Pregnant women in the Indo-Gangetic plains face more than double the risk of preterm birth when exposed to unusually high summer heat or low winter temperatures, a study across Bengal, Bihar and Uttar Pradesh has found.
The study found that pregnant women exposed to extreme temperatures for just days to weeks have a 2.26-fold increased risk of preterm birth compared with those whose average gestational temperature (AGT) remained between 25°C and 27°C.
Researchers at the Indian Institute of Public Health (IIPH), Bhubaneswar, who analysed the monthly average temperatures during the gestational periods of 36,851 women and their pregnancy outcomes, noted the lowest risk of preterm birth at AGTs between 25.9°C and 26.5°C.
When AGTs rose to between 27°C and 30°C or dipped to between 21°C and 25°C, the risk of preterm birth was 2.26 times higher than the baseline level, the researchers have said, describing their findings in the journal BMC Pregnancy and Childbirth.
“A three-degree change may not seem much, but the AGT reflects temperature exposure during the entire eight or nine months of pregnancy,” Ambarish Dutta, professor of epidemiology at the IIPH, who led the study, told The Telegraph.
Dutta said even two to three weeks of exposure to temperatures above 40°C during gestation could elevate the AGT to around 30°C. Exposure to temperatures below 10°C for similar brief periods could lower the AGT to 21°C.
Multiple earlier studies in the US, Australia and other countries have documented higher preterm birth rates following heat waves.
Researchers say extreme heat can reduce placental blood flow, cause dehydration and trigger inflammatory responses that increase the risk of early labour. Extreme heat has also been associated with the premature rupture of membranes.
“But the increased risk of preterm births from exposure to cold temperatures was unexpected,” Dutta said.
Winters in the Indo-Gangetic plains are relatively mild and similar strong links between preterm birth and extreme cold have not consistently emerged from much colder regions such as northern Europe or Siberia.
The researchers have proposed that the elevated winter risk may not stem directly from low temperatures but from the severe air pollution in winter across the Indo-Gangetic plains.
They said the basin-like geography of the Indo-Gangetic region, low wind speeds and other weather conditions contribute to high concentrations of fine particulate matter during the winter months.
A 2021 study led by researchers in the US had estimated that air pollution contributes to six million premature births worldwide.
Health researchers estimate that India records around three million preterm births annually, or about 9.9 per cent of all births. Children who are born premature or with low birth weight face an enhanced risk of developmental delays, impaired motor skills, and non-communicable diseases in adulthood.
The IIPH study also found that the winter risk was higher (2.38-fold) among the poorest households than among the richest households (1.6-fold), reinforcing concerns that socio-economic factors also influence pregnancy outcomes linked to weather.
Dutta said the findings had relevance for both the public and policymakers and underscored the need for efforts to minimise exposure of pregnant women to extreme heat or cold and air pollution.
The study’s co-authors are Aisurya Samal, Jagyenswar Rath, Jyoti Ghosal at IIPH and Subha Shankar Das at the Mamta Health Institute for Mother and Child, a maternal and child health research entity.