Heat Exposure During Pregnancy Linked to Autism Risk in New Study

Higher nighttime temperatures during pregnancy may be associated with an increased risk of autism diagnosis in children, according to a new study led by researchers at Tulane University. The research, published in the journal Science of The Total Environment, examined nearly 295,000 mother-child pairs in Southern California from 2001 to 2014.

Elevated Heat Tied to Autism Risk

The study found that warmer overnight temperatures were linked to increased autism risk during two key periods of pregnancy: early pregnancy (weeks 1-10) and late pregnancy (weeks 30-37). Children of mothers exposed to higher than typical nighttime temperatures during the first 10 weeks of pregnancy had a 15% higher risk of being diagnosed with autism. Exposure during weeks 30-37 was associated with a 13% higher risk.

Researchers classified extreme temperatures as those above the 90th percentile (3.6°F hotter than average) and 99th percentile (5.6°F higher than average). These increases resulted in a “significant” association between hotter-than-average nights experienced by pregnant women and autism diagnoses in their children.

Rising Nighttime Heat in California

The findings add to a growing body of research exploring how environmental factors—including air pollution and wildfire smoke—may influence fetal neurodevelopment. This study is the first to specifically examine the impact of temperature on this development as global temperatures rise. According to California’s Office of Environmental Health Hazard Association, nighttime temperatures in California have risen three times faster than daytime temperatures. Nine of the state’s ten hottest years on record have occurred since 2014.

What Researchers Measured and Found

“Heat waves are becoming more frequent, and people may only think of the dangers of daytime heat exposure,” said Mostafijur Rahman, assistant professor of environmental health sciences at the Celia Scott Weatherhead School of Public Health and Tropical Medicine at Tulane University. “These results indicate a strong association between high nighttime temperatures during pregnancy and autism risk in children and show that we need to think about exposure to heat around the clock.”

The study compared weekly outdoor temperature estimates at residential addresses with autism outcomes in children. The association remained even after researchers accounted for factors such as neighborhood conditions, vegetation, and fine-particle air pollution. The study was unable to account for access to air conditioning.

Interestingly, researchers did not find the same association with daytime temperatures. This may be because people spend more time away from home during the day, making it more difficult to accurately measure actual heat exposure.

Possible Explanations and Next Steps

“A key takeaway is that we identified specific windows when a mother and her developing child can be most affected by exposures to higher nighttime temperatures,” said lead author David Luglio, a post-doctoral fellow with the Celia Scott Weatherhead School of Public Health and Tropical Medicine. “This is critical and hopefully can help mothers prepare accordingly.”

The study did not directly examine how higher temperatures at night could impact prenatal neurodevelopment. Luglio suggested that higher nighttime temperatures may disrupt sleep for pregnant mothers. A previous study has linked insufficient sleep during pregnancy with a higher risk of neurocognitive delays in children.

“Extreme heat exposure during pregnancy has been linked to a range of adverse health outcomes, including prenatal neurodevelopmental delays and complications with an embryo’s development of a central nervous system,” Luglio said. “The goal of our study was to specifically explore the link between prenatal heat exposure and autism diagnoses for the first time.”

Further research is needed to understand the biological mechanisms linking nighttime heat exposure to autism risk. This study highlights the importance of considering environmental factors, alongside genetic predispositions, in understanding the complex causes of autism spectrum disorder.