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“Thanks for the informative podcast! Can you address heat exposure when pregnant? I live in New Orleans and have previously enjoyed runs outside. At what temperature and duration is it safe to run/exercise outside in heat? How about the use of heating pads or baths?” – Laura

Exposure to heat in hot weather and activities like working out can cause your body temperature to go up. When you are expecting there is concern of an elevated core temperature causing neural tube defects, spontaneous abortion, and other abnormalities. There are cautions you should take when practicing Bikram yoga, going in a hot tub or sauna, and even taking a hot bath. Learn about heat exposure and how to make sure you and your baby stay safe and do not overheat.

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Exposure to heat in hot weather and activities like working out can cause your body temperature to go up. Your normal body temperature is 98.6 degrees Fahrenheit, or 37 degrees Celsius. Hyperthermia occurs when you have a high temperature (102 degrees Fahrenheit, or 38.9 degrees Celsius) for an extended period of time without an actual fever.

Pregnancy and Hyperthermia

Hyperthermia is the reason you want to avoid overheating. When you are expecting there is concern of an elevated core temperature causing neural tube defects, spontaneous abortion, and other abnormalities. Many of the studies relating to hyperthermia and pregnancy are older than I would like. I try to find the most recent research available and really prefer it is from the last 10 years.

A study done in 1988 on mice also showed a strong correlation between hyperthermia and neural tube defects. It seems it was established decades ago that there was a correlation between hyperthermia and neural tube defects and as a result there has not been a need for more recent research on it. A meta-analysis of 15 studies done in 2005 found maternal hyperthermia in early pregnancy is associated with increased risk for neural tube defects.


Your body’s temperature set-point is increased when you run a fever. Your hypothalamnus, acts as your body’s thermostat and is responsible for increasing your body’s temperature. When your set-point increases your hypothalamus triggers changes in your body to either raise or lower your temperature. Your hypothalamus is responsible for creating muscle contractions and shivering, which causes your body to produce more heat and conserve heat so your temperature will increase to match your internal set point. It also releases epinephrine (or adrenaline) which can increase your temperature.

Fevers during pregnancy can be stressful and there is more information on that topic in the episode on Fevers During Pregnancy. Heat exposure doesn’t cause a fever, because your set point has not actually increased. As long as you are able to cool down, and you are not overheating, you and your baby should be fine.

Hot Weather

It is important to stay cool in hot weather. You need to pay more attention to your comfort and make sure you are not getting overheated in higher temperatures. This isn’t just a theoretical risk. Recent research has shown that hot weather and exposure to high temperatures can increase the risks for stillbirth. Another study linked higher temperatures to lower birth weights.

You may want to spend more time indoors with air conditioning and limit how long you are outside, especially in the sun. If you are planning to be out in hot weather for long periods plan ahead by dressing in cooler clothing and stick to the shade when you can.

Exercise in Hot Weather

Both working out and being exposed to hot weather can put you at risk for overheating. When you combine the two you need to be especially careful. If you are in an area that gets especially hot you may want to plan any outdoor exercise for early mornings or in the evenings when it is cooler outside. On especially hot days you may want to take your workout indoors, where you will be cooler and potentially have air conditioning.

Your body is constantly changing during pregnancy and as your pregnancy progresses you may need to modify your workouts. Pay attention to how you feel and if you think you may be overheating take it down a notch, have a break, or call it a day. Also be sure you are staying hydrated and drinking plenty of water.

Bikram Yoga

A study done by the American Council on Exercise took a group of participants who swallowed a core body temperature sensor, so researchers could remotely monitor core temperatures. Participants went through a 60 minute yoga class in a room heated to 70 degrees Fahrenheit, or 21.1 Celsius. Then 24 hours later the same participants went through a yoga class in a room heated to 92 Fahrenheit, or 33.3 Celsius. The researchers did not find a difference in the increase of core temperature between the two rooms. The highest recorded temperature of all of the participants was 102.4 Fahrenheit, which is just over the 102 F or 38.9 C that we prefer to see it during pregnancy.

In a room heated to a temperature higher than 92 degrees F, or 33.3 C, it is possible you could see more of an increase in core temperature but I cannot find any research showing that. For more information on practicing yoga during pregnancy check out the episode on Prenatal Yoga.

Hot Tubs & Saunas

There is evidence of an increased risk of neural tube defects among fetuses exposed to excessive heat during the first trimester. This study was primarily focused on hot tubs and saunas and was from the early 1990’s. It is generally recommended that you limit hot tub use to less than 15 minutes, or avoid it altogether. Your body tends to reach a higher core temperature in a hot tub than you would in a sauna, because in a sauna or yoga studio you are perspiring which can help to lower your core temp, rather than being submerged in a hot tub.

You can take a bath during your pregnancy, just don’t make it scalding hot. You know when you are overheated. Pay attention to the temperature and how you are feeling, but please enjoy a warm bath if you would like to.

Heating Pads

Using heating pads during pregnancy should be safe, as long as you are not burning your skin. You wouldn’t want to use a heating pad on your belly and you may want limit how long you have a heating pad on your lower back, due to the proximity to your baby. As with all things pregnancy related, you can always run things by your doctor or midwife for their expertise.

Other Risks with Heat Exposure

There are a couple of other risks that can come up with heat exposure. If you are spending time outdoors you want to be careful to avoid getting a sunburn. Wear clothing or sunscreen to protect your skin and take advantage of the shade when you can. During pregnancy a sunburn will not harm your baby, but it can increase the visibility of stretch marks or make melasma (dark spots that can appear on your face, aka the mask of pregnancy) appear darker. Check out this episode for more on Sunburns and Sun Exposure During Pregnancy.

The second thing that you want to pay attention to is how much water you are drinking. Staying hydrated is one of the easiest ways to improve the health of you and your baby. In warmer weather you need more water. Make drinking water a priority, and make it easy. Take a water bottle along with you when you go out. I love my Hydroflask and I take it everywhere with me.

Talking to Your Doctor or Midwife

If you have questions about your health or the safety of specific exercises for your baby please bring them up with your doctor or midwife.


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