Exercise Intensity During Pregnancy: How Much is Too Much?

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Welcome to our blog series Bad Prenatal Fitness Advice in which we look at the most common recommendations given to pregnant women about exercise and call out the good, the bad and the seriously wrong. This series is about learning the history of these recommendations, chasing down the research and boiling it down to what you really need to know about safe, healthy exercise during your pregnancy. 

As always, make sure you are cleared to exercise during pregnancy by your doctor. See our post about times when exercise during pregnancy is not safe or appropriate.

Last time, we discussed the recommendation to never exercise flat on your back. Up next, we’re examining common language used around exercise intensity and pregnancy.


Bad Prenatal Fitness Advice: “Don’t Overdo it”

Why this Advice is Bad

It is so vague that it is entirely meaningless. It provides nothing that informs or empowers a pregnant woman making decisions about her own health and wellness. What does overdoing it refer to? Does it refer to heart rate, soreness, injury, post-workout energy levels or some other metabolic metric like blood glucose or cortisol? How do you know if you’ve overdone it? Are there signs or symptoms to look out for to signal that you’re “overdone?” What happens if you accidentally overdo it? What are the consequences?


Background and Research

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There is currently no well-defined safe upper limit when it comes to exercise intensity. This is because the overwhelming majority of research on prenatal exercise looks at women who exercise at a low to moderate intensity- defined as 30-60 minutes, 2-4 times per week. The wealth of information about moderate exercise during pregnancy definitively shows that it’s perfectly safe and richly beneficial. For this reason, moderate exercise has been defined as the standard guideline by the American College of Obstetricians and Gynecologists (ACOG).  Any doctor who chooses to not follow ACOG recommendations for her very active or competitive pregnant athlete assumes some amount of additional liability. 

Many recommendations suggest using the talk test or keeping your heart rate below a certain beat per minute. This doesn’t take into consideration the physiological changes the body undergoes during pregnancy. With less space in the abdomen for lung expansion, many pregnant women are out of breath walking from their cars to our front door. That hardly counts as overdoing it despite failing the talk test. There are abundant studies done on pregnant women reaching heart rates of 64-85% predicted maximum and having no adverse outcomes. This exercise intensity would exceed the talk test rule for many women. So it’s not always a very useful metric. 

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There are two very small studies that examine the effect of very strenuous exercise on fetal well-being. One study was comprised of pregnant Olympic-level athletes. The second included competitive athletes, moderate exercisers and sedentary women. Both studies show that fetal heart rate (FHR) experienced decelerations when the mother reached >90% maximum heart rate on a peak treadmill test. Interestingly, only the competitive athletes experienced FHR decelerations even though the moderate and sedentary test groups all achieved >90% reserve. All babies recovered quickly and there were no adverse fetal outcomes.

These studies were so small that it’s hard to draw conclusions. It may suggest that elite athletes are able to exert at a level much higher than the average woman, and that level may be above what a fetus can comfortably tolerate. But the average exercise enthusiast, even working at >90% capacity experienced no negative fetal outcomes, temporary or permanent. 


The Takeaway

Unless you are a professional or elite athlete, it’s unlikely you should be concerned about exercise intensity and your baby’s well-being.  Some days you may feel like getting your heart rate up and getting a little breathy. That’s okay to do! Some days you may feel like taking it slow and easy. That’s great too! 

Reaching >90% maximum heart rate is extremely difficult to achieve without sprinting or jumping, so it’s unlikely you’ll be able to reach that intensity, or even feel like doing so during pregnancy. Frankly, we don’t recommend sprinting or jumping during pregnancy, not for the safety of the baby, but for the health and longevity of your pelvic floor.  So enjoy your workout with peace of mind.


References

Beetham KS, Giles C, Noetel M, Clifton V, Jones JC, Naughton G. The effects of vigorous intensity exercise in the third trimester of pregnancy: a systematic review and meta-analysis. BMC Pregnancy Childbirth. 2019 Aug 7;19(1):281.

Petrov Fieril K, Glantz A, Fagevik Olsen M. The efficacy of moderate-to-vigorous resistance exercise during pregnancy: a randomized controlled trial. Acta Obstet Gynecol Scand. 2015 Jan;94(1):35-42. 

Szymanski LM, Satin AJ. Strenuous exercise during pregnancy: is there a limit? Am J Obstet Gynecol. 2012 Sep;207(3):179.e1-6.

ACOG Committee Opinion No. 650: Physical Activity and Exercise During Pregnancy and the Postpartum Period. Obstet Gynecol. 2015 Dec;126(6):e135-42. 

Salvesen KÅ, Hem E, Sundgot-Borgen J. Fetal wellbeing may be compromised during strenuous exercise among pregnant elite athletes. Br J Sports Med. 2012 Mar;46(4):279-83.