LAHORE: A study of nearly 47,000 births in four Pakistani cities has found that extreme heat during pregnancy has, at most, only a small and uncertain connection to premature birth, far weaker than initial numbers suggest.

The study was led by Dr Shaper Mirza of the Lahore University of Management Sciences (LUMS) and Dr Faridah Amir Ali of the Indus Hospital and Health Network. Together, they examined records of 46,773 births delivered between June 2021 and July 2024 across Karachi, Lahore, Muzaffargarh, and Badin.

Premature birth, delivery before 37 weeks, is the leading direct cause of newborn death worldwide, responsible for more than 900,000 deaths annually, with roughly 40% occurring in South Asia. Pakistan carries one of the highest premature birth rates globally while simultaneously enduring some of the region’s most extreme summers, with temperatures regularly exceeding 40°C across Sindh and southern Punjab.

While earlier research from wealthier nations, including the United States, Israel, and parts of Western Europe, had established links between hot weather and premature birth, almost no such evidence existed for South Asia, a gap this study directly addresses.

New study across four cities examined 46,773 births from 2021 to 2024

The researchers also highlight that a large proportion of Pakistani women of childbearing age are full-time homemakers whose heat exposure comes primarily from indoor sources: cooking on gas stoves in poorly ventilated kitchens and household work in small, crowded homes with little or no cooling. Indoor temperatures in such households can match or exceed outdoor levels, particularly in rural and semi-urban areas. Since individual indoor heat exposure is rarely measured, the team used city-wide outdoor temperatures as a proxy.

Across the full study, 25.8% of births were premature, but this varied dramatically between cities, ranging from 13.1% in Lahore to 36.0% in Badin. An initial comparison suggested that mothers who experienced at least one hot, humid day above 33°C during the final trimester had roughly 33% higher odds of an early birth. However, this apparent link almost entirely disappeared once researchers accounted for which city a mother lived in and each city’s seasonal birth patterns. After these adjustments, the difference narrowed to just 0.06 percentage points, statistically negligible.

The explanation, the researchers found, centres largely on Badin. Nearly 96% of births there were preceded by at least one such hot, humid day, and Badin also had by far the highest premature birth rate, which fluctuated sharply with the seasons, from 27.6% in November to 46.9% in August. A simple analysis without city and seasonal controls therefore confused “living in Badin” with “being exposed to heat.” The researchers attribute Badin’s unusual seasonal pattern to factors such as the timing of hospital referrals linked to the agricultural calendar, seasonal infections, and fluctuating nutritional status among mothers, not heat itself.

To rigorously test for a heat effect, Dr Shaper Mirza, Dr Faridah Amir Ali, and their team applied five independently designed analytical approaches, each run on all four cities combined, on the dataset excluding Badin, and on Badin alone. Two approaches found no link to premature birth in any configuration. Two others showed an apparent link only when Badin was included, which vanished or reversed once Badin was removed, confirming these were Badin-specific patterns rather than a general heat effect.

Only one method produced a consistent result. Mothers who experienced more days of 40°C or higher during the middle months of pregnancy had a slightly elevated risk of early delivery, equivalent to roughly a 4% increase in odds for a typical level of exposure. This effect held across all city combinations, suggesting it was not a single-city quirk.

Even so, the researchers caution this signal is weak. A hidden factor affecting both heat exposure and premature birth by as little as 5%, such as neighbourhood-level heat disparities, seasonal nutrition, or seasonal infections, would be sufficient to erase the link entirely. A more advanced statistical model tracking delayed effects of heat found no lasting impact on premature birth at all.

Dr Mirza and Dr Ali conclude that in countries like Pakistan, facing both extreme heat and high premature birth rates, the core challenge for future research is not merely detecting a connection, but rigorously ruling out the role of location and season before attributing causality to heat.

They recommend that future studies apply multiple analytical methods simultaneously, test whether results change when individual cities are removed, and collect richer data on mothers’ health, circumstances, and actual heat exposure.

Published in Dawn, July 18th, 2026