A large new study suggests that unusually warm nights during pregnancy could be linked to a higher chance of autism diagnosis in children.
Researchers from Tulane University analyzed nearly 295,000 mother–child pairs in Southern California and found that exposure to higher-than-normal nighttime temperatures was associated with increased autism risk during two specific stages of pregnancy.
The study, published in Science of The Total Environment, focused on overnight temperatures rather than daytime heat.
Scientists discovered that exposure to warmer nights during early pregnancy, particularly weeks 1 through 10, was linked to a 15 percent higher likelihood of autism diagnosis.
Exposure later in pregnancy, during weeks 30 through 37, was associated with a 13 percent increase.
The researchers defined extreme nighttime heat as temperatures significantly above the local average.
Even relatively small increases—just a few degrees warmer than usual—were enough to show a measurable association.
Scientists emphasized that the study found a link, not proof that heat directly causes autism.
Nighttime temperatures have been rising rapidly in recent years, especially in places like California, where the research was conducted. In fact, nighttime warming has increased faster than daytime temperatures in the region.
As climate change continues, scientists are increasingly concerned about how environmental factors such as heat, air pollution, and wildfire smoke might affect fetal development.
To conduct the study, researchers estimated outdoor temperatures at the residential addresses of pregnant women and compared those exposures with later autism diagnoses in their children. The association remained even after accounting for factors such as neighborhood characteristics, greenery, and air pollution.
However, the study could not measure certain personal factors, including whether families had access to air conditioning.
Interestingly, the researchers did not find the same connection with daytime temperatures. One possible explanation is that people are often away from home during the day, making actual exposure harder to measure. At night, people are usually at home and more directly affected by indoor temperatures.
Scientists are still exploring why nighttime heat might influence fetal brain development. One possible explanation is sleep disruption. Warm nights can make it harder for pregnant women to sleep well, and poor sleep during pregnancy has previously been linked to developmental challenges in children. Heat exposure may also place stress on the body, which could affect the developing nervous system.
Experts say the findings highlight the importance of considering environmental conditions around the clock, not just during the day. Identifying sensitive periods during pregnancy could help families take precautions during heat waves, such as improving cooling at night or adjusting routines to reduce exposure.
While more research is needed to confirm the findings and understand the underlying causes, the study adds to growing evidence that environmental conditions during pregnancy may influence long-term child health. As global temperatures continue to rise, understanding these risks could help protect vulnerable populations and guide future public health strategies.
Researchers stress that autism has many contributing factors, including genetics, and that no single environmental exposure determines whether a child will develop the condition. However, the study offers new insight into how climate-related changes might affect early development.
