Ambient Temperature and Markers of Fetal Growth: A Retrospective Observational Study of 29 Million U.S. Singleton Births

Abstract

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 (>90th percentile) during the entire pregnancy were associated with higher risk of term SGA {odds ratio [OR] = 1.041 [95% confidence interval (CI): 1.029, 1.054]} and lower term birth weight [standardized to −15g (95% CI: −17g, −13g) reduction in birth weight for infants born at 40 weeks of gestation]. Low temperatures (≤10th percentile) during the entire pregnancy were not associated with SGA [OR = 1.003 (95% CI: 0.991, 1.015)] but were associated with a small decrement in term birth weight [standardized to −6g (95% CI: −8g, −4g)]. 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

Taking the Heat: Potential Fetal Health Effects of Hot Temperatures

“…In a second study using this data set, the researchers looked at the relationship between average ambient temperatures across the entire pregnancy and markers of fetal growth 9. They concluded that among full-term babies, those whose mothers were exposed to outdoor temperatures above the 90th percentile, averaged across the pregnancy, tended to have a slightly lower birth weight—equivalent to a mean 15g (about 0.5 oz) 17. Similar associations were reported for the second and third trimesters.…”

“…However, the work continues. Most recently, in 2019, researchers at the Brown University School of Public Health published two reports based on data from the National Center for Health Statistics on approximately 30 million singleton births across 403 U.S. counties 9 , 17. In their study of preterm birth,9 the researchers defined “extreme heat” as temperatures above the 95th percentile of each county-specific temperature distribution.…”

Associations between high temperatures in pregnancy and risk of preterm birth, low birth weight, and stillbirths: systematic review and meta-analysis

“…The review was not designed to assess associations between low temperature and birth outcomes, though these were documented in several studies, most of which reported U shaped associations between temperature and outcomes. 46 51 61 62 64 79 86 87 Links between exposures to cold and other outcomes, such as mortality, are well documented. Little attention is paid to these links when climate change is discussed, even though the changes in climate which are occurring encompass both increases in temperatures and periods of low temperature extremes.…”

Post-conception heat exposure increases clinically unobserved pregnancy losses

“…Estimations of the impacts of temperature exposure during early pregnancy should be especially affected. Indeed, the most reliable estimates show that the effect of first trimester exposure to a hot temperature on birth weight is much weaker than the effect of exposure to a hot temperature during the second and third trimesters 45 – 47 , which is consistent with a selection-induced upward bias, although other factors may also play a role. More importantly, our empirical approach offers a tool that can be used for the estimation of a corrected scarring effect.…”