When it comes to cardio exercise, Fit to Deliver co-authors Karen Nordahl, M.D., an OB-GYN in Vancouver, British Columbia, and Renee Jeffreys have a rule of thumb: "If you were really active before pregnancy, stay really active. If you weren't, now is a good time to become active."
For beginners, Nordahl recommends 30 minutes of walking three days a week.
During pregnancy you'll need to scale back on the intensity. Gauge your intensity using the Rate of Perceived Exertion (RPE) scale from 0 to 10: Aim for an intensity between 3 and 5 (you should be able to talk but not belt out show tunes).
Ignore heart-rate readouts on the cardio machines; since your blood volume increases during pregnancy, and your resting heart rate is considerably higher than normal, heart rate isn't an accurate gauge of intensity.
Which cardio machine is best during pregnancy?
Try them all. That way, if one becomes uncomfortable, your body will already be accustomed to the alternatives.
On the treadmill is ideal since you can control the terrain. Add moderate hills when you're up to it; go flat when you're not or if hills trigger calf cramps. If you're a runner, let your body tell you when it's time to switch to walking; nearly everyone does.
The elliptical trainer places little stress on your joints. However, the motion may feel uncomfortable if you're experiencing symphysis pubic dysfunction (SPD), a pain in the front of your crotch.
The recumbent and upright bikes are both good options. Many women like the back support the recumbent offers, though in the third trimester your belly might get in the way of your knees.