Six cases of prolonged fetal bradycardia occurred during high-intensity treadmill running in pregnant athletes but resolved within 8 minutes postexercise without lasting effects, according to a recent study.
High-intensity interval training (HIIT) wasn't associated with sustained harm to fetal well-being in a study of pregnant athletes. In the study, researchers investigated fetal responses to acute high-intensity exercise during pregnancy.
Lead study author Emilie Mass Dalhaug, of the Norwegian School of Sport Sciences in the Department of Sports Medicine in Oslo, and colleagues evaluated 60 healthy pregnant athletes—10 elite and 50 recreational—between 26- and 35-weeks’ gestation. The participants completed two laboratory sessions involving five 5-minute bouts of high-intensity treadmill running and cycling, with 4-minute recovery intervals. Exercise intensity targeted a rating of 17 on the Borg Rating of Perceived Exertion (RPE) scale and 90% of estimated maximal maternal heart rate (MHRmax). Fetal heart rate (FHR) and uteroplacental blood flow were assessed during rest periods using Doppler ultrasound.
During treadmill exercise, the mean RPE was 16.4 and the mean maternal heart rate was 89.4% of MHRmax. During cycling, the mean RPE was 16 and maternal heart rate averaged 84.6% of MHRmax. Mean FHR was 140.9 beats per minute (bpm) during recovery from running and 148.9 bpm during cycling recovery. Six instances of prolonged fetal bradycardia—defined as FHR below 110 bpm for more than 3 minutes—occurred during running. These cases involved maternal effort levels of RPE 17 to 18 and MHRmax ranging from 86.4% to 92.6%. Each episode resolved within 8 minutes following exercise cessation.
No prolonged bradycardia events were recorded during cycling. Fetal tachycardia, defined as FHR of 180 bpm or greater for more than 4 minutes, prompted exercise termination in three running sessions and two cycling sessions. All cases were transient and resolved without medical intervention.
Among participants, without prolonged FHR changes, mean FHR increased by 6.9 bpm during running and 7.6 bpm during cycling. Uterine and umbilical artery pulsatility index values remained within normal ranges throughout the study.
The researchers concluded that while transient fetal heart rate deviations occurred, healthy pregnant athletes tolerated acute HIIT sessions without sustained adverse effects on fetal well-being. These findings contributed important data to informing evidence-based exercise guidelines for pregnant athletes.
The authors reported no conflicts of interest.