The dangers of heat during pregnancy

Stefania Papatheodorou

January 12, 2023 – Higher temperatures were linked to impaired fetal growth, according to a study from Stefania Papatheodorou, lecturer on epidemiology at Harvard Chan School, and colleagues. Papatheodorou discusses the implications of the study—and what pregnant people can do to protect themselves during days of extreme heat driven by climate change.        

What were some of the specific findings of your study? Will you be pursuing this line of research?

We studied a large pregnancy cohort from eastern Massachusetts, looking at nearly 9,500 pregnancies. Adjusting for air pollution as well as characteristics of the mother, such as maternal age, race, education, smoking, and socioeconomic status, we found that temperature has a profound effect on fetal growth. Fetal growth is measured by head parameters, abdominal circumference, and femur length. We found that the head parameters, which characterize brain size, were particularly affected by temperature, and more so early in pregnancy. They were more sensitive than the rest of the structures in the body.

We don’t yet know what the clinical importance of these findings are. That needs to be answered. Just because babies are born smaller doesn’t necessarily mean there will be a clinical outcome. However, changes in brain structures have been found to point towards neurodevelopmental outcomes, such as autism.

To see if these heat-related changes in fetal structures result in actual neurodevelopmental outcomes, we are now doing a larger study. We got a large grant to do a national analysis of more than 1.4 million pregnancies between 2000 and 2014, using Medicaid data. This study will be aimed at teasing out the association between climatic factors, air pollution, and neurodevelopmental outcomes. We hope to have results within a year.

How does your recent study fit in with your overall research interests?

My main research is about the intersection between air pollution and climate change. But climate change overall is a very big concern to many scientists across many different fields. For me and my colleagues, climate change translates to changes in factors such as temperature, humidity, precipitation, barometric pressure—anything that can change in terms of climate parameters that we can actually measure. It’s not adequate to just look at the effects of particulate matter air pollution [PM2.5] and other pollutants. We always want to see if negative health effects are truly related to air pollution or to something else that is related to climate change. We always want to parse this out. It’s complicated, but we do have the tools to do it.

Last year we published a study that looked at the effect of PM2.5 on fetal growth. It took temperature into account. That study showed an independent effect of temperature on fetal growth, higher than the effect of PM2.5. That was a surprise. We were expecting to see some effect from heat but not higher than the effect of air pollution. Given that finding, we wanted to highlight the effects of heat in a separate paper, because the impact of heat on fetal development can have huge implications for later life.

What can pregnant people do to avoid the potentially negative impacts of heat on fetal growth?

Pregnant people can avoid spending a lot of time outdoors when it’s extremely hot, and use air conditioning. Clinicians could use the information from our study to make these sorts of recommendations to their patients. But it’s not that simple, because there are disparities. For instance, some people may not have air conditioning, or may not be able to afford to put it on 24/7. Maybe they have to work outside regardless of the weather. Maybe they can’t afford to lose days of work.

Given these realities, and as temperatures are likely to become more extreme, we really need to find solutions now, because they can take years to implement. We need to have policy-level solutions to support people in protecting themselves from climate change.

Karen Feldscher