Wondering Why Your Output Is Lower During Pregnancy? Here’s Why—and What it Means for You and Your Baby

Your performance might be waning, but you’re actually improving your cardiovascular fitness and health with each workout—and so is your baby.

By Aimee Layton, PhDUpdated June 2, 2022


Along with the myriad changes that come with pregnancy, you might have trouble hitting your usual output on the Peloton Bike or Peloton Tread, despite putting in the same effort. I get how frustrating that can be. During both of my pregnancies, it took all I had to stick to my workouts. I was more tired than normal, but I showed up, which made it so discouraging when my output on the Bike didn’t match my perceived effort.

As an exercise physiologist, I had to remind myself of an important fun fact most expectant mothers don’t know: Even if it feels like you’re not at your peak of exercise performance when you’re pregnant, you’re still improving fitness and teeing yourself up to come back stronger after you give birth.

Your Cardiovascular System, Explained

To help you understand why your output, or other measurements of fitness efforts, may be lower when you’re expecting—and why those metrics might not actually matter—it’s important to have a basic understanding of how your heart works.

For starters, with every beat, the left ventricle of the heart fills with blood and pushes it out to circulate throughout your body. The left ventricle is made to get bigger and stronger if stressed; it’s built to stretch and grow to accommodate demand. When you exercise, more blood is pumped from your muscle back up to that left ventricle, so it fills up with more blood. As exercise demand increases, your heart also pumps faster (read: your heart rate increases). Between pumping more volume with each beat and pumping faster, that left ventricle gets stronger. The result is a boost to your cardiovascular fitness.

When you’re pregnant, the amount of blood in your body (aka, your blood volume) rises to meet the demands of your growing baby. This increased blood volume means you’re pumping a lot more blood through the left ventricle of your heart each time your heart beats.

The outcome? When you exercise while pregnant, your left ventricle stretches even more and needs to contract with more force. This result is your cardiovascular system getting stronger despite your power output or speed staying the same or even decreasing a little. Added bonus: Research shows these cardiovascular gains that occur during pregnancy last up to a year after your baby is born if you continue exercising.

Exercising During Pregnancy Is Good for Your Baby Too

When you work out, blood is shunted away from certain parts of the body, like the digestive system. It’s part of your body’s MO to conserve energy where it can to make sure it can meet the demands of whatever you’re doing. However, there are two organs that require a consistent volume of blood even if you push your body to its limits: your brain and the placenta.

Blood flow to the placenta doesn’t decrease when exercising during pregnancy. In fact, it can increase. Your growing baby may see a surge in blood flow when you exert yourself because of your heart increasing cardiac output (described above). Combine that with the added blood volume circulating throughout your body during pregnancy and the result is more blood going to your baby when you exercise relative to what the baby gets when you are not active.

You can think of it as giving your baby a little boost because that extra blood flow is super beneficial. You’re sending more oxygen and nutrients—all the good stuff that travels in the bloodstream—to your baby. This is why numerous studies show exercise during pregnancy is beneficial for both the mom and fetus during gestation, with benefits that persist for the mom up to a year after giving birth, if she continues exercising, and for the child.

Another research review found that exercise during pregnancy can also help prevent excessive weight gain that might lead to health problems, reduce the risk of maternal high blood pressure, and lower the likelihood of a cesarean section and newborn breathing problems. (While a lot of research supports exercising during pregnancy, it’s important to speak to your doctor and get clearance to exercise.)

OK, So What Gives With the Lower Output?

Think of your growing baby as another muscle. Just as your heart has to increase how much blood goes to your legs and glute muscles when you’re on the Peloton Bike or Peloton Tread, it also has to send blood to your placenta. In this case, however, instead of feeding your muscles all that extra blood in circulation, it’s also feeding your baby. The tradeoff is that your legs may tire sooner, which may lower your total output.

What’s more, you may see a higher heart rate than you did pre-pregnancy. This is due to that increased blood demand on your heart to feed both the baby and your body. Your ticker may have to pump faster than usual to accommodate the extra fluid. (Endurance athletes tend to be an exception; their hearts have been trained to pump so efficiently that their heart rates may stay the same or even dip during pregnancy.) There’s a good chance that seeing a surge in heart rate or feeling your heart pumping faster will prompt you to take your effort down a notch, which will also reduce your output.

As your baby gets bigger in your second and third trimesters, you’ll also find your respiratory rate (how quickly you breathe) increases. You’ll move the same amount of total air, but you’ll do it by taking more shallow breaths rather than taking deeper ones. That’s because your baby is positioned against your diaphragm, making it harder for it to expand and contract. This means you may have trouble moving as much volume of air per breath, so your body compensates by increasing the rate of breaths you take to still move the same amount of air. This may lead to your being more aware of your breathing or feeling short of breath earlier in your workout than you used to.

So next time you’re breathing heavily during a workout when you’ve got a baby on board, it may simply be because you have to accommodate and sustain a tiny developing human inside you.

3 Tips for Getting the Most Out of Your Perinatal Workouts

Once you understand the benefits of exercising during pregnancy for both you and your baby, it can be a big motivator. Here’s my best advice for exercising when you’re expecting, once you’ve obtained clearance from your doctor.

1. Aim to stick to your pre-pregnancy workout schedule for as long as possible.

If you don’t feel as strong as you did pre-pregnancy or if your spot on the Leaderboard has dropped significantly, remind yourself that your cardiovascular conditioning is still improving. In fact, after you give birth and your doctor clears you to exercise again, you may notice that your endurance is better than it was pre-pregnancy. That’s because if you followed an exercise regimen during pregnancy, you likely strengthened your cardiovascular system. As a result, you’ll see the benefits of those gains now that your heart is super-efficient at pumping blood throughout your body and you don’t have to share that blood with a placenta. Just remember that if you are breastfeeding, you will need more water/fluids when you work out to balance the loss of fluids that are going to the baby in the form of breast milk.

2. Cut yourself some slack if you simply can’t stick to your pre-pregnancy workout schedule.

Pregnancy can make you feel more fatigued than you thought was humanly possible. If you’re experiencing this, try not to beat yourself up. Simply continue to keep moving as much as you can, even if your workout is very different from your pre-pregnancy days. Often you will feel tired before you exercise and have more energy afterward, so working out may actually give you a boost. Some of my patients find they can cycle throughout their entire pregnancies, but can’t run; others may be uncomfortable biking, but can happily walk or run on the Peloton Tread. During the third trimester of my pregnancy, I used the Peloton App and took Peloton Bike classes while I was on an elliptical machine. My belly was so big, I couldn’t reach the handlebars on the Bike! It was a great way to get the motivation and music from my favorite instructors and also adapt the workouts so they suited my needs. Whatever you do, listen to your body, and make adjustments to your best-laid training plans accordingly.

3. Focus on having a healthy pregnancy.

When you realize that exercise gives both your baby and you a major health advantage, it can be a big inspiration to follow through with an exercise plan, even when you’d rather sit on the couch. That said, I know it won’t be easy. Exercising when you’re pregnant can feel like a slog, and you may not look forward to your workouts like you used to. Your doctor may also suggest that you scale way back or stop exercising altogether in certain situations. And that’s OK. Remember that giving your body rest and caring for your mental well-being is incredibly important when it comes to having a healthy pregnancy. However, if you can muster up the time and the energy—and as long as you’re feeling good, staying hydrated and have the green light from your doctor to keep going—you may just find yourself stronger and fitter than ever after your bundle of joy arrives.

Aimee Layton, PhD is a Peloton Member and paid consultant member of the Peloton Health & Wellness Advisory Council. Dr. Layton is an assistant professor of Applied Physiology in Pediatrics in the Division of Pediatric Cardiology and the Director of the Pediatric Cardiopulmonary Exercise Laboratory at Columbia University Medical Center / New York Presbyterian Hospital, where she plays an important role as one of the lead exercise physiologists in testing, exercise counseling and research.

This content is for informational and educational purposes only and does not constitute individualized advice. It is not intended to replace professional medical evaluation, diagnosis, or treatment. Seek the advice of your physician for questions you may have regarding your health or a medical condition. If you are having a medical emergency, call your physician or 911 immediately.