Why doctors say pregnant people should be extra careful during a heatwave

With extreme heat and humidity creating dangerous conditions across the Greater Toronto Area, doctors are warning pregnant people about the serious health risks heat waves pose to them. 

When temperatures soar above 30 C for several days, pregnant people are especially vulnerable to heat-related illnesses, says Dr. Tali Bogler, chair of family medicine obstetrics at Saint Michael's Hospital.

"We do have studies that have shown that there is certainly risk for the pregnancy and for both mom and baby," she told CBC Toronto.  Environment Canada says this summer's first heat wave is expected to create daytime highs of 33 to 36 C into Tuesday night, with humidex values of 42 to 46. 

Bogler says that kind of heat can impact someone who's expecting, no matter what stage of pregnancy they're in. 

During the first trimester of pregnancy, there is some limited evidence that shows prolonged heat exposure can result in congenital birth defects, she said, such as neural tube or heart defects. In the second trimester, extreme heat can create issues around hypertension and gestational diabetes, and in the final trimester, it could result in a low birth weight, preterm delivery or stillbirths, Bogler said. 

If you get heat rash or feel unwell, get somewhere cool, but if you're experiencing more severe symptoms such as vomiting or feeling faint, then that should be treated as an emergency, Bogler said. 

For pregnant people, staying hydrated means drinking about three litres of water a day or more, says Dr. Cynthia Maxwell, a maternal fetal medicine specialist at Mount Sinai Hospital. "Because you're losing a lot of the moisture through the skin, and then of course, the requirements for the baby and for the circulation to the baby requires constant replenishment," she said.

Mom-to-be Julia Lee in Toronto is 23 weeks pregnant, and says she's been careful about how much time she spends outside. "I've been really feeling it. I feel like summer came a little late, and then it kind of just hit us like a truck."

From 2020 to 2024, Toronto had 15 days of extreme heat risk for pregnant people, according to a recent study by non-profit Climate Central.

Its analysis found that of those days, half were added by climate change, says Andrew Pershing, the organization's chief program officer.

"Heat is only going to grow as a problem for us as we continue to burn coal, oil and natural gas and pollute the atmosphere with carbon dioxide," he said.