There is a long-standing belief of a high risk of overheating during pregnancy because pregnant women have a higher core body temperature than non-pregnant women. This is particularly a concern when exercising in hot weather. However, recent research shows pregnant women may not be at a greater risk for overheating after all though they are for dehydration. I caution my pregnant athletes to pay extra close attention to how they are feeling when running for longer than 45 minutes in temperatures above 85 degrees.
Google running in hot weather and pregnancy and you’re likely to be terrified! There is a lot of information out there that makes it seem as if you ran for a minute in the heat, you will seriously harm your baby.
I was incredibly scared to run while pregnant in hot weather—especially at higher intensities or for longer than half an hour.
The truth is there is not a lot of research on pregnant women exercising in the heat, especially on trained pregnant women. Pregnant women have been assumed to be vulnerable to extreme heat because ambient heat exposure has been linked to pregnancy complications including preterm birth and low birth weight. But the physiological reasons why have not been well studied.
New research is showing that pregnant women can regulate their body temperature better than previously thought and therefore safely withstand exercising in heat for longer than believed.
I dive into the research and talk with runner and OB-GYN Dr. Jaclyn van Nes about running while pregnant in hot weather.
Reminder: I am a running coach, not a doctor, so talk to yours about your running plans, always!
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What Outdoor Temperature Is Too Hot for Pregnancy?
As a running coach, I advise extra caution for my athletes, including my pregnant athletes, when running in temperatures above 85 degrees for longer than 45 minutes.
But how hot is too hot for pregnancy? There is no specific outdoor temperature for pregnant women to avoid outside as every pregnancy and every BODY is different. There’s also a lack of research on athletic women exercising during their pregnancy in heat. For these reasons, there’s no hard and fast rule as to what is too hot for a pregnant woman to exercise and when is too long. Thus, I have my own rules.
One of the greatest risks for running in the heat during pregnancy is dehydration, says Dr. van Nes–especially during the second trimester. “I usually say to patients, think of your kidneys as working twice as fast! This can lead to getting dehydrated much more quickly than usual and more frequent electrolyte imbalances,” she explains. “Therefore, it’s especially important to push the fluids and make sure you’re getting enough electrolytes.”
Related: The Complete Guide to Running While Pregnant
- That said, a recent study found that pregnant women can exercise up to 45 minutes of moderate-intensity in 90 degrees Fahrenheit and 45-degree relative humidity with minimal risk of overheating.
- Another study found that women pregnant women can safely engage in exercise for up to 35 min at 80%–90% of their maximum heart rate at 25°C (77 degrees Fahrenheit and 45% relative humidity.
Because pregnant women have a higher core body temperature, it may be easier for that temperature to rise above 102.2 degrees which is dangerous for pregnant women and their babies.
The mothers are at risk for heat stroke, heat exhaustion, and preeclampsia. The babies are at risk for increased risk of preterm and early term birth, low and decreased birth weight, birth defects, stillbirth, high blood pressure, and harmful newborn stress.
Everyone adjusts to heat differently in a process called thermoregulation. Because our body temperature is 98.6 degrees and about .2 degrees Fahrenheit higher during pregnancy, it is possible for our core body temp while running pregnant to rise to 103.
Related: Can You Run a Marathon While Pregnant?
Signs of When to Stop Running During Pregnancy in the Heat
I know this sounds scary and risk to run in the heat. However, the good news is that your body is a good communicator. It will send you overt signs of heat stroke and heat exhaustion that you are getting too hot.
- dizziness and confusion
- loss of appetite
- nausea
- excessive sweating
- pale, clammy skin
- muscle and stomach cramping
- fast heart rate and breath
- intense thirst.
Running, Heat and Pregnancy in First Trimester
There is potentially an increased risk for birth defects if running in the heat while pregnant during your first trimester.
This is primarily due to two reasons. First, some women do not feel the effects of their pregnancy yet and therefore may push their limits, raising their core body temperature above the 102.2-degree mark. As our pregnancies progress, body weight can lead to a higher Rate of Perceived Exertion (RPE) and a corresponding higher heart rate. This means we are working harder to maintain a slower pace. This lowers our rate of overheating as we enter the second and third trimesters.
Related: Baby Benefits of Running While Pregnant
Second, our pregnant bodies are still figuring out how to regulate temperature in the first trimester, and therefore not as able to keep cool. One study shows that our bodies are smart and figure out how to adapt to pregnancy and therefore are at a lower risk of overheating as the pregnancy progresses. In fact, peak temperature decreased from 8 weeks to 37 weeks while exercising at 64 percent VO2 max.
Related: Best Belly Bands for Runners
Running, Heat and Pregnancy in Second Trimester
If you are running in the second trimester, awesome! Bear in mind that while your risk for overheating may lower—your body is getting hotter (and bigger) and thus, running may feel harder.
Research shows women exhibited on average, a 7-8 percent increase in heat production per trimester of pregnancy which can be accounted for by weight gain. This heat increase does not mean you are at greater risk for birth defects or health concerns. In fact, your body is better able to thermoregulate in the second trimester.
Your risk for dehydration during pregnancy peaks in the second trimester, notes Dr. van Nes. So be sure to be drinking your electrolytes (at least 400 mg of sodium before, per hour of running, and after–more if it is really hot).
The natural progression of your pregnancy will have a reverse progression of your exercise program. You’ll likely be running slower and shorter, which will help keep your body temperature lower.
So, what can you do? Read the tips below to stay safe when running in hot weather and pregnancy.
Related: Your Pregnant Running Nutrition Guide
5 Tips to Stay Safe When Running in Hot Weather and Pregnancy
Run in the morning.
Avoid running in the hottest part of the day from 10 a.m. to 4 p.m. The best time to run is in the morning before the sun rises or in the evening after the sun sets. Seek shady routes if you are running as the sun rises.
Adjust your effort—and your schedule.
This is the golden rule for running in heat while pregnant or NOT pregnant. Adjust your effort so that you stay within your prescribed intensity bounds. Walk as needed. Also, consider changing your off day and consider something like yoga or swimming instead.
Hydrate!
Hydration is so important while running pregnant. I go into detail about your hydration and electrolyte needs while pregnant here. But ensure you are running with an icy water bottle, pre-hydrate before you run, drink every mile during your run, and replenish what you lost after your run.
Related: Runner’s Hydration Guide: from an RD
Dress light.
Aim to wear as little clothing as possible and what clothing you do wear—make sure it is light, breathable, sweat-wicking, and light-colored.
Leverage ice.
Put ice in whatever you can including your water bottle, your sports bra, your hat, or your hydration vest. Also, invest in a cooling towel which can work wonders in keeping your core body temperature down.
Check out my 21 tips to beat the heat here!
Beat the Heat
Hot weather and pregnancy mean doubling down on what runners already should do—paying attention to their bodies. Pushing our bodies to the red line while running in the heat is never a good idea and comes with risks—this is especially true when running in hot weather while pregnant.
If you are experiencing a heatwave when pregnant, look for other options for running including on a different day, or secure access to a treadmill from a gym or friend’s house. The risks are not worth it.
Working a running coach such as myself can also help you adjust your schedule so that you continue to work towards your goals while staying safe.
In most cases, the benefits of exercising while pregnant far outweigh the risks so someone like me can help you keep working on your health goals for you and your baby!
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