of stillbirth: systematic review and meta-analysis of the empirical evidence.
abstract: 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 10 ppb (EE=1.066, 95% CI 0.965 to 1.178, n=3), CO per 0.4 ppm (EE=1.025, 95% CI 0.985 to 1.066, n=3), SO2 per 3 ppb (EE=1.022, 95% CI 0.984 to 1.062, n=3,), PM2.5 per 4 μg/m(3) (EE=1.021, 95% CI 0.996 to 1.046, n=2) and PM10 per 10 μg/m(3) (EE=1.014, 95% CI 0.948 to 1.085, n=2). The effect estimates for SO2, CO, PM10 and O3 were highest for the third trimester exposure. Two time series studies used a lag term of not more than 6 days 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.
authors.affiliation: Faculty of Medicine, Center for Environmental and Respiratory Health Research, University of Oulu, Oulu, Finland.
authors.forename: Nazeeba
authors.initials: N
authors.lastname: Siddika
authors.affiliation: Faculty of Medicine, Center for Environmental and Respiratory Health Research, University of Oulu, Oulu, Finland.
authors.forename: Hamudat A
authors.initials: HA
authors.lastname: Balogun
authors.affiliation: Faculty of Medicine, Center for Environmental and Respiratory Health Research, University of Oulu, Oulu, Finland Department of Biomedical and Forensic Sciences, University of Cape Coast, Cape Coast, Ghana.
authors.forename: Adeladza K
authors.initials: AK
authors.lastname: Amegah
authors.affiliation: Faculty of Medicine, Center for Environmental and Respiratory Health Research, University of Oulu, Oulu, Finland Medical Research Center, University of Oulu and Oulu University Hospital, Oulu, Finland.
authors.forename: Jouni J K
authors.initials: JJ
authors.lastname: Jaakkola
mesh_terms: D000393:Air Pollutants / Q000009:adverse effects*; D000397:Air Pollution / Q000009:adverse effects*; D002248:Carbon Monoxide / Q000009:adverse effects; D004784:Environmental Monitoring; D005260:Female; D006801:Humans; D018811:Maternal Exposure / Q000009:adverse effects*; D009585:Nitrogen Dioxide / Q000009:adverse effects; D010126:Ozone / Q000009:adverse effects; D052638:Particulate Matter / Q000009:adverse effects; D011247:Pregnancy; D011263:Pregnancy Trimester, Third; D012307:Risk Factors; D050497:Stillbirth / Q000453:epidemiology*; D013458:Sulfur Dioxide / Q000009:adverse effects
chemical_list: D000393:Air Pollutants; D052638:Particulate Matter; D013458:Sulfur Dioxide; D010126:Ozone; D002248:Carbon Monoxide; D009585:Nitrogen Dioxide