It's Getting Hot in Here: Considerations for Practicing Heated Yoga While Pregnant

I'm often asked about practicing heated yoga during pregnancy- what my thoughts are and whether or not our clients should be continuing their practice throughout.

This is a hot topic (pun very much intended). And frankly, there's no consensus- even among physicians- those making the rules for their patients and their patient's safety. Anecdotally, I personally know of people who have practiced throughout pregnancy and have heard from colleagues whose clients have being given the green light, by their OBs, to continue their heated practice. While others have been told to stop immediately, per ACOG (American College of Gynecology) guidelines.

What is the fear?

A research article published in 1992 showed an increased risk for Neural Tube Defects (NTDs) in women with elevated core temperature- specifically through exposure to hot tubs, saunas, or fever, in the first trimester. Thus, the American College of Gynecology recommends that pregnant women keep their core body temperature below 102.2º F.

The thought process is that exposure to a heated yoga room (especially in the 1st trimester) may predispose a pregnant woman for certain genetic abnormalities.

What do we know?:

Despite the lack of scientific evidence looking at heated yoga during pregnancy, we know a lot about how the body works during pregnancy, during exercise, and while exercising in deviations from thermoneutral environments (an environment where a person will experience neither heat nor cold stress (26.2 degrees C/ 79 degrees F).

Our body is pretty awesome at heat and cold self-regulation (thermoregulation):

Our normal internal temperature is around 98.6º F and, on average, increases about .4 degrees during pregnancy. Thermoregulation is the process by which the body works to keep it's core temperature at homeostasis- during exercise and when we are in heated (or cool) environments, the body implements cooling mechanisms- sweating and vasodilation (which blood moves faster to the skin- allowing it to release heat faster) to cool itself.

This process does not change during pregnancy.

Of the very limited studies looking at prenatal exercise, there is little direct evidence to show that exercise raises a woman's core temperature into the range that could cause defects as long as the woman is exercising in a thermoneutral environment where she is also not experiencing dehydration (here, here & here) However, no studies have been performed on non-athletic populations exercising at high intensity, prolonged, or enough to induce dehydration.

Average temperatures of saunas & hot tubs and heated yoga rooms

A quick google search tells us the average temperature of a hot tub is between 100°F-102°F (the standard maximum temperature 104°F), the average sauna temperature ranges from 160-200°F, and the average temperature, in a hot yoga room, based on the style is between 92-105°F.

In a study published in 1981, researchers looked at how long it took the core temperatures of pregnant women (measured vaginally) to raise to 38.9°C (102°F). The small sample (20 women) of healthy pregnant women sat in hot tubs of 39°C (102.2°F) or 41.1°C (105.98°F) or a sauna set to 81.4°C (178.52°F) and were asked to remain until their temperature reached 38.9°C (102°F) or until they were too uncomfortable to continue.

The mean exposure time to reach 38.9°C (102°F) in the 39°C was 15.3 minutes yet, subjects began to leave the tub due to discomfort after 10 minutes. Subjects began to leave the 41.1°C after just 5 minutes due to discomfort however, of those who stayed, it took an average of 18.5 minutes before their temperatures reached 38.9°C. ALL of the sauna subjects left before their temperature reached 38.9°C, staying an average of 9 minutes.

Core temperature can rise to 'dangerous' levels during hot yoga

In a research article published in 2015, 20 healthy male and female subjects (ranging from 28-67 years old), who regularly practiced Bikram were asked to particpate in a 90-minute Bikram class, taught by a certified Bikram instructor, where target room temperature and relative humidity were 105ºF and 40%, respectively. Participants swallowed a CorTemp Ingestible Core Body Temperature Sensor and core temperature was collected throughout class. While core temperatures rose throughout the class and peaked towards the end, average core temperatures for men were 103.2 ± .78ºF while average temperatures for women were 102.0 ± .92ºF. "One man in the study had a core temperature of 104.1°F by the end of the 90-minute class, and 7 other participants had core temperature greater than 103°F." Interestingly, those who consumed water had significantly lower core temperatures (102.2ºF ± 1.01ºF) than those who did not (103.2ºF ± .93ºF).

In another study by the same group that looked at the core temperatures of individuals during a regular Hatha yoga class, with an average temperature of 92.7°F and 35% relative humidity, the average core temperature was 99.7°F and the highest core temperature recorded for an individual was 102.4°F

Heat acclimatization/acclimation

There are plenty of studies that show that we can acclimate to environmental variables- in a heated scenario that would mean, the temperature thresholds for vasodilation and the onset of sweating are lowered also usually improving our exercise response (eg lowered heart rate) (here, here, here, here, here). Whether or not would transfer to, or would present similarly in a heated yoga scenario would be interesting as most of these studies looked at more "intense" forms of exercise (running, & cycling).