Fetal wellbeing may be compromised during strenuous exercise among pregnant elite athletes

Salvesen, KA, Hem, E, Sundgot-Borgen, J. Br J Sports Med. 2012 46:279-283.

The recommendations for exercise in pregnancy have changed during the last few decades. Exercise has gone from being something pregnant women were supposed to avoid to now being recommended as an integral part of a healthy pregnancy. Moderate physical activity is generally accepted as safe during uncomplicated pregnancies; however, there has been very little research into the effects of strenuous exercise on maternal or fetal health. This study aimed to measure the impact of strenuous treadmill running on fetal wellbeing and uteroplacental bloodflow during the second trimester. Six pregnant Olympic-level endurance athletes took part in the study. The women were asked to run three to five submaximal workloads on a treadmill of 60 to 90% VO2 max. Uteroplacental bloodflow was assessed with Doppler ultrasound of the uterine arteries, before, during, and after testing. Maternal heart rate (MHR) and fetal heart rate (FHR) were recorded throughout the study. Maternal blood lactate levels (a measure of anaerobic metabolism) were also analyzed. The results showed that FHR remained in a normal range as long as maternal heart rate was < 90% of maximal MHR. When 2 women exercised at an intensity that resulted in maximal MHR > 90%, substantial drops in mean uterine artery blood flow were seen (less than 50% of the initial value) and in both cases, the fetuses experienced bradycardia (slowed heart rate; a sign of fetal distress) which resolved on stopping exercise. In 4 women, blood lactate was elevated at > 3 nmol/l; however, this did not seem to be correlated with fetal distress as only the 2 women with maximal MHR > 90% and uterine volume blood flow < 50% experienced fetal bradycardia.

This study is remarkable because it is one of the few studies to look at the physiologic effects of strenuous exercise in pregnant women. Although the study size was small (lets face it, the number of women who are willing to exercise at an intensity of greater than 90% maximal MHR during pregnancy is not a large number), it is important because it provides us with some specific data for a population of pregnant women (i.e., elite athletes) who have previously been treated by assumption and educated guessing. Most of the previous studies on exercise in pregnancy were done in sheep. During exercise, blood is shunted away from organs, including the placenta, to the working muscles. It is assumed that the placenta responds to this relative oxygen deprivation by developing a system of collaterals to increase blood flow both during and after exercise. This is much the same mechanism that the heart uses to get stronger and is a known effect of endurance training. This study suggests that if a pregnant woman exercises hard enough (> 90% maximal MHR in an elite athlete) blood flow to the placenta is reduced enough to cause fetal bradycardia – a sign of fetal distress. Interestingly, elevated blood lactate levels alone do not seem to cause fetal bradycardia. It seems to be more dependent on uteroplacental blood flow. It is impossible to draw conclusions regarding pregnancy outcomes from this study. We don’t know if fetal bradycardia during exercise is necessarily harmful, however, in the world of obstetrics, any indication of fetal distress is very concerning. Given this new data, it makes sense to counsel pregnant elite athletes not to exercise at intensities greater than 90% of maximal MHR. It is important to note that this may not necessarily translate to recreational athletes. There may be a lower threshold for non-elite athletes. How do you counsel your pregnant athletes?

Written by: Hallie Labrador, MD and Marc Harwood, MD
Reviewed by: Jeffrey Driban

Salvesen KÅ, Hem E, & Sundgot-Borgen J (2012). Fetal wellbeing may be compromised during strenuous exercise among pregnant elite athletes. British Journal of Sports Medicine, 46 (4), 279-83 PMID: 21393257