Introduction: Maternal wildfire exposure has been associated with poor pregnancy outcomes with effects potentially mediated through air pollution and psychosocial stress. We conducted a systematic review that evaluated the scientific evidence on the association between wildfire exposure during pregnancy and the risk of adverse birth outcomes (PROSPERO # 42018094103).
Methods: Comprehensive searches in nine databases were conducted up to July 2019. Observational epidemiological studies that evaluated the relationship between adverse birth outcomes and exposure to wildfire smoke during pregnancy were included in the review. Screening of retrieved articles, data extraction, and methodological quality assessment were performed by two independent reviewers. Data were analyzed descriptively.
Results: Seven studies published from four countries (U.S [n=4], Indonesia [n=1], Australia  and Brazil ) involving 1,255,291 births met the inclusion criteria for the review. Heterogeneity in study populations, timing of exposure (first/second/third trimester), duration of exposure, and exposure ascertainment methods precluded data pooling. In-utero exposure to wildfire smoke was assessed by measurement of wildfire-generated air pollutants (particulate matter 2.5 [n=1], particulate matter 10 [n=1], aerosols [n=2]) and proximity of maternal residence to disaster area (n=3). The methodological quality of primary studies was fair. Maternal wildfire exposure was consistently associated with birth weight reductions (range 3.3 gm -18.0 gm), particularly when mothers were exposed to wildfire smoke in the second and third trimesters. Sex differences in birth weight were also reported with baby boys being heavier than girls and their peers born in non-wildfire areas. Wildfire exposure during late pregnancy was also linked with a slight increase in the incidence of preterm births.
Conclusion: Maternal exposure to wildfire during late pregnancy is linked with birth weight reductions and preterm birth. These findings have implications for disaster management programs and perinatal service providers. Further research is needed to elucidate the biological mechanisms through which wildfires impact pregnancy outcomes.