Associations between heat wave during pregnancy and term birth weight outcomes: The PARIS birth cohort.

Background: Climate change will make extreme weather events more frequent in the 21st century. Extreme ambient temperatures during the prenatal period have been associated with adverse pregnancy outcomes such as preterm birth. It is unclear, however, whether heat waves during pregnancy impact fetal growth in apparently healthy term newborns.

Objectives: We aimed to investigate associations between heat wave during pregnancy and birth weight outcomes in term newborns from the PARIS birth cohort, and to explore meteorological conditions and air pollution as possible intermediate factors.

Methods: We examined data on 3,359 newborns born between 37 and 42 weeks in Paris, France, between 2003 and 2006. Associations of maternal exposure to heat wave (during whole pregnancy and each trimester) with birth weight and small for gestational age (SGA) at term were studied using linear and logistic regression models adjusted for potential confounders. Maternal characteristics were investigated as possible modifiers. We explored the mediating role of ambient temperature, relative humidity, and air pollution levels in the relationship between heat wave during the first trimester and term SGA.

Results: Mothers who were pregnant during the 2003 French heat wave (n = 506, 15 %) were more likely to have a term SGA baby (aOR = 2.70; 95 %CI: 1.38, 5.28) compared to mothers who did not experience heat wave during pregnancy. The association was stronger when heat wave occurred during the first trimester (aOR = 4.18; 95 %CI: 1.69, 10.35). Primiparous women were identified as more vulnerable than multiparous women. Average ambient temperature and air quality index explained about 36 % and 56 % of the association between heat wave during the first trimester and term SGA, respectively.

Conclusions: This study suggests prenatal exposure to heat wave, especially during the first trimester, may adversely affect fetal growth of term newborns, which could be explained by both increasing ambient temperatures and worsening air quality.