Opportunities and challenges in studying air pollution and pregnancy loss

Abstract

BACKGROUND AND AIM: Pregnancy loss is common (spontaneous abortion/miscarriage: 30%, stillbirth: 1%) but its cause is largely unknown. Air pollution has a wide range of reproductive health effects but its effects on pregnancy loss is unclear. We summarize current epidemiologic research and highlight key challenges and opportunities for future research on this important but understudied topic. METHODS: A comprehensive literature review of original epidemiologic studies (before March 2021) on ambient air pollutants and pregnancy loss at all gestational age (i.e., miscarriage, spontaneous abortion, stillbirth) was performed. We excluded studies that a) did not focus on specific outdoor pollutants (i.e., indoor smoke) or b) assessed occupational exposures. RESULTS:Thirty-five (n=35) epidemiologic studies were identified, most of which were conducted in the US (35%), China (17%), and Iran (11%); and estimated air pollution exposures using fixed air stations near maternal residence (66%). The majority of the studies evaluated stillbirths ≥20 weeks (n=19). Pollutants including SO2, PM10, and PM2.5, CO, NO2, and O3 are positively associated with both spontaneous abortion/miscarriage and stillbirth. Critical windows of exposure are unclear but both acute (prior week) and chronic exposures (average over the gestational length and trimester specific) have been suggested. Major challenges include a) lack of personal exposure or data on residential mobility/daily activities, b) exclusion of early losses in retrospective data, c) small clinical samples seeking infertility treatment, d) inconsistent definition of the outcome, e) lack of information on time of event, and f) confounding. CONCLUSIONS:There is growing evidence suggesting that exposures to ambient air pollution, even in areas with moderate concentrations, may increase the risk of pregnancy loss. While awaiting larger preconception studies to address current challenges and further understand mechanisms of effects, it is prudent to minimize exposures among pregnant women and those trying to conceive. KEYWORDS: air pollution, pregnancy loss, pregnancy outcomes, stillbirth, spontaneous abortion