Maternal ambient air pollution exposure and risk of stillbirth in Georgia, USA

Evidence suggests maternal exposure to ambient air pollution increases the risk of stillbirth, but few studies conducted in the United States have evaluated temporally varying exposures or susceptibility across gestational windows. Moreover, the generalizability of existing findings is often limited by restricted geographic coverage or reliance on selected study populations.

Using Georgia vital records from 2005 to 2014, we conducted a matched case-control study including 8,384 stillbirths and 33,459 live birth controls matched on maternal county of residence and conception month. We used stratified Cox proportional hazards models with time-varying covariates to estimate hazard ratios (HRs) for 10 air pollutants across five exposure windows (first month, weekly, and first, second, and third trimester). Our primary analysis included all stillbirths combined, with subgroup analyses separating second- and third trimester losses.

Stillbirths had a median gestational age of 27 weeks (interquartile range [IQR], 6.67) compared with 38 weeks for live births (IQR, 2.13). Particulate matter showed strong associations in the second trimester exposure window for all stillbirths (PM₁₀: HR = 1.07; 95% confidence interval [CI] = 1.04, 1.11; PM_2.5: HR = 1.05; 95% CI = 1.01, 1.09). This pattern was consistent for NO₂ and NH₄, which also exhibited positive associations across early and entire pregnancy exposure windows (first month, first trimester, and weekly), with the strongest associations for the second trimester exposures. Associations were larger for second-trimester stillbirths, whereas estimates for third-trimester stillbirths were largely null or negative.

In this population-based study in Georgia, time-varying ambient air pollution exposures during pregnancy were associated with an increased risk of stillbirth, particularly for second-trimester exposures and for stillbirths occurring earlier in pregnancy. These findings highlight the importance of considering gestational timing when evaluating environmental risk factors for stillbirth.