Association Between Proximity to Animal Agriculture During Pregnancy and Spontaneous Preterm Birth Among Nulliparous Women in a California Cohort

BACKGROUND: Environmental exposures during pregnancy may influence risk of spontaneous preterm birth, a serious adverse outcome. Concentrated Animal Feeding Operations (CAFOs) near maternal residences may emit pollutants associated with preterm birth. METHODS: We analyzed n = 811,953 births to nulliparous women in California (2007-2011), including 7487 spontaneous very preterm (20-31 weeks) and 37,026 spontaneous moderate preterm (32-36 weeks) births. Locations of n = 2121 CAFOs were obtained from a near-census of facilities. Using time-to-event and Bayesian methods, we estimated hazard and odds of preterm birth as a function of distance, density, and wind-based CAFO exposure, adjusting for maternal, temporal, socioeconomic, and environmental covariates. Bayesian models allow inference on both the distance range of association and the association strength. RESULTS: Greater exposure to CAFOs is associated with small increases in hazard of all preterm birth (adjusted hazard ratios, per-unit exposure change - inverse distance: 1.003 [95% confidence interval [CI] = 1.000, 1.005]; downwind-weighted: 1.020 [95% CI = 1.005, 1.035]) with stronger associations for very preterm birth (adjusted hazard ratios - inverse distance: 1.007 [95% CI = 1.002, 1.012]; downwind-weighted: 1.043 [95% CI = 1.008, 1.080]). Among exposed births, less downwind exposure is associated with lower preterm birth hazard compared with being predominantly downwind. Bayesian models suggest births with at least one CAFO within 25.23 km (95% highest density interval [HDI]: 18.21, 32.74 km) have 1.16 times the odds of being very preterm (95% HDI: 1.10, 1.22), with similar distance ranges but smaller odds for moderate preterm birth (adjusted odds ratio: 1.08 [95% HDI: 1.05, 1.10]). CONCLUSIONS: Living near a CAFO may increase risk of spontaneous preterm birth in California, with stronger and more consistent associations for very preterm births, particularly when combined with greater downwind exposure.