Have you been told to avoid the heat during pregnancy — perhaps skip the sauna, cool down the bath, don’t push it outside when it’s warm — but never really understood why?
The concern is real, but so is the science showing that the pregnant body is better equipped to handle heat than many people realize. This article breaks down where the caution comes from, what the research actually says, and how to exercise safely in warmer conditions.
Read on to learn:
- Where the concern about overheating during pregnancy comes from
- How the pregnant body naturally dissipates heat
- What the research says about exercising in warmer conditions
- Guidelines for safely exercising in the heat during pregnancy
Quick Answer: Exercising in the heat during pregnancy can often be safe when the right precautions are in place. The main concern is avoiding a significant, sustained rise in core body temperature, especially in early pregnancy. Because the pregnant body naturally adapts to release heat more efficiently, many pregnant people can continue outdoor exercise safely by staying hydrated, wearing breathable clothing, avoiding prolonged high-intensity exercise in hot and humid conditions, and choosing cooler times of day.
Where the Concern about Overheating During Pregnancy Comes From
The concern around overheating during pregnancy comes from one important fact: fetal body temperature is influenced by maternal body temperature. This matters most in the first trimester, when critical brain and organ development is occurring.
Research from the 1960s, conducted in animals, found that when maternal body temperature rose above 102°F, it increased the risk of birth defects, especially during early pregnancy (Edwards, 1967).
This is where much of the guidance to limit heat exposure during pregnancy comes from. The goal is not to avoid all warmth or outdoor activity. The goal is to avoid conditions that cause a significant and sustained rise in core body temperature.
The Fascinating Ways the Pregnant Body Dissipates Heat
While the concern around overheating is understandable, the pregnant body is not defenseless. In fact, it naturally adapts in several ways that help release heat more efficiently.
Specifically, the following changes occur:
- Sweating begins at a lower body temperature. This helps the body start cooling itself sooner
- More heat is lost through respiration. Breathing rate increases by 50% during pregnancy (ACE, 2011), helping to release heat through the breath.
- More heat transfers from the skin to the environment. Increased blood flow during pregnancy helps move heat toward the skin, where it can be released.
So, while exercise does raise body temperature, these natural adaptations help protect against excessive heat buildup. In fact, exercise alone (even high-intensity exercise) has never been shown to pose a problem (provided the safety guidelines listed below are followed). The key is to avoid conditions that interfere with the body’s ability to dissipate heat effectively.
What the Research Says About Exercise in Warmer Conditions
Newer research suggests that pregnant people may be able to safely exercise in warmer conditions than previously thought.
A 2019 review found that pregnant people can safely exercise for up to 35 minutes at 80–90% of max heart rate in 77°F heat with 45% relative humidity (Ravenelli et. al, 2019). The same review also found that water exercise and sauna exposure can be safe when specific time, temperature, and humidity guidelines (outlined below).
That does not mean heat exposure is risk-free or that every person will tolerate it the same way. It means the guidance is more nuanced than “avoid heat altogether.” The key is to understand the conditions that make heat dissipation more difficult, and then use the guidelines below to reduce risk.
Guidelines for Safe Heat Exposure During Pregnancy
While the pregnant body has built-in mechanisms to release heat, the goal is to avoid conditions that make heat dissipation more difficult.
Based on the research cited above, it is recommended to avoid (Ravenelli et. al, 2019):
- Prolonged, high-intensity exercise in heat: More than 35 minutes of high-intensity exercise above 77°F and 45% relative humidity
- Hot water immersion: Water hotter than 92°F for more than 45 minutes
- Saunas: More than 20 minutes above 158°F and 15% relative humidity
- Hot tubs or hot baths: More than 20 minutes above 104°F
- Hot yoga
To further reduce the risk of overheating:
- Drink water before, during, and after exercise.
- Wear clothing that allows sweat to evaporate.
- Exercise during cooler parts of the day when possible, especially before 10 a.m. or after 4 p.m.
If you or your client has an uncomplicated pregnancy and follows the guidelines above, exercising in warmer conditions can often be done safely. As always, individual medical guidance matters, especially if there are any pregnancy complications or pre-existing conditions.
Exercising in the heat during pregnancy does not have to be an automatic no. The concern about overheating is valid, especially in early pregnancy, but the pregnant body also has built-in mechanisms to dissipate heat more efficiently.
The key is knowing where the true risk lies: prolonged, high-intensity effort in hot and humid conditions, hot tubs, hot yoga, and other environments that inhibit the pregnant body’s natural ability to dissipate heat.
With the right precautions, many pregnant people can continue enjoying outdoor movement safely and confidently.
Sources:
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American Council on Exercise. (2011). ACE Group Fitness Instructor Manual: A Guide for Fitness Professionals. San Diego, Print.
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Edwards. M.J. (1967). Congenital defects in guinea pigs. Following induced hyperthermia during gestation. Arch Pathol., 84(1):42-8.
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Ravanelli, N., Casasola, W., English, T., Edwards, K.M., Jay, O. (2019). Heat stress and fetal risk. Environmental limits for exercise and passive heat stress during pregnancy: a systematic review with best evidence synthesis. British Journal of Sports Medicine, 53(13), 799-805.