As your uterus grows bigger, your body's center of gravity changes. It's easier to lose your balance. Exercise helps you adapt to your changing shape. Normal hormone changes cause ligaments and joints to relax. That increases the chance of sprains.
Exercise increases muscle strength and flexibility.
Because of the extra effort your heart makes during pregnancy, your heart beats faster. Even moderate exercise can make your heart stronger. During exercise, keep your heart rate lower than 140 beats per minute. Expect to feel out of breath sooner than you normally would. If you can talk while you exercise, you're probably exercising at the right level.