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04-03-2012 | Gynaecology | Article

Fetal wellbeing ‘compromised’ by strenuous exercise

Abstract

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MedWire News: Strenuous exercise may compromise fetal wellbeing, suggest findings of a study on pregnant elite athletes.

Lead study author Kjell Salvesen (Trondheim University Hospital, Norway) recommends that athletes should stay below 90% of their maximal heart rate when pregnant.

"At higher levels we observed signs of fetal distress (bradycardia) if a maximal maternal heart rate (MHR) of more than 90% occurred simultaneously with a 50% reduction of volume blood flow to the uterus," he told MedWire News.

Pregnant women are recommended to be physically active during pregnancy, but to stay below a heart rate of 140-150 beats per minute (bpm), he explained. However, elite athletes must train at higher intensities if they want a quick return to their sport after pregnancy.

As there are few studies exploring the intensity levels of training which can be considered safe for the fetus, Salvesen and co-investigators examined the effects of strenuous treadmill running on fetal wellbeing in six pregnant Olympic-level athletes. The women were aged between 28 and 37 years and were training for 15-22 hours per week before the pregnancy.

After warming up for 10 minutes the women, who were 23-29 weeks pregnant, ran three to five submaximal workloads on a treadmill, with a range of 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.

The researchers report that 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) remained within the normal range (110-160 bpm) as long as women exercised below 90% of maximal MHR. But when the mean uterine artery blood flow was less than 50% of the initial value and the exercise intensity was greater than 90%, fetal bradycardia (FHR <110 bpm) occurred.

After 10 minutes of rest, the mean volume blood flow returned to approximately 90% of the initial value in four of the women. Recovery of blood flow was slightly slower in two women.

"A limitation of the study is the small sample size," note the researchers in the British Journal of Sports Medicine. "We do not know if the study results can be generalized to other groups of women who may be physically fit and train regularly, but do not have 'elite' status."

Nevertheless, Salvesen thinks that the current recommendations to stay below a MHR of 140-150 bpm are probably "too conservative." Women may exercise at higher intensities without risk of fetal compromise, he said. However, they should stay below a heart rate maximum of 90%.

"More studies of both elite athletes and physically active pregnant women are needed to establish intensity values above which exercise is potentially harmful to the fetus," concludes the team.

By Nikki Withers

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