How Heat Storms Put Pregnant Women in Danger

Scientists and doctors have made a surprising and alarming discovery: heat waves are dangerous for pregnant women and their unborn babies. Studies from many parts of the world show that when temperatures rise above normal, pregnant women face a much higher risk of losing their baby early, having a miscarriage, or developing serious health problems. This is not just a danger for women who live in the hottest places on Earth — it can happen anywhere temperatures climb above what is normal for that area. Researchers are now racing to understand exactly how heat harms pregnancies, and what can be done to protect mothers and babies.

One of those researchers is Dr. Tarylee Lakhoo, a scientist in Johannesburg, South Africa. As part of her work, she studied nearly 200 scientific papers to learn how heat affects pregnant women. While she was doing that research, she became pregnant herself — with twins. During her pregnancy, Johannesburg suffered through 74 days of extreme heat, called 'heat storm' days. That was more than a quarter of a full-term pregnancy spent in dangerous temperatures.

Heat storms are large heat waves that cover at least 1% of the Earth's surface at their peak — roughly half the size of Canada. In the 1960s, there were only about 50 heat storms, and the largest covered about 6% of the planet. So far this decade, there have been more than 665. The biggest one stretched across nearly 85% of the Earth during its year-long lifespan, affecting almost every person on the planet. Scientists define a heat wave as five days in a row where temperatures are in the top 10% of what is normal for that area.

Lakhoo and her fellow researchers found something important: what matters most is what a pregnant woman is used to. For every 1 degree Celsius rise in the average temperature where she lives, the risk of losing the baby in the weeks that follow increases significantly. Studies from four continents show that the same 1-degree rise leads to a 4% higher chance of the baby being born too early — what doctors call a preterm birth. In the week after a heat wave ends, the odds of a preterm birth jump by 26%.

Being born too early can cause serious, lifelong health problems for a child, including slower growth, severe asthma, learning difficulties, and cerebral palsy. Heat can also harm the mother herself. Higher temperatures are linked to a dangerous condition called preeclampsia, which can cause kidney failure, heart attacks, and strokes. Preeclampsia is responsible for more than one in seven deaths of mothers around the world.

Reuters studied more than 8 million births in Mexico between 2008 and 2025. The data showed that when heat storms or heat waves struck in the four weeks before a baby was due, the rate of preterm births rose by about 6.5%. That adds up to more than 1,400 Mexican babies born too early every year during the hot seasons. At the highest level of heat exposure, the rate of preterm births jumped by 18%.

Heat does not affect all pregnant women equally — it makes existing risks even worse. Women who are older, poorer, or who have health conditions like diabetes or high blood pressure face even greater dangers from heat. Researchers also found that women carrying twins are especially vulnerable. Matthew Chersich, a professor at South Africa's University of the Witwatersrand, said clearly: 'Twin pregnancies are very vulnerable to the heat.'

Lakhoo's own pregnancy showed how serious these dangers can be. Three months in, doctors found problems with blood flow to her placenta. At eight months, her blood pressure rose and she needed medication. The following week, she showed signs of preeclampsia, and when a stretch of unusually hot days arrived in early November, her babies stopped growing. Before doctors could schedule a planned delivery, her water broke, and her twins were delivered by emergency C-section — so premature they needed help just to breathe.

Chersich had tried to help Lakhoo by sending her a portable air conditioner as a gift. 'Air conditioning is lifesaving equipment,' he said. But Lakhoo used it only rarely, thinking the electricity cost was too high on her researcher's salary. She was also more focused on the challenges of carrying twins than on the heat itself — a mistake she later said she learned from.

In Mexico City, a doctor named Juan Rivero Corona noticed something similar during a record-breaking heat storm in spring 2024. He began seeing more pregnant patients with preeclampsia and dangerous placental problems. When he pulled hospital records, he found that preterm births and dangerous complications consistently spiked every spring — the hottest season there. He started telling his patients to drink more water and wear loose clothing, and scheduled their appointments in the cooler morning hours.

Scientists are still working to understand exactly how heat harms pregnancies. Some think heat causes the body to send blood away from the placenta to help cool the mother down, while others think it disrupts the hormones and genes that guide a baby's development. A hospital in Johannesburg called Rahima Moosa is running an ambitious research study to find answers, collecting blood, breastmilk, placentas, and umbilical cords from volunteer mothers. Dr. Renate Strehlau, who leads the study, compared the work to the early days of researching HIV: 'The science was definitely behind reality.'

Strehlau and Lakhoo believe fighting this danger will require big changes in how society treats pregnant women. Nurses should ask about heat exposure as routinely as they ask about alcohol use. Maternity leave policies should encourage women to rest during pregnancy, not just after birth. Clean water and mobile cooling clinics should be widely available to all women, regardless of income.

In Miami, Florida — one of the most heat-storm-affected cities in America — a doctor named Cheryl Holder is working to protect pregnant women. Her team warns mothers about heat dangers and hands out water bottles, fans, wet towels, and umbrellas. For some families, they even provide air conditioning units. Most people Holder speaks with have no idea that heat is a threat during pregnancy.

After weeks in the hospital, Lakhoo's twin daughters were finally healthy enough to go home. Nearly two years later, the girls are growing and active, though they are much smaller than most children their age, and their skin carries tiny scars from the tubes that kept them alive. Lakhoo said the experience changed her completely, and she now uses air conditioning without hesitation. 'Everyone is at risk,' she said — a lesson she learned the hardest possible way.