Every extra day of extreme heat a pregnant woman endures in India raises the risk of harm to her baby. That is the finding of the largest study yet to link heat and pregnancy outcomes across the country, and it covers more than 209,000 births.
Researchers tracked births in 707 districts from 2015 to 2020. Long stretches of extreme heat raised the odds of low birth weight, preterm birth, and stillbirth. Timing mattered as much as severity: heat in the first trimester raised preterm birth risk, heat in the second trimester lowered birth weight, and heat in the third trimester carried the strongest link to stillbirth.
Scientists at the University of Queensland–Indian Institute of Technology Delhi Academy of Research led the work, with IIT Delhi, the University of Queensland, the University of New South Wales, the University of Exeter, and Korea University. Environmental Research Communications published the study.
How the Study Measured Heat
Earlier research leaned on average or peak temperatures. This team counted consecutive “sweltering” days — days when the India Heat Index, which combines temperature, humidity, wind speed, and sunlight, climbed above the 90th percentile for that area’s climate history.
Researchers matched hourly heat data to each mother’s exact pregnancy window, using her household location, then adjusted for air pollution, cooking fuel, wealth, education, maternal age, weight, and anemia. The final sample held 209,266 births from India’s National Family Health Survey.
The Numbers
Low birth weight affected 12.8% of the births, the most common problem. Preterm birth affected 1%; stillbirth, 0.74%.
Each additional sweltering day raised the odds of low birth weight by about 1%, after adjustment for other factors.
“While the per-day increase is small, it compounds over a long, hot pregnancy and affects a very large population,” said Vineetha Vincent, the study’s lead author at UQIDAR.
Who Faces the Greatest Risk
The danger fell unevenly. Mothers with a low body mass index — under 18.5 — faced a sharply higher risk of low birth weight from heat than mothers of normal weight.
Women in mountainous regions faced the highest risk from prolonged heat, likely because their bodies are less acclimated to extreme conditions. Low birth weight was also strongly linked to heat in humid subtropical and arid regions.
Poorer households and mothers with little or no schooling faced higher risks of low birth weight and stillbirth as heat exposure rose.
“The mothers most exposed to extreme heat are often those with the fewest resources to cope with it: the poorest, the least educated, the under-nourished,” said Sagnik Dey, co-author and professor at IIT Delhi. “Any adaptation response that ignores this social gradient will miss the women who need protection most.”
Why Heat Harms a Pregnancy
Pregnancy already strains the body. Cardiac output rises by 30% to 50% by late pregnancy, and core temperature shifts.
Extreme heat pushes that temperature higher still. Researchers point to two pathways of harm: rising fetal temperature, which can affect development, and dehydration, which cuts blood flow to the uterus and can trigger early contractions.
Other research backs the pattern. A 2023 Chinese cohort study of more than 205,000 births found that temperatures near the 95th percentile in the week before delivery sharply raised preterm labor risk. A 2024 South Asian cohort study found pregnant women doing physically demanding work in high heat faced double the risk of miscarriage.
What Comes Next
The authors want pregnant women named as a priority group in India’s city heat action plans, with cooling, hydration support, and antenatal advice reaching high-risk districts.
“India already has heat action plans in several cities,” said Nicholas J. Osborne, co-author and researcher at the University of Queensland’s School of Public Health. “The next step is to make sure pregnant women are named as a priority group within them.”
The team is calling for cohort studies that track pregnant women over time and collect biological data, to map exactly how heat damages a developing pregnancy — before India’s summers get hotter still. A related multi-center effort, the Heat in Pregnancy study, is already under way in India.
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