Over the last 65 years, heat waves have morphed from intermittent irritations into globe-spanning events that can last months or even years. While these supersized heat waves affect virtually everyone, among the most vulnerable are pregnant women and fetuses.
Medical research has found that heat can correlate with weaker babies, sicker mothers, and more babies born prematurely. Heat storms have become a major global threat.
Until now, there has been no universally accepted definition of a heat wave. By our definition, a heat wave is five consecutive days of heat in the top 10% of the historical range for an area.
We call the largest heat waves - those that cover at least 1% of the globe at their peak - heat storms. By analyzing temperature data for every hour of every day since 1960, researchers found that the 1960s saw just 50 heat storms; this decade, there have already been more than 665.
The biggest heat storm drifted over nearly 85% of the planet in its year-long lifespan. By contrast, the largest heat wave of the 1960s reached about 6% of the planet and lasted 46 days.
Heat storms are especially risky for pregnant women. Recent research has tied heat waves to severe pregnancy complications.
Spikes in heat are associated with higher odds of dangerous hypertension, birth defects, miscarriage, and stillbirth. Researchers are particularly concerned about unusual heat: Every 1 degree Celsius (1.8 degrees Fahrenheit) above what a woman is used to has been found to correlate with harm.
The clearest connection is with preterm birth, which can have severe lifelong health effects. We found that the rate of preterm births increased with more exposure to hot-season heat storms and heat waves during the last month of pregnancy.
Among women with no exposure to heat storms or heat waves during the hot season, 7.2% of births were preterm. The threat from heat storms is global.
Scientific studies have identified heat-related complications among pregnant women from China and Australia to South Africa, Mexico, and the United States. Because the science is still new, researchers are trying to determine how heat hurts pregnancies.
Some suggest high temperatures prompt the body to redirect blood from the placenta to cool a mother, or that heat increases the strain on a fetus's heart. While we wait for clear answers, health workers from Johannesburg to Miami are stepping in with protections that range from wide-brimmed hats and white-painted roofs to air conditioning units.
"Everyone is at risk," an expert noted. One of the doctors studying the risk that heat storms pose to pregnant women is Dr. Darshnika Lakhoo, who co-wrote a 2024 review article for Nature Medicine.
While writing that paper, Lakhoo herself sweltered through a high-risk pregnancy.