During pregnancy, no exercise should be performed in the supine position after the fourth month. Target heart rate should not exceed 140 bpm. Avoid exercises that incorporate extreme flexed or extended joint positions. Joints are looser in the latter part of pregnancy. Also avoid jumping movements due to joint and tissue laxity. It is important to maintain the current fitness levels during pregnancy and not try to increase or improve the fitness level. The time to do this is before pregnancy not during. Keep Aerobic/Step moves basic and simple. High step heights can become dangerous due to the body's change in the center of gravity. Recommended step heights are 4 to 6 inches. Keep strenuous activities down to a duration of 15 minutes at a time. It is also very important to avoid the Valsalva maneuver (holding the breath) during exercise. It robs not only the baby of oxygen but oxygen starved muscles can cramp easily.
