Pregnant women can still do cardio exercises. Although this is important for women who were really physically active before their pregnancy, in general, it is a good way for women to maintain a certain level fitness.
One recommended form of cardio exercise is the power walk. It keeps your heart rate low and strong. If you do it correctly (i.e. moving your arms at the same time), you can even get a full body workout. It’s the safest and easiest form of cardio workout.
Running is also another option, but this is generally more so for women who regularly ran before their pregnancy. Consult your doctor about until how far into the pregnancy you can run.
Another good cardio exercise is aqua aerobics. There are those which cater to pregnant women, making it more low impact the usual.
Just remember to always consult your doctor about the type of exercise you can do. As you are not working out for the Olympics, it is always best to not to push yourself too hard, and concentrate on keeping fit and healthy.
