Is exercising in hot weather safe during pregnancy?

What does the evidence say about exercising in hot weather during pregnancy?

1. MotherToBaby recommends avoiding or minimizing strenuous activity during extreme heat, defined as a period of high heat and humidity with temperatures above 90°F for at least two to three days. 1
2. Heat exhaustion symptoms include heavy sweating and cold, pale, clammy skin along with nausea, vomiting, or dizziness, while heat stroke is marked by a body temperature of 103°F or higher; pregnant people need extra water to avoid dehydration, which can affect newborn weight and head circumference if chronic. 1
3. MotherToBaby advises taking precautions and limiting time in the heat generally, alongside limiting hot tub or sauna use, when visiting or living in a hot climate during pregnancy. 2
4. In a study of pregnant athletes running high-intensity intervals, core body temperature stayed below the 39.0°C concern threshold even during vigorous exertion, and researchers found pregnancy came with enhanced sweating and blood flow adjustments that help dissipate heat; outdoor exercise benefits from wind aiding heat loss, while indoor exercise may be the better choice in hot, humid conditions. 3
5. A meta-analysis of maternal hyperthermia found roughly double the risk of neural tube defects with heat exposure in early pregnancy, the same underlying concern that makes extreme-heat exercise, not just hot tubs and saunas, worth moderating in the first trimester specifically. 4

Is exercising in hot weather safe in each trimester?

• First trimester. This is the highest-stakes window for heat-related risk, since overheating early in pregnancy has been linked to a higher chance of certain birth defects. Keep sessions shorter and lower-intensity on very hot or humid days.
• Second trimester. Research on pregnant athletes running at high intensity found their core temperature stayed below the level of concern even during hard efforts, which is reassuring for continuing a moderate outdoor routine with normal precautions.
• Third trimester. Same general guidance continues, with less margin for error as your body already works harder to manage heat late in pregnancy. Indoor, air-conditioned workouts are the more comfortable choice on extreme heat days.