Study links prolonged heat exposure during pregnancy to adverse birth outcomes

A nationwide analysis of data from the National Family Health Survey (NFHS) quantifies the link between prolonged exposure to extreme heat during pregnancy and adverse birth outcomes in India. It was associated with a measurable increase in the risk of low birthweight, preterm birth and stillbirths among the mothers, and was tied to the trimester during which exposure occurred.

The paper ‘Effect of extreme heat stress on adverse pregnancy outcomes in India’, analysed 209,266 births recorded between 2015 and 2020, and matched each birth to local heat conditions during the pregnancy. It used nationally representative NFHS-5 data (2019–21) linked to high-resolution heat exposure estimates (India Heat Index) for 2015–20 across 707 districts. It uses ‘consecutive sweltering days’ instead of average temperature.

The researchers, Vineetha Vincent et al, showed that trimester-specific windows: heat extremes during the first trimester were most linked to preterm birth, the second trimester to low birth weight, and the third trimester showed a higher risk for stillbirth.

The risks were also higher among mothers with low Body Mass Index (<18.5), with lower education levels, and from poorer wealth groups, the study concluded.

It also found that urban residents showed slightly higher low birth weight rates than their rural counterparts.

Climate zones

The study also found associations between climatic zones, heat exposure, and pregnancy outcomes — mountainous zones showed stronger associations, while humid subtropical and arid zones also showed significant low birth weight occurrences. This is presented as the first population-scale evidence from India tying prolonged extreme heat stress (not just temperature) to adverse pregnancy outcomes, using a heat index that incorporates humidity, wind, and radiation.

The authors say research evidence already shows the potential biological effects of high ambient temperature: increased maternal core body temperature raises foetal core temperature, leading to birth defects, and dehydration decreases uterine blood flow and foetal growth, and due to reduced blood flow, there is a heightened chance of uterine contractions and an increased risk of preterm birth. Low and middle income countries experience higher birth rates and therefore, a larger absolute number of adverse birth outcomes.

The study was led by researchers at the University of Queensland – IIT Delhi Academy of Research, in association with the Indian Institute of Technology Delhi (Centre for Atmospheric Sciences and School of Public Policy) and the University of Queensland’s School of Public Health, Brisbane, University of New South Wales, Sydney; the University of Exeter, U.K.; and Korea University, Seoul.

Strategies for future

With extreme conditions expected to increase due to global warming, India should prepare to address the heat challenges with more adequate adaptation planning, the authors say. It is crucial to raise awareness of the risks of heat exposure for pregnant women from marginalised populations, particularly those with lower education and in rural areas, they urge.