Objectives To study fetal wellbeing and uteroplacental blood flow during strenuous treadmill running in the second trimester.

Methods Six pregnant Olympic-level athletes in endurance events aged 28–37 years and training 15–22 h per week before the pregnancy were tested once at 23–29 weeks of pregnancy. The women ran three to five submaximal workloads on a treadmill with approximately 60–90% of maximal oxygen consumption. The maternal–fetal circulation was assessed with Doppler ultrasound of the uterine and umbilical arteries before, during and after exercise.

Results Mean uterine artery volume blood flow was reduced to 60–80% after warming up and stayed at 40–75% of the initial value during exercise. Fetal heart rate (FHR) was within the normal range (110–160 bpm) as long as the woman exercised below 90% of maximal maternal heart rate (MHR). Fetal bradycardia and high umbilical artery pulsatility index (PI) occurred when the woman exercised more than 90% of maximal MHR and the mean uterine artery volume blood flow was less than 50% of the initial value. FHR and umbilical artery PI normalised quickly after stopping the exercise.

Conclusions Exercise at intensity above 90% of maximal MHR in pregnant elite athletes may compromise fetal wellbeing.