A new national study has found that prolonged extreme heat during pregnancy raises the risk of adverse birth outcomes, and that it does not take a long heatwave to do harm. For Andhra Pradesh and Telangana, where the long summer brings repeated bursts of intense heat across the coast and the interior, even the shorter spells appear to matter.
In a closer look at how the length of hot spells affected risk, the study found that exposure to three to four consecutive sweltering days was associated with a measurable and statistically significant rise in the odds of low birth weight. The finding suggests that recurring short spells, not only prolonged heatwaves, can add up over the course of a pregnancy.
The study, titled Effect of extreme heat stress on adverse pregnancy outcomes in India, analysed 209,266 births recorded between 2015 and 2020 and matched each one to local heat conditions across the mother’s pregnancy. Rather than relying on average or peak temperatures, the researchers counted the number of consecutive ‘sweltering’ days a woman experienced, defined as days when the India Heat Index exceeded the 90th percentile of the long-term local climate.
It was published in the peer-reviewed journal Environmental Research Communications. For low birth weight, defined as a live baby weighing under 2,500 grams, each additional consecutive sweltering day during pregnancy increased the odds by about 1% after accounting for other factors.
The timing of exposure mattered. Heat in the first trimester was linked to a higher risk of preterm birth, and heat in the second trimester to lower birth weight. The study was led by researchers at the University of Queensland–IIT Delhi Academy of Research (UQIDAR), with IIT Delhi and the University of Queensland's School of Public Health, and collaborators at UNSW Sydney, the University of Exeter and Korea University.
“While the per-day increase is small, it compounds over a long, hot pregnancy and affects a very large population, making it a meaningful public health concern in a country where extreme heat is becoming more common,” said Vineetha Vincent, lead author of the study from UQIDAR. The study also found a clear social gradient. Mothers with no schooling or only primary education were more likely to have a low-birth-weight baby or a stillbirth as their heat exposure increased, compared with better-educated mothers.
“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. Any adaptation response that ignores this social gradient will miss the women who need protection most,” said Sagnik Dey, co-author and professor at IIT Delhi.
About the research
Research led by: Vineetha Vincent, Darsy Darssan, Nicholas J Osborne, and Sagnik Dey.
Institutions: University of Queensland–IIT Delhi Academy of Research (UQIDAR); Centre for Atmospheric Sciences and School of Public Policy, IIT Delhi; and the School of Public Health, University of Queensland, Australia. Additional collaborators are from the University of New South Wales, Sydney; the University of Exeter, United Kingdom; and Korea University, Seoul.