Can active women continue to work out hard in late pregnancy without compromising the wellbeing of their unborn babies? Or does the diversion of blood and oxygen to working muscles put the fetus at a dangerous disadvantage? These question are of great interest to many female athletes and other physically active women, who would prefer to maintain their fitness during pregnancy. Studies of the effects of moderate exercise have resulted in less conservative guidelines for physical activity in healthy women with normal pregnancies. And now a small study of intensive exercise suggests it poses no significant risk.
The study, from Ontario, Canada, was designed to examine the effects of maximal maternal exercise testing on fetal heart rate (FHR) responses, which are important indicators of wellbeing or distress in unborn babies. The researchers were working on the theory that FHR responses to a single bout of strenuous exercise by aerobically-conditioned women in late pregnancy would be minimal and transient. They were persuaded of the existence of protective mechanisms that exist to ensure an adequate supply of oxygen to the baby, even when the mother is engaged in strenuous exertion.

The 23 participants were all healthy women aged between 20 and 40, 31-38 weeks into a singleton pregnancy throughout which they had continued to be physically active. The women exercised on a constant work-rate cycle ergometer at 20w for four minutes and then at an increased rate of 20w per minute until voluntary fatigue. Their babies' heart rates were monitored by an experienced obstetric nurse for 20 minutes before and 20 minutes immediately after the maximal exercise test, with any significant abnormalities referred to an obstetrician for further assessment.

The resultant fetal heart rate tracings were analysed for baseline FHR, number of accelerations (abrupt increases in FHR) from baseline, number of decelerations (abrupt decreases) and degree of FHR variability - the latter being an indicator of adequate oxygenation and fetal wellbeing. The key results were as follows:

* Mean baseline FHR rose significantly after the exercise test from 139 to 145 beats per minute, with fewer accelerations;
* FHR decelerations were uncommon, with no significant differences before and after testing;
* There was a significant reduction in FHR variability after the test;
* Post-test tachycardia (baseline FHR of more than 160 beats per minute) was noted on two tracings;
* A single episode of transient bradycardia (baseline FHR of less than 110 beats per minute) was seen when FHR fell to 60 beats per minute immediately after the test and increased gradually to 120 beats over a period of six minutes. The woman concerned underwent further testing and was found to have a growth-retarded baby who needed inducing three days after the test.

'The results of this study confirm that the most common FHR response to an acute bout of strenuous exercise is an increase in FHR immediately after exercise,' the researchers report.

They were surprised that, with the exception described above, there were no episodes of fetal bradycardia, since this has been reported to occur in 15-20% of fetuses after strenuous exercise. Bradycardia is a reflex response to oxygen shortage, which protects the fetus by preserving blood flow and oxygen delivery to vital organs, including the brain and heart. The researchers suggest that, since their subjects were 'conditioned', there may have been maternal and fetal compensatory mechanisms to prevent fetal hypoxia. 'Such women might be able to perform at a higher work rate before inducing fetal hypoxic stress, as less cardiac output is redistributed toward skeletal muscle and away from the placenta, and they might have greater placental volume.' Also the exercise protocol of this study was shorter than that used in previous studies and involved the use of a cycle ergometer instead of modes of activity requiring greater muscle mass.

'The present study results suggest that maternal exercise testing using a brief cycle ergometer testing protocol is safe in this group under carefully controlled conditions,' they conclude, 'but additional study involving the testing of a larger number of subjects is necessary to confirm this hyothesis.

'The value of the single episode of bradycardia is that it demonstrates the need for screening of fetal wellbeing, including estimates of weight, before women embark on intensive exercise in late pregnancy. Obviously a compromised fetus will be less able to withstand the effects of reduced uterine flow than a healthy one.' And the researchers warn that their study does not prove the safety of regular intensive exercise in late pregnancy. This 'might not be safe and could result in altered fetal growth'.

Obstet Gynaecol 2000 Oct;96(4):565-570

Isabel Walker