There are two primary findings from this study. First, overall fetal well-being was reassuring after strenuous exercise in both exercisers and non-exercisers. Second, a small subset of highly active women demonstrated transient fetal heart rate decelerations and alterations in umbilical and uterine artery Dopplers immediately post-exercise.
With strenuous exercise, all participants, regardless of activity status, reported perceived exertion ratings consistent with strenuous exercise and peak maternal heart rates were similar. Umbilical artery S/D ratio, a common measure used to evaluate fetal well-being, fell within the 25th
percentile for gestational age according to reference values11
and did not significantly change with exercise. Similarly, uterine artery Doppler measures, a reflection of maternal blood flow, were near the 50th
percentile for gestational age according to reference intervals12
and did not significantly change with strenuous exercise. Furthermore, fetal heart tracings were classified as Category I and met established criteria for reactivity after exercise in all women and BPPs were reassuring. Although one participant’s time to a BPP of 8/8 was 32:57, this is not likely clinically significant.
An intriguing finding in this study is the transient fetal heart rate decelerations experienced by a subset of participants. Interestingly, each woman who experienced an immediate post-exercise fetal heart rate deceleration was in the Highly Active group. The subgroup numbers are small; thus, the data must be interpreted cautiously. It is also important to note that these decelerations were of short duration (mean 2:37 minutes), not technically meeting the definition of a bradycardia, defined as lasting 10 minutes or more.9
However, in addition to the fetal heart rate decelerations, changes were also seen in umbilical and uterine artery parameters. In the Highly Active women with fetal heart rate decelerations, umbilical artery S/D ratio increased with exercise, which is a different and potentially more concerning response. Additionally, uterine artery PI increased to the 90th
percentile according to reference data.12
This may indicate a brief relative reduction in maternal blood flow to the uterus in some women immediately after strenuous exercise. This brief alteration in resistance indices did not appear to affect overall fetal-well being, as all scored 8/8 BPPs shortly after exercise and all fetal heart tracings were reactive. Importantly, we recently reported no fetal heart rate decelerations or untoward fetal responses after vigorous exercise, defined as 60–84% of heart rate reserve,5
suggesting this response may only occur with exercise intensity over the “vigorous” threshold.
Our results are similar to a recently published study evaluating fetal well-being after strenuous exercise between 23–29 weeks gestation in Olympic-level athletes.4
In 2 of the 6 athletes, fetal “bradycardias” were noted when maternal heart rate exceeded 90% of maximum. Additionally, elevated umbilical artery PI was seen in these 2 women. Uterine artery Doppler studies determined overall flow was less than 50% of the initial value in these women. Their fetuses recovered quickly with no signs of sustained bradycardia or elevated Dopplers in the following 10 minutes. The overall conclusions by the authors were that fetal well-being may be compromised when exercise intensity exceeds 90% of maximum maternal heart rate and that uterine artery blood flow was reduced 25–60% during intensive exercise. However, similar to the current study, they recognize the sample size is small and results should be interpreted with caution. It is difficult to compare our findings to Salveson et al.4
in terms of percent of predicted maximum maternal heart rate achieved during exercise. Because they did not report maternal heart rate data, we are unable to determine how percent of maximum heart rate achieved was calculated. Using standard prediction equations in the present study, all Highly Active participants achieved over 90% of predicted maximum heart rate. Although those with fetal heart rate decelerations achieved a slightly higher percentage (95.9 vs 90.3), this was not statistically significant. Importantly, the sample sizes are small, making it difficult to draw conclusions. Additionally, the accuracy of predicting maximum heart rate in pregnant women is unclear as existing literature indicates conflicting results with most studies reporting no change in maternal maximum heart rate with pregnancy13
while others suggest an attenuation.14
In the present study, the mean peak maternal heart rates achieved by all activity groups were significantly greater than 140 beats per minute, the threshold heart rate many providers advise women not to exceed during exercise.15
Notably, ACOG removed this restriction from their exercise recommendations in 1994.16
More data are needed before we are able to provide evidence-based threshold heart rates for exercising women.
A strength of our study is that a variety of fetal well-being tests were performed, all of which were reassuring in all women after strenuous exercise. Additionally, we evaluated both exercisers and non-exercisers, which is important since exercise recommendations differ depending upon a woman’s activity status.1,2
An obvious limitation to this study is that the fetal well-being measures were not evaluated during exercise. Monitoring fetal heart rate during exercise is technically difficult and previous early investigations that reported fetal bradycardias during exercise17
likely reported artifact from exercise movement.18,19
We believe the immediate post-exercise results are a good representation of fetal well-being. It is often hypothesized that maternal hypoxemia contributes to fetal bradycardia. We did not measure maternal oxygenation status or lactate with exercise. However, if the fetus was hypoxic during the exercise, post-exercise measures would likely be nonreassuring. We did not see any nonreassuring fetal responses to exercise and the decelerations were all transient. Moreover, Salveson et al.4
did not find differences in lactate levels between the exercisers with and without fetal bradycardias. It has also been speculated that fetal bradycardias with maternal exercise20,21
may be related to maternal catecholamine release, leading to a reduction in uterine blood flow. Importantly, in these studies, similar to the current study, the brief “bradycardic” episodes appeared to be well-tolerated by the fetuses.
Another limitation to the present study is that it only involves healthy women of normal weight prior to pregnancy. Responses may be different in obese women and women with medical complications, such as hypertension or diabetes. We also only evaluated responses to strenuous exercise at one time-point in the third trimester. Responses could differ at different gestational ages. Additionally, this study was not powered to evaluate neonatal outcomes. However, all delivery data was reassuring and all deliveries were uncomplicated. All delivered at term, with the exception of two participants, one non-exerciser (36-1/7 weeks) and one highly active participant (36-6/7 weeks). Both of these neonates were discharged home on day two with their mothers.
In conclusion, overall fetal well-being is reassuring after short-duration, strenuous exercise in both active and inactive pregnant women. However, a subset of highly active women experienced transient fetal heart rate decelerations and alterations in umbilical and uterine artery Dopplers immediately after exercise. Although all of the fetuses subsequently showed reassuring fetal testing responses and the decelerations were short in duration, further research is needed on exercise in pregnant athletes to determine if an upper limit of exercise exists that, if exceeded, places their fetus at risk. Pregnant athletes, particularly elite athletes, may benefit from individualized exercise prescriptions as they may push themselves beyond a threshold where measures of fetal well-being may be compromised. However, the clinical significance of a transient “bradycardia” is unclear. Whether or not this translates into adverse neonatal outcomes is not known. No available neonatal data suggests adverse outcomes.