Ambient air pollution and the sex ratio at birth: a systematic review and narrative synthesis

Ambient air pollution is associated with adverse pregnancy outcomes such as stillbirth, miscarriage, and preterm birth. However, its effects on the viability, implantation, and survival of conceptuses--largely unobserved before the clinical detection of pregnancy-- remain poorly understood. Shifts in the sex ratio at birth (SRB) provide a sensitive, population-level indicator of sex-biased conception and pregnancy loss. This systematic review examined whether maternal exposure to ambient air pollution before conception and during pregnancy is associated with deviations in the SRB. We searched MEDLINE, Embase, and Global Health from inception to 12 February 2025 for observational studies assessing both ambient air pollution exposure and SRB. Eleven studies met the inclusion criteria, representing data from eight countries spanning 1970-2023. PM10 was the most frequently investigated pollutant, followed by PM2.5. Six studies reported no significant association, while four observed increased feminisation with higher exposure levels. Air pollution may plausibly influence SRB through effects on gamete quality, conceptus viability, and placental development. The SRB therefore provides a valuable proxy for detecting early biological vulnerability, revealing effects of air pollution that are otherwise hidden due to the selective observation of clinically recognised pregnancies. However, current evidence remains limited, heterogeneous, and inconclusive. As ambient air quality continues to deteriorate, more standardised research is needed to improve comparability and quantify the impact of air pollution on reproductive outcomes.