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Background We conducted the first umbrella review of meta-analyses on the association between in utero exposure to ambient air pollution and adverse birth outcomes. Methods We systematically searched for meta-analyses on criteria air pollutants (NO2, CO, O3, SO2, and PM2.5 and PM10) and adverse birth outcomes (preterm birth, stillbirth, reduced birth weight, low birth weight, and small-for-gestational age) from PubMed, CINAHL, Scopus, MEDLINE/Ovid, EMBASE/Ovid, Web of Science, systematic reviews repositories, electronic grey literature, and references on 21st September 2020 with weekly alerts thereafter. We graded the overall direction and confidence of cause-and-effect as per our pre-specified protocol (Doi:10.3390/ijerph17228658). Results Sixteen (16) meta-analyses, pooling 192 unique primary studies with moderate (9.0%) overlap qualified for the final synthesis. Consistent positive associations were reported between whole pregnancy exposure to PM2.5/PM10 and birth weight reduction and between CO/SO2 and low birth weight and were graded as strong ( ++). There were less consistent associations between whole pregnancy PM2.5/NO2 exposures with PTB, and for all criteria pollutants with stillbirth, hence graded moderate (+). Evidence for associations with trimester specific exposures were also moderate (+). However, meta-analyses observed high heterogeneity, high imprecision, and lacked experimental studies. Consequently, taken together, the current observations indicate ‘probable evidence’ of causation. Conclusions Current observations indicate ‘probable evidence’ of causation. Further studies with standardised designs would help elucidate reasons for heterogeneity of associations. Key messages In the absence of randomised controlled trials, the strong observational evidence for associations between ambient air pollution and adverse birth outcomes warrants adoption of the precautionary principle.

1371Ambient air pollution and adverse birth outcomes: a systematic synthesis of meta-analyses of epidemiological studies