BackgroundThe public health impact of tropical cyclones (TCs) is expected to increase due to the continued growth of coastal populations and the increasing severity of these events. We aimed to estimate the association between prenatal exposure to TC and risk of preterm birth (PTB) in the eastern United States (US).MethodsWe included data on 19,529,748 spontaneous singleton births from 1989 to 2002 across 378 US counties. In each county, we classified days as exposed to a TC when TC-associated peak sustained winds at the county’s population-weighted center were >17.2 m/s (gale-force winds or greater). We used distributed lag log-linear mixed-effects models to estimate the relative risk (RR) and absolute risk difference (ARD) for TC exposure by comparing PTBs occurring in TC-periods (from 2 days before to 30 days after) to matched non-TC periods. We conducted secondary analyses using different exposure metrics and thresholds.Results During the study period, there were 1,981,797 (10.1%) PTBs and 58 TCs that affected at least one studied US county. The risk of PTB was positively associated with TC exposure defined as peak sustained wind speed >17.2 m/s [RR: 1.01 (95% CI: 0.99, 1.03); ARD: 9 (95% CI: -7, 25) per 10,000 pregnancies], distance to storm track <60km [RR: 1.02 (95% CI: 1.01, 1.04); ARD: 23 (95% CI: 9, 38) per 10,000 pregnancies], and cumulative rainfall >100mm [RR: 1.04 (95% CI: 1.02, 1.06); ARD: 36 (95% CI: 16, 56) per 10,000 pregnancies]. Results were comparable when considering other wind, distance, or rain thresholds. The association was more pronounced among early PTBs and mothers living in more socially vulnerable counties but did not vary across strata of other hypothesized risk factors.ConclusionsPrenatal exposure to TC was associated with a higher risk of PTB. Our findings provide initial evidence that severe storms may trigger PTB.1