Hot, Humid Prenatal Exposure Linked to Greater Growth Stunting, Study Finds

Exposure to hot and humid weather during pregnancy can stunt children's height later in life, according to a new study analyzing about 200,000 children across South Asia. The researchers linked prenatal heat exposure to lower height-for-age scores and warned that climate change could expand hot-humid conditions in more regions, including parts of the United States.

Using a fine-scale quasi-experimental approach, the study tracked two weather thresholds during pregnancy: days with maximum temperature above 35°C and days with high wet-bulb globe temperature (WBGTmax) above 29°C, which combines heat and humidity. A one-standard-deviation rise in hot-humid days during the third trimester was tied to a 5.1 percent drop in height-for-age, while an equivalent rise in dry-heat days (Tmax above 35°C) corresponded to a 1.3 percent drop. That implies the growth-harm from humid heat is nearly four times that from dry heat.

The researchers project future impacts: under projected climate scenarios, hot-humid extremes could push 3 to 3.7 million more children into stunting, 2.7 to 3.3 million more than if only dry heat were considered. The analysis linked health data to location-specific weather records and compared children born in the same communities across different years to account for seasonal trends.

Humidity matters because the body’s cooling system relies on sweat evaporation, which is impaired when the air is humid. For pregnant people, hormonal and metabolic shifts heighten sensitivity to heat and dehydration, increasing the risk of heat stress, preterm birth, low birth weight and other complications at birth. These immediate effects can translate into long-term health and developmental challenges for children.

The climate context underscores why hot and humid conditions are rising in many places. As the planet warms, more regions are reclassified as subtropical, including parts of the northeastern United States. Warmer air carries more moisture, making humid heat days more common. In the United States, a Washington Post analysis of dew point data since 1979 found that more than 120 million people across about 1,500 counties endured one of their three most humid summers on record, with the Mid-Atlantic, Southeast and Plains regions among the hardest hit.

Beyond growth, the study notes that outside temperature can influence a baby’s risk of birth defects affecting the brain, spine and spinal cord. Projections for 2025 to 2035 indicate regional increases in certain heart conditions, such as abnormalities in the arteries leaving the heart and atrial septal defects, which could rise by about one-third in some regions. The researchers emphasize that the critical hazard is the combination of heat and humidity, not temperature alone, and that protecting pregnant people from oppressive heat will be essential as climate trends unfold.