You might envision pregnancy as a time when your body slows down and becomes less capable given that your physiological resources are directed toward growing a baby. However, you may be surprised to learn that you actually develop an increase in capacity. Specifically, your cardiorespiratory system develops almost 50% greater capacity by the end of pregnancy (Klein and Pich, 2003).
In other words, pregnancy causes a natural “doping effect” (increasing blood oxygen saturation) that gives people a training advantage.
This is an astonishing adaptation that, if you choose to, can be taken advantage of to increase your aerobic system’s functioning. Let’s explore how this pregnancy change happens and how you can safely push your body in order to capitalize on this unique window of time, should you wish to do so.
How the Cardio-Circulatory System Changes During Pregnancy
Cardiac output is the number of times your heart beats per minute multiplied by how much blood is pumped with each beat. During pregnancy, cardiac output increases due to 2 main adaptations:
- The heart pumps more blood with each beat (about 40% more by the end of the third trimester)
- The heart beats more times per minute (about 15-20 beats more per minute by the end of the third trimester) (Clapp, 1998; Artal, 1992).
This increase in cardiac output confers an aerobic advantage to those expecting. In fact, some athletes have achieved personal records during pregnancy due to this adaptation.
The advantage is typically felt most during the second trimester – after the fatigue and/or nausea of the first trimester, and before third trimester weight gain.
It is important to note, however, that just because pregnant people have this added capacity does not mean that every person can, or will want to, take advantage of it. There are other factors that may mitigate this adaptation (pain, injuries, fatigue, etc.).
How You Can Safely Utilize This Increase In Cardiac Output
If you are interested in taking advantage of this unique opportunity to increase your aerobic system’s functioning, how do you ensure you do so in a safe way? While you might consider monitoring your heart rate, this is actually not a recommended strategy during pregnancy. Why?
Heart rate is NOT an accurate way to gauge exercise intensity during pregnancy.
There are a few reasons for this:
- Resting heart rate progressively increases throughout pregnancy
- Heart rate response to exercise is variable throughout pregnancy, and from one individual to another
- Blunted, exaggerated, and normal linear responses may all be seen at different stages during the same pregnancy (ACE, 2011)
Unfortunately, the guideline to “keep your heart rate below 140 beats per minute” is still one of the biggest prenatal exercise myths out there (and regrettably, we hear it most often from doctors). This is actually an outdated guideline from the American College of Obstetricians and Gynecologists (ACOG) from 1985, which was developed when almost no research existed about exercise during pregnancy.
ACOG eliminated the heart rate guideline in 1994 (due to research findings on heart rate variability during pregnancy) and replaced it with the guideline to use Rate of Perceived Exertion (RPE) instead. RPE relies on listening to your body and gauging your exertion by how you feel, which is a much more accurate method.
So, how do you use RPE? While there are different RPE scales, we like to use a 1 – 10 scale because it’s the simplest:
- 1 = Extremely Easy: This is minimal-to-no exertion, like gentle walking.
- 10 = Extremely Difficult: This is maximal exertion, like sprinting or lifting something so heavy you could only do 1 repetition.
In general, a good guideline for pregnancy is to work between a 6 – 8 on the RPE scale. This means that on a scale of 1 – 10 (with the parameters noted above), you work in what feels like a 6 – 8 range for YOU. If you are accustomed to higher intensity exercise, you can go up to an 8.5, provided that feels ok for you. There is no research to date at RPE levels higher than 8.5.
A Few Important Tips on Using RPE
First, it is important to note that you should always follow your doctor’s guidance.
Second, remember that RPE is all about how you feel. Therefore, this will vary highly by individual and for you during different stages of your pregnancy (and even from day-to-day). For example, given this increased aerobic capacity, you may be able to do more than you could in the past while not feeling as winded. However, as your pregnancy progresses, increased weight gain and other changes will make movements that felt like a lower intensity earlier on feel much higher. In other words, those same squats that felt like a 6 in your first trimester may feel like a 7 or 8 by your third.
Bottom Line: If you listen to your body, and stay within what feels like a 6 – 8 (up to 8.5 for those accustomed to higher intensity work), then you will be within a safe range.
Training Programs and Other Resources
If you are interested in safe and effective total body workouts for all stages of pregnancy, check out our prenatal training programs and services. Or, if you are a health and fitness professional interested in learning how to coach pre & postnatal clients, explore our pre & postnatal professional education.
ACOG Committee on Obstetric Practice (2020, April). Physical Activity and Exercise During Pregnancy and the Postpartum Period. American College of Obstetricians and Gynecologists. Opinion 804.
American Council on Exercise. (2011). ACE Group Fitness Instructor Manual: A Guide for Fitness Professionals. San Diego, Print.
Klein, H.H., & Pich, S. (2003). Cardiovascular changes during pregnancy. Herz, 28 (3).173-174.