A new study suggests that Extreme Heat during pregnancy may have only a small and uncertain effect on premature births in Pakistan. Researchers found that the initial link between heat exposure and early delivery became much weaker after accounting for seasonal and regional differences.
The study examined 46,773 deliveries recorded between June 2021 and July 2024. It 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.
Researchers collected data from hospitals in Karachi, Lahore, Muzaffargarh, and Badin. They wanted to understand whether Extreme Heat during pregnancy increased the risk of premature birth.
In the first analysis, women exposed to at least one hot and humid day above 33°C during the final trimester appeared to have around 33 percent higher odds of giving birth prematurely.
However, the results changed after researchers adjusted for differences between cities and seasonal birth trends. After these changes, heat exposure was linked to only a 0.06 percent difference in premature births. Researchers said this difference was statistically insignificant.
The study found that 25.8 percent of all recorded births were premature. However, the rate varied widely across the four cities.
Lahore had the lowest premature birth rate at 13.1 percent. Badin recorded the highest rate at 36 percent.
Researchers found that Badin played a major role in the initial findings. Nearly 96 percent of births in the city were preceded by at least one hot and humid day above 33°C.
Badin also showed large seasonal changes in premature birth rates. The rate ranged from 27.6 percent in November to 46.9 percent in August.
According to the researchers, this made it difficult to separate the effects of Extreme Heat from other local factors. They suggested that hospital referral patterns, seasonal infections, agricultural work, and changes in maternal nutrition may have influenced the results.
To better understand the relationship, the research team applied five different analytical methods. They tested the complete dataset, removed Badin from the data, and also examined Badin separately.
The findings were mixed. Two methods found no connection between heat and premature birth. Two others found a link only when Badin was included in the analysis. When the city’s data was removed, the relationship either disappeared or reversed.
Only one analytical method found a consistent association. It suggested that women exposed to more days with temperatures of 40°C or higher during the middle months of pregnancy had about 4 percent higher odds of premature delivery.
Even then, researchers described the result as weak. They said that an unknown factor affecting both heat exposure and premature birth by as little as 5 percent could completely remove the observed relationship.
The study also used an advanced model to examine delayed effects of heat exposure. This model found no lasting impact on premature birth.
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The researchers concluded that future studies should carefully consider regional and seasonal differences before linking Extreme Heat to pregnancy outcomes. They also recommended collecting more detailed information about mothers’ health, living conditions, and actual heat exposure to improve future research.
