Heat Wave Safety During Pregnancy

What does the evidence say about heat wave and extreme heat safety during pregnancy?

1. A systematic review and meta-analysis found the odds of preterm birth rose about 5% for every 1°C rise in temperature and about 16% during heatwaves specifically, compared with cooler periods across pregnancy or by trimester. 1
2. The same review found stillbirth odds rose about 5% per 1°C increase and about 46% during heatwaves across the studies that examined it, and heat exposure in the final weeks before birth appeared most important for both preterm birth and stillbirth. 1
3. CDC lists pregnant women among the groups facing greater vulnerability during extreme heat, alongside young children, older adults, and people with asthma or heart disease. A fan only helps when the indoor temperature stays below 90°F (32°C); above that, a fan can raise body temperature instead of lowering it, so a cooling center reached by dialing 211 is the safer option during a power outage or a home without air conditioning. 2
4. CDC's occupational heat guidance lists pregnancy among the individual risk factors for heat-related illness, alongside dehydration, high humidity, direct sun exposure, physical exertion, and limited air movement, meaning the same conditions that raise heat-illness risk generally raise it further during pregnancy. 3
5. MotherToBaby describes hyperthermia as the body absorbing more heat than it releases, from causes including heat stroke, hot tubs, saunas, and prolonged exposure to temperatures of 86°F (30°C) or higher, and notes that a sustained elevated body temperature in early pregnancy, particularly before 6 weeks, has been studied for a possible link to neural tube defects, with some conflicting results across studies. 4
6. Pregnancy brings real heat-adapting changes, including a lower threshold for when sweating starts, more blood flow near the skin, and greater plasma volume, but an expert review found the current evidence insufficient to say whether these adaptations remain protective during prolonged or extreme heat exposure, particularly for people without reliable access to cooling. 5

Is heat wave and extreme heat safety safe in each trimester?

• First trimester. Try to avoid prolonged overheating in early pregnancy. Research on fever and hyperthermia before 6 weeks has looked at a possible link to neural tube defects, and while an ordinary warm day is not the same as a sustained rise in core body temperature, a multi-day heat wave with no cooling plan is worth taking seriously this early.
• Second trimester. As pregnancy continues, the same heat wave carries a measurably higher chance of preterm birth in the research behind this page. Pregnancy's own heat-adapting changes, a lower threshold for sweating and more blood flow to the skin, are real, but researchers say the evidence on whether they hold up under prolonged extreme heat is still limited.
• Third trimester. In the final weeks, heat exposure appears to matter most for both preterm birth and stillbirth risk in the studies behind this page, which is part of why having a cooling plan, since a fan alone stops helping above 90°F, and knowing your nearest cooling center matter most during any heat wave this late in pregnancy.

Staying safe during a heat wave