Exercise intensity while pregnant (why you’re so tired)

Posted on December 7, 2011

(Last Updated On: November 24, 2017)

So you’ve decided it’s good for you to exercise while pregnant, you know it’s alright to run, bike, and you know it’s alright for you to lift weights. How hard can you do these activities? Can you exercise too intensely while pregnant?

Beginner versus regular exerciser

Your pre-pregnancy exercise levels are going to help dictate how intense your exercise should be. If you’re just starting you are going to want to gradually up your intensity. Just like a non-pregnant person would.

If you are in good, or really good shape already, I hope it would be obvious that now is not the time you are going to try and greatly improve your fitness. You may be able to improve things a bit, but don’t expect a lot. You can definitely maintain most of what you have, if not all, but there are some changes your body is going to go through that are working against you here.

With that said though, there is really nothing to suggest that exercising too intensely will hurt you or your baby. Other than doing a select few things, which I’ll discuss, there’s not much to worry about.

Getting more specific:

Intensity regarding aerobic exercise

The following is a quote from the Guidelines of the American College of Obstetricians and Gynecologists for exercise during pregnancy and the postpartum period:

“In a meta-analysis study of exercise and pregnancy, it was reported that, with exercise intensities of 81% of heart rate maximum, no significant adverse effects were found.”

In other words, for women who exercised at 80% of their maximum abilities, which is a high intensity, no ill effects were found.

Don’t get caught up in using your heart rate as a barometer for how hard you’re working though. It is a poor tool for pregnant women.

Throughout pregnancy, and especially in the first trimester, a pregnant woman’s resting heart rate will increase due to the extra work making a baby entails. Your resting heart rate is a great indicator for how much work your heart has to do just to keep you sitting around and breathing. Overweight, out of shape people have high resting heart rates while in shape people have lower ones. Lance Armstrong is an example of someone with a very low resting heart rate.

Now everyone has a certain limit to how many beats their heart can generate in a minute. If you have a high resting heart rate you don’t’ have as many beats left over for exercise as someone with a low resting heart rate. For example:

  • Person A has a resting heart rate of 50 and maximum heart rate of 200 = 150 beats left for exercise
  • Person B has a resting heart rate of 100 and maximum heart rate of 200 = 100 beats left for exercise

Person B does not have as many heart beats available as Person A. Thus, they are most likely not going to be able to exercise as intensely as Person A.

Since a pregnant woman has a higher resting heart rate than she would normally have if she wasn’t pregnant, she doesn’t have as many beats left over for exercise as she may have been used to before conception. This is the long way of saying the longer you are pregnant the less heart beats you have available for exercise. Which means the intensity you can exercise with diminishes.

As I mentioned, this spike in resting heart rate is most prominent the first trimester. I bet this is why so many pregnant woman report feeling so tired in the first trimester but feel better in the second and third: They are getting used to all of the extra work they are doing in the first trimester but by the second they have adapted fairly well.

Also, a pregnant woman’s max heart rate capabilities decrease the more pregnancy advances. You are actually losing your ability to exercise by decreasing the amount of beats you have available for exercise and decreasing how much your heart can beat overall. Therefore, using your heart rate as a barometer becomes less and less accurate. You no longer know what your actual max heart rate is.

Just keep this in mind when you’re exercising. If it feels harder than it used to, it’s supposed to. That doesn’t mean you can’t exercise intensely. It just means your definition of intense is going to change throughout the course of your pregnancy. It’s much better to judge that intensity by how you feel than by what a number is telling you.


Intensity regarding weight lifting

More quotes. A couple I have issues with:

“Although supporting data are lacking, it would be prudent to limit repetitive isometric or heavy resistance weightlifting and any exercises that result in a large pressor effect during pregnancy. “

For those of you don’t know the “pressor effect” is another term for increasing your blood pressure.

My issue with the first quote is that any exercise can produce a large increase in blood pressure. Whether you’re lifting for 20 reps or 5 reps. The important factor is how you are loaded and how you’re breathing.

A common thing to do when someone is lifting weights and they are straining, whether it’s rep 3 or rep 20, is to hold their breath more than normal. Ordinarily this can be a good thing; it helps accomplish the lift. However, when you’re pregnant you’re better off just not going for that last rep and avoiding the breath holding all together. Holding the breath can cause even greater increases in blood pressure.

This is where there are some differences regarding intensity between a pregnant woman and non-pregnant woman: The pregnant woman is better off not shooting for those grind it out reps. Overall we’re talking about an intensity reduction of maybe 10-20%. Not much.

It’s amusing there are still doctors advocating against lifting weights while pregnant. This despite the fact the organization that is probably providing their guidelines, the one quoted above, advocates resistance training. And even though they say only lift light weights, they are saying this despite admitting the fact “supporting data are lacking.” Basically any doctor saying you shouldn’t lift weights or shouldn’t lift anything heavy is giving their completely unscientific guess.

Another quote:

“The use of lighter weights to avoid overloading joints that are loosened by the hormones of pregnancy, along with more repetitions will assist in maintaining muscle mass without stressing the joints. “

This is a bogus rationale for not lifting “heavy” while pregnant. First off, women’s joints are notoriously lax to begin with (compared to men). Does that mean men should lift weights while women shouldn’t? Of course not.

Second, if anything strength work is going to help this situation. Lax ligaments / tendons / joints are typically weak. Like I said, women are already lax. And no offense, but women are typically already weak. Strength work can help make sure the joints don’t become too lax as well as strengthen the surrounding muscles to help offset the weaker ligaments and tendons. This is one reason why strength work helps prevent knee injuries in women: Women need that extra strength more than anyone.

Third, light weights and high repetitions don’t maintain muscle. If you already have some decent strength and muscle on you you’re going to need to lift at least 80% of what you’ve been doing in order to maintain that. Going to the pink dumbbells and doing all sets of 15 is only going to cause you to lose that hard earned muscle you built. Giving you even more work to do once you have your baby and you’re trying to get back to your normal body.

Finally, there has been a bunch of research done on high level athletes while pregnant. There hasn’t been any noteworthy differences between those women who train very hard even while pregnant, and women who don’t. If high level athletes don’t need to worry too much about how much weight they’re lifting I doubt the average (no where near as strong) mom-to-be does.

Listen to your body

Another quote:

“Pregnancy induces profound alterations in maternal haemodynamics. Such changes include an increase in blood volume, heart rate, and stroke volume as well as cardiac output, and a decrease in systemic vascular resistance.” 

Here are some of the things exercise does to the body: Increases blood volume, heart rate, and stroke volume. As as well as cardiac output, and it decreases vascular resistance.

Just being pregnant is an exercise in itself! Again, a lot of these changes are most prominent in the first trimester. Think of yourself as exercising all day once you get pregnant. Even just sitting around has become an exercise for your body. No wonder you’re so tired!

This is important because exercise is a stressor too. If you’re completely exhausted you are going to want to scale back your exercise intensity. Listen to how you’re body is feeling!

While exercise can be a great way to deal with stress, you don’t want to be exercising to the point where it becomes an additional source of stress in your life. Do this often enough, long enough, and now we’re talking about a detrimental influence to you and your baby.

BUT…don’t use this as an excuse to do nothing. There are too many benefits of exercising while pregnant to ax it completely.

This is where having someone else with you while exercising can be very beneficial. Someone who knows you well, like a workout partner, spouse, your trainer, is going to know by looking at you how tired you already are and can help you give you a more objective opinion on how intense things should be. Plus, having somebody else with you can make the exercise seem like less of something you just have to check off for the day and more like something you enjoy.

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Posted in: Pregnancy