Myths about pregnancy and exercise can be confusing and daunting, but the good news is many are over-cautious and in some cases simply not true.

Two guidelines most hotly contested are: 1) exercise levels should not cause the heart rate (HR) to exceed 140 beats per minute; and 2) pregnant women should refrain from any supine exercise (lying on their backs) after the first trimester. For healthy, active, women with an established exercise routine, these guidelines are conservative and in most cases simply not the case.

1) The previously outdated guidelines for exercise during pregnancy were to keep the HR at or just below 140bpm. Given the variability in maternal HR responses to exercise, target heart rates cannot be used to monitor exercise intensity during pregnancy.

The American College of Obstetrics and Gynaecology (ACOG) released new guidelines in Jan 2002 recommending use of the Borg Scale – A rate of perceived exertion in which pregnant women are encouraged to work at a “somewhat hard” pace in which they are self monitoring and can adjust the pace as pregnancy progresses. This allows for individual differences in fitness and resting heart rates. This is a far more accurate reflection for individuals of how hard to workout as an unfit woman may have to keep her HR well below 140 whilst someone fitter could go as high as 170 with no effect.

HR is higher in the first trimester because the vessels are dilated and there is not enough blood to fill them. As pregnancy progresses the HR decreases and by the end of pregnancy it’s at its lowest and is not that easy to elevate during exercise.

2) Supine Hypertensive Disorder (SHD) refers to positional low blood pressure that may result when the weight of the uterus compresses the vena cava (the largest vein of the torso) to the point that it restricts blood flow back to the heart.

When blood pressure falls low enough, a woman will immediately feel intensely dizzy. (This can also occur with rapid changes of position, i.e. from lying to standing and vice versa.)

To prevent SHD the ACOG advises women not to exercise on their backs after the first trimester; However researchers have documented that true SHD occurs in less than 10 percent of all pregnancies. Additionally women who experience these symptoms always feel an urgent need to either roll over or sit up, which quickly relieves symptoms.

No documentation yet exists to support the premise that exercising on the back could negatively impact foetal development or that resting or sleeping on the back restricts blood flow.

Rhythmic movement of the limbs, particularly the legs, acts like a pump, which actually increases the rate of blood flow return to the heart while exercising on the back. Additionally, gravity enhances the blood flow rate during exercises that incorporate raised legs.

Unfortunately, miscomprehension about SHD has led some women to unnecessarily fear that they might harm their babies while sleeping on their backs. However both mum and baby can rest easy. Sleeping on the back has never been shown restrict blood flow to the placenta and baby.

Still, because controversy about SHD persists, during pregnancy you should be cautious if you choose to exercise while lying on your back. Exercise on the back for short durations only- especially after 20 weeks of pregnancy.  Watch out for feelings of dizziness, and if you experience dizziness cease exercising on your back. Lying on your side after 20 weeks is OK, but the left side is preferable for long periods.

Click here for Part 2 of this myth busting article


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