Scientists from Inserm and the University of Grenoble Alpes, in collaboration with Santé publique France, studied nearly 21,000 pregnant women to examine the impact of heat—combined with air pollution, proximity to green spaces, and socioeconomic factors—on birth weight in France. Their findings, to be published in Environmental Science & Technology, highlight how exposure to heat early in pregnancy, influenced by these other factors, could have a significant impact on newborn weight by increasing the likelihood of low birth weight.

A growing body of scientific research suggests that high temperatures may increase perinatal risks such as low birth weight (less than 2.5 kg), preterm birth (before 37 weeks of gestation), and even neonatal mortality. In 2020, approximately 15% of newborns worldwide had low birth weight.

Studies to date have focused on examining the effect of a single environmental factor (temperature, pollution, etc.) on a child’s birth weight. Thus, the effect of heat had been studied in isolation, without considering the role of air pollution, the presence of green spaces, or social stressors (individual social status and the socioeconomic context of residence).

A research team led by Johanna Lepeule, Inserm Research Director at the Institute for the Advancement of Biosciences (Inserm/CNRS/Université Grenoble Alpes), and Mathilde Pascal, project manager at Santé publique France, sought to identify specific periods of fetal growth during which maternal exposure to heat could have a significant effect on the child’s future birth weight. The study also aimed to explore how air pollution (PM2.5 fine particles, nitrogen dioxide, and ozone), vegetation density, and social stressors might themselves modulate the effects of heat on birth weight.

Using data from four French mother-child cohorts led or co-led by Inserm¹, the researchers studied the exposure to these factors among 21,000 pregnant participants, recruited between 2002 and 2017 and followed throughout their pregnancies, and its impact on the babies’ birth weight.

The results highlight critical periods of pregnancy during which exposure to moderate, severe, or extreme heat had a significant effect on children’s birth weight. Thus, heat exposure during the first two trimesters of pregnancy was significantly associated with a reduced birth weight of -40 to -200 g. Conversely, heat exposure toward the end of pregnancy (weeks 32 to 35) was associated with an increase of approximately 60 g in birth weight.

“Our results support existing studies showing a potential role of heat exposure in the occurrence of low birth weight,” comments Lucie Adélaïde, an epidemiologist at Inserm and co-first author of this study. “Studying these links between heat and birth weight is particularly important in the current context of climate change, as studies show that low birth weight is a risk factor for complications, even mortality, in newborns, but also for the development of lifelong conditions such as diabetes or hypertension,” she adds.

Finally, when exposure to pollution, the presence of vegetation, and social stressors were taken into account, the study shows that these factors modulate the impact of heat exposure on birth weight. The results thus suggest that these effects may be more pronounced among women living in areas with few green spaces and/or subject to increased social stress.

“This is the first time a study has taken into account the role of these various factors in the effects of heat on birth weight,” notes Maximilien Génard-Walton, a postdoctoral researcher at Inserm and co-first author of the publication. “Our results underscore the importance of implementing targeted measures to protect pregnant women and their unborn children from the very beginning of pregnancy, particularly by greening living environments to reduce heat exposure,” he concludes.