Air pollution and adverse birth outcomes: A systematic review of fine critical windows

Epidemiological studies have reported associations between prenatal exposure to outdoor air pollution and adverse pregnancy outcomes. However, most analyses assessed exposure over the entire pregnancy or by trimester, potentially missing specific critical windows of vulnerability that recent statistical methods can identify. The objective of this study was to summarise studies examining critical windows of exposure to outdoor air pollution in relation to adverse pregnancy outcomes. This systematic review (PROSPERO-CRD42024523345) followed PRISMA recommendations using PubMed, Embase and CENTRAL databases. The search was performed on April 2024 and udpated on February 2025. Eligible studies analysed critical windows of association between air pollutants (PM2.5, PM10, NO₂, O₃) and the following outcomes: prematurity, low birth weight, intrahepatic cholestasis of pregnancy, gestational diabetes, pre-eclampsia, eclampsia, gestational hypertension and stillbirth. Two independent reviewers screened titles, abstracts and full texts. Results were synthesised narratively. Out of 3631 articles, 20 studies were included. All were conducted in China or the United States and relied almost exclusively on distributed lag models (DLM or DLNM). Heterogeneity was observed across pathologies and between studies addressing the same outcome. The most consistent finding concerned preterm birth, for which the end of the second trimester (weeks 17-28) was often identified as a critical window. No studies addressed intrahepatic cholestasis of pregnancy or stillbirth. Conclusions. The variability in results reflects distinct pathophysiological mechanisms, differences in exposure levels, or individual characteristics. Precisely identifying critical windows of environmental exposure during pregnancy appears essential to improve perinatal environmental prevention and guide public health action.