This episode explores how various sources of heat exposure can impact your core temperature and your baby during pregnancy. Get evidence-based information on everything from hot weather and physical activity to hot tubs, saunas, and heating pads. This episode breaks down the nuance in the research to go beyond the typical conservative advice to avoid everything. Learn how to stay cool, protect your baby, and confidently navigate heat exposure. Whether the heat comes from something you can control, such as a hot yoga class, or something you cannot, like a summer heatwave, this episode will help you make informed decisions and stay safe.

Listen Now

This episode is made possible with support from our sponsors. I appreciate your support for the brands that help power this podcast.

Article and Resources

Hyperthermia and Pregnancy

Activities such as working out or spending time in hot weather can increase your body temperature. Hyperthermia refers to an elevated body temperature, typically above 102 degrees Fahrenheit (38.9 degrees Celsius), that is not caused by a fever and lasts for an extended period. For reference, the average normal body temperature is around 98.6 degrees Fahrenheit (37 degrees Celsius).

When you are pregnant, there is concern that an elevated core body temperature may increase the risk of complications, including neural tube defects, miscarriage, and other abnormalities. These risks are especially heightened during the first trimester, which is the most sensitive period for fetal development. While I always aim to reference the most recent research, ideally from the past decade, many studies on hyperthermia and pregnancy are older. For example, a 1988 study on mice showed a strong correlation between hyperthermia and neural tube defects. This connection was well established decades ago, which may explain the lack of more recent research specifically on this topic. A 2005 meta-analysis of 15 studies also concluded that maternal hyperthermia in early pregnancy is associated with an increased risk of neural tube defects.

Fevers

Your body has a temperature set point regulated by your hypothalamus, a part of your brain that acts like a thermostat. When your set point rises, such as in response to an infection, your body works to match it, resulting in a fever. The hypothalamus triggers processes such as shivering and muscle contractions to generate and conserve heat, and it can also release epinephrine (adrenaline), which further raises your body temperature. Fevers during pregnancy can be stressful. If you are looking for more details on this topic, check out the episode on Fevers During Pregnancy. It is essential to understand that heat exposure from the environment or physical activity does not cause a fever, because your internal set point has not changed. As long as you can cool down and avoid overheating, you and your baby should be okay.

Hot Weather

As outdoor temperatures rise, it is crucial to monitor your comfort and take measures to prevent overheating. This is not just a theoretical concern. Research has shown that high environmental temperatures can increase risks during pregnancy. One study found an association between extreme heat and a higher risk of stillbirth. Another study linked higher temperatures to lower birth weights.

To stay safe, consider spending more time indoors in air-conditioned spaces and limiting your time outside, especially during the hottest parts of the day. If you need to be outside, wear lightweight, breathable clothing, stay in the shade, and stay well-hydrated.

Heat Exhaustion and Heat Stroke

During pregnancy, your body is already working harder to regulate its temperature, making you more vulnerable to overheating. In high temperatures or with prolonged exposure to heat, there is an increased risk of developing heat-related illnesses, such as heat exhaustion or heat stroke. Heat exhaustion can cause symptoms like dizziness, headache, nausea, fatigue, and excessive sweating. If not addressed, it can progress to heat stroke, a serious medical emergency. Signs of heat stroke include confusion, a rapid pulse, flushed skin, and loss of consciousness. If you suspect heatstroke, seek medical attention immediately. That said, your body usually gives you plenty of warning signs long before things become serious. Paying attention to these early signals can help you avoid complications and stay safe.

Exercise in Hot Weather

Both physical activity and hot weather can raise your risk of overheating. When you combine the two, it is crucial to be mindful of your body temperature. If you live in a hot climate, consider planning outdoor workouts for early mornings or evenings when the temperature is cooler. On particularly hot days, consider exercising indoors, where you can enjoy air conditioning and better temperature control. As your pregnancy progresses, your body undergoes numerous changes, and you may need to adjust your workouts accordingly. Always pay attention to how you feel. If you start to feel too warm or think you might be overheating, slow down, take a break, or stop altogether.

Bikram Yoga

ACOG advises against hot yoga due to the risk of overheating. However, some research shows it may be safe. An American Council on Exercise study took a group of participants who swallowed a core body temperature sensor. Researchers could remotely monitor core temperatures while participants underwent a 60-minute yoga class in a room heated to 70 °Fahrenheit (21.1 °Celsius). Then, 24 hours later, the same participants underwent a yoga class in a room heated to 92°F (33.3°C).

The researchers found no difference in the core temperature increase between the two rooms. The highest recorded temperature among all participants was 102.4°F, just over the 102°F (38.9°C) threshold that we prefer to keep temperatures below during pregnancy. In a room heated to a temperature higher than 92°F (33.3 °C), it is possible you could see more of an increase in core temperature.

It is essential to note that actual Bikram or hot yoga classes may have room temperatures exceeding 100°F (38°C), which could lead to a greater increase in core temperature than observed in this study. If you are comfortable and continue heated yoga during pregnancy, please remember to leave the room and cool off if you ever feel overheated. For more information on practicing yoga during pregnancy, check out the episode on Prenatal Yoga.

Hot Tubs and Saunas

One of the earlier studies, conducted in the 1990s, demonstrated an increased risk of neural tube defects among fetuses exposed to excessive heat during the first trimester. This research originated from investigations into the use of hot tubs and saunas. As a result, most official recommendations from organizations like ACOG are to avoid hot tubs and saunas during pregnancy. Some other sources recommend limiting hot tub use to less than 10-15 minutes. It is much easier for organizations to issue blanket recommendations. As with many pregnancy safety topics, when you delve into the research, you see that there is nuance to this topic.

One study measured vaginal temperatures in 20 nonpregnant women while they sat in hot tubs, aiming to estimate the potential temperature a fetus might be exposed to. Researchers asked participants to remain in the hot tub until they either felt uncomfortable or their temperature reached 102.02°F (38.9°C). In hot tubs set to 102.2°F (39°C), participants began to exit after about 10 minutes, although five women stayed in longer than 15 minutes before reaching the target temperature. In hotter tubs set to 105.98°F (41.1°C), participants began exiting after five minutes. Six women remained in this higher-temperature tub for over 10 minutes before reaching the same temperature threshold.

Researchers also tested a sauna with an average temperature of 178.5°F (81.4°C). None of the women voluntarily remained in the sauna long enough to reach a core temperature of 102.02°F (38.9°C). Saunas allow your body to sweat, helping you cool down. Submersion in a hot tub inhibits that cooling mechanism, so your core temperature may rise more quickly in a hot tub than in a sauna.

This research highlights two key points. First, normal use of hot tubs or saunas is unlikely to raise your body temperature to levels that pose a risk to your baby. Second, you should always listen to your body. If you start feeling overheated, get out and cool down. There is a big difference between occasionally spending a few minutes in a hot tub and doing so daily for extended periods.

Lastly, it is a myth that you should not take baths during pregnancy. You can enjoy a bath, but make sure it’s not scalding hot. You will know if you start to overheat. Pay attention to how you feel, and go ahead and relax in a warm bath if it is comfortable.

Heating Pads and Electric Blankets

Using heating pads or electric blankets during pregnancy is generally considered safe, as long as you do not feel overheated. These devices provide localized warmth and are unlikely to raise your core body temperature significantly when used appropriately.

One study that included electric blanket use during pregnancy found no material association between exposure and an increased risk of neural tube defects. If you want to be cautious, consider keeping heating pads and electric blankets on a low setting and limiting their use to a short duration. If you use an electric blanket in bed, consider warming the bed before getting in and then turning it off to avoid prolonged exposure. Heating pads can be beneficial for managing localized aches and pains, such as back or round ligament discomfort. Just avoid placing them directly on your belly.

Other Heat-Related Risks

Spending time in hot weather or under direct sunlight can present other risks beyond overheating. Two key concerns to be aware of are excessive sun exposure and dehydration.

Sun Exposure

Sun exposure does have benefits, and you should feel free to enjoy it, especially during summer months. A review found that exposure to UV radiation in the first three months of pregnancy had beneficial effects on fetal growth and blood pressure during the pregnancy period. Sunlight can increase your vitamin D levels and positively impact your mood. However, your skin tends to be more sensitive to everything during pregnancy, including sun exposure. Some common skin issues during pregnancy, such as melasma and stretch marks, can worsen with exposure to the sun.

To protect yourself, consider wearing a hat to keep the sun off your face, wearing clothing that protects your skin, or spending time in the shade. If you have exposed skin, you should be using sunscreen. The American Academy of Dermatology recommends an SPF of at least 30, which blocks 97% of UVB rays. You want to use a broad-spectrum sunscreen for protection against UVA rays. You should reapply every two hours or after going in the water or sweating. For a deeper dive into safe sun practices during pregnancy, see this episode.

Staying Hydrated

When you are pregnant, your body needs more water to support increased nutrient demands, help with digestion, and transport nutrients to your baby. Water is also necessary to make amniotic fluid, support blood volume expansion, and regulate body temperature. These needs become even more critical when you are exposed to heat, as you lose additional fluids through sweating and increased respiratory rate.

The American College of Obstetricians and Gynecologists recommends drinking 64–96 ounces (1.9–2.8 liters) of water per day. If you spend time in hot weather or exercise, you should drink even more to stay properly hydrated.

Your body gives you many signs when it needs more fluids. Feeling thirsty or having a dry mouth are early clues. Other signs of dehydration include urinating less often, having dark-colored urine, headaches, fatigue, dizziness, or lightheadedness. According to ACOG, you should call your care provider if you have the following signs and symptoms of dehydration:

• You have a small amount of urine that is dark in color.
• You are unable to urinate.
• You cannot keep down liquids.
• You are dizzy or faint when standing up.
• You have a racing or pounding heartbeat.

One of the easiest ways to monitor your hydration is to observe the color of your urine. Ideally, it should be a light yellow. Evidence shows this is a reliable marker of hydration in pregnant and lactating individuals. If your urine is darker, it may be a sign that you need to increase your fluid intake. See this episode for more in-depth information on hydration and drinking water.

Listen to Your Body

Every pregnancy is different, and your tolerance to heat can change as your pregnancy progresses. Pay close attention to how you feel when you are in a hot environment. If you start feeling lightheaded, dizzy, nauseous, or overly warm, it is vital to take a break, cool down, hydrate, or move to a cooler location. These are all signs that your body may be having trouble regulating its temperature. Trust your instincts. If something feels off, do not try to push through it; instead, take a step back. Staying cool and comfortable is not just about your well-being; it also helps protect your baby.

Talking to Your Doctor or Midwife

If you have any questions or concerns about heat exposure during pregnancy, please do not hesitate to discuss them with your doctor or midwife. Your care provider can offer personalized guidance tailored to your specific needs and circumstances. Remember, many pregnancy recommendations are designed to be a one-size-fits-all, but that is not realistic. Your provider can help you navigate the nuances and find safe, practical solutions that fit your lifestyle.

Thank you to the brands that help power this podcast.