Table 4.

Practical Guidance for Monitoring Physical Activity Intensity During Pregnancy.

Topic	Literature	Practical guidance for providers
Relative intensity – heart rate
Accuracy	Overall, the standard error was 12 bpm for the Fox formula and 11 bpm for the Tanaka formula, indicating that approximately 95% of the estimated maximal heart rate values will fall within +/−24 and +/−22 bpm, respectively. 35	There is substantial error around age predicted maximal heart rate used to calculate intensity of physical activity. This error is even larger among older ages, Blacks, those with lower cardiorespiratory fitness, and those with higher body mass indices. At the intersection of ranges between light-to-moderate or moderate-to-vigorous intensity, errors due to individual variation could place a woman in the incorrect intensity category.
Changes At rest	Resting heart rate in pregnancy is higher than prepregnancy heart rate,45,46,47 even as early as 2-5 weeks’ gestation. 49	Target heart rate ranges for physical activity that account for resting heart rate (Karvonen) will usually be higher for a pregnant woman compared to her pre-pregnancy state starting as early as 2-5 weeks’ gestation.
Submaximal effort	During submaximal weight-bearing and nonweight bearing exercise, heart rate is higher during pregnancy compared to the nonpregnancy state.47,50	Target heart rate ranges appropriate before pregnancy will meaningfully differ during pregnancy, since submaximal heart rates are higher with pregnancy.
Upper limit	The 140 bpm limit moreso restricts (i) younger women who generally have higher maximal heart rates than older women and (ii) women with higher resting heart rate compared to a lower resting heart rate. At 140 bpm, most pregnant women would not reach vigorous intensity unless deconditioned.	Using a single number as a heart rate cap is not a useful restriction, given the decline in maximal heart rate with age and the individual variation depending on resting heart rate.
External impacts	Heart rate is impacted by environmental factors (e.g., stress, heat, and emotions), as well as certain medications, which can obscure using heart rate to guide intensity.	Learning to use perceived intensity along with heart rate is important since heart rate can be impacted by many external factors.
Relative intensity – perceived exertion
Differential misclassification	Perceived exertion may either under- or over-estimate measured intensity level.64,65	There is individual variation in perception of physical activity during pregnancy. Using perceived exertion may be better for some and heart rate may be better for others to gauge exercise intensity. A combination of both heart rate and perceived exertion may be the best way for a pregnant woman to monitor exercise intensity.
Individual variation	Changes to a woman’s physiology during pregnancy, her baseline pre-pregnancy cardiorespiratory fitness, and the type of physical activity performed may all make it difficult for women to accurately estimate perceived exertion during pregnancy.	There are many reasons why perception of physical activity among pregnant women may not match with intensity based on either heart rate or measured oxygen consumption. Intensity could be guided by the “talk test” wherein if a woman can carry on a conversation then she should not be overexerting.
Pre-pregnancy activity levels	Physically active participants have a lower level of perceived exertion during exercise than untrained participants during the same exercise at the same level of oxygen consumption.66,70,78	Women who were active before pregnancy may be more likely to underestimate their actual intensity during exercise and exceed 140 bpm.
Natural changes over the course of pregnancy	Several studies, on average, indicate that women reduce the intensity of a given physical activity as pregnancy progresses.70,73,74,75	Pregnant women may naturally, and even unconsciously, alter the intensity of a given activity as pregnancy progresses.
Perception with activity types	The type of physical activity that women engage in (i.e., aerobic and strength) is likely to impact their perception of how hard they are working.76,77,80,86	Different types of physical activities may produce different perceived responses by a pregnant woman. If a woman engaged in a specific physical activity before pregnancy, then during pregnancy she may report a lower perceived intensity compared to a woman engaging in that activity during pregnancy without prior experience.