The relationship between extreme ambient temperature and small for gestational age: A cohort study of 1,436,480 singleton term births in China

Studies have shown that exposure to extreme ambient temperature can contribute to adverse pregnancy outcomes, however, results across studies have been inconsistent. We aimed to evaluate the relationships between trimester-specific extreme temperature exposures and fetal growth restriction indicated by small for gestational age (SGA) in term pregnancies, and to assess whether and to what extent this relationship varies between different geographic regions. We linked 1,436,480 singleton term newborns (2014-2016) in Hubei Province, China, with a sub-district-level temperature exposures estimated by a generalized additive spatio-temporal model. Mixed-effects logistic regression models were employed to estimate the effects of extreme cold (temperature & LE;5th percentile) and heat exposures (temperature >95th percentile) on term SGA in three different geographic regions, while adjusting for the effects of maternal age, infant sex, the frequency of health checks, parity, educational level, season of birth, area-level income, and PM2.5 exposure. We also stratified our analyses by infant sex, maternal age, urban-rural type, income categories and PM2.5 exposure for robustness analyses. We found that both cold (OR:1.32, 95% CI: 1.25-1.39) and heat (OR:1.17, 95% CI: 1.13-1.22) exposures during the third trimester significantly increased the risk of SGA in the East region. Only extreme heat exposure (OR:1.29, 95% CI: 1.21-1.37) during the third trimester was significantly related to SGA in the Middle region. Our findings suggest that extreme ambient temperature exposure during pregnancy can lead to fetal growth restriction. Governments and public health institutions should pay more attention to environmental stresses during gestation, especially in the late stage of the pregnancy.