Background: Emerging studies suggest that ambient temperature during pregnancy may be associated with fetal growth, but the existing evidence is limited and inconsistent.
Objectives: We aimed to evaluate the association of trimester-specific temperature with risk of being born small for gestational age (SGA) and birth weight-markers of fetal growth-among term births in the contiguous United States.
Methods: We included data on 29,597,735 live singleton births between 1989 and 2002 across 403 U.S. counties. We estimated daily county-level population-weighted mean temperature using a spatially refined gridded climate data set. We used logistic regression to estimate the association between trimester-specific temperature and risk of SGA and linear regression to evaluate the association between trimester-specific temperature and term birth weight z-score, adjusting for parity, maternal demographics, smoking or drinking during pregnancy, chronic hypertension, and year and month of conception. We then pooled results overall and by geographic regions and climate zones.
Results: High ambient temperatures ([Formula: see text] percentile) during the entire pregnancy were associated with higher risk of term SGA {odds ratio [OR] [Formula: see text] 1.041 [95% confidence interval (CI): 1.029, 1.054]} and lower term birth weight [standardized to [Formula: see text] (95% CI: [Formula: see text], [Formula: see text]) reduction in birth weight for infants born at 40 weeks of gestation]. Low temperatures ([Formula: see text] percentile) during the entire pregnancy were not associated with SGA [OR [Formula: see text] 1.003 (95% CI: 0.991, 1.015)] but were associated with a small decrement in term birth weight [standardized to [Formula: see text] (95% CI: [Formula: see text], [Formula: see text])]. Risks of term SGA and birth weight were more strongly associated with temperature averaged across the second and third trimesters, in areas the Northeast, and in areas with cold or very cold climates.
Conclusions: Above-average temperatures during pregnancy were associated with lower fetal growth. Our findings provide evidence that temperature may be a novel risk factor for reduced fetal growth. https://doi.org/10.1289/EHP4648.