Is fetal well-being jeopardised during high-intensity interval training?

Objectives To explore the acute effects of high-intensity interval training on fetal heart rate (FHR) and uteroplacental blood flow. Method Elite (n=10) and recreational athletes (n=50) participated in an experimental laboratory study involving 5×5 min intervals of high-intensity exercise on both a treadmill and cycle ergometer, with a 4 min pause between intervals for measurement of FHR and uteroplacental blood flow. Target intensity was 17 on Borg’s rating of perceived exertion (RPE) scale and 90% of estimated maximal maternal heart rate (MHR max ). Results Mean exercise intensity was 16.4 (SD 1.0) RPE and 89.4 (SD 3.8) % of MHR max during running and 16.0 (SD 1.0) RPE and 84.6 (SD 5.0) % of MHR max during cycling. Mean FHR was 140.9 (SD 27.4) beats per minute (bpm) during pauses between running bouts and 148.9 (SD 16.0) bpm during pauses between cycling bouts. Six cases of prolonged fetal bradycardia (<100 bpm for >3 min) occurred during running, leading to the termination of exercise. The mothers exercised at 17–18 on Borg’s RPE scale and 86.4%–92.6% of MHR max in these cases. All cases were normalised within 8 min of ending the protocol. There were no cases of prolonged fetal bradycardia during cycling. Exercise was terminated for three women during running and two women during cycling due to fetal tachycardia (≥180 bpm for >4 min during rest). Conclusion Despite cases of fetal bradycardia and tachycardia, our results indicate that highly active women can engage in multiple intervals of high-intensity interval training without apparent harm to fetal well-being.