Usually women find that they naturally alter their exercise program to accommodate these changes, especially in the last trimester of pregnancy. The joints also undergo changes during pregnancy. Your body releases a hormone called relaxin which loosens up the joints of the pelvis to make room for the birth of your child. Because all of the joints in the body are more lax, there is a greater chance of spraining or straining muscles and joints during pregnancy. During pregnancy your body uses carbohydrates more quickly.
Exercise also increases the metabolism of carbohydrates.
These two factors can lead to low blood sugar reactions during exercise. Increasing caloric intake to shift your carbohydrate balance is very important for pregnant athletes. This allows nutrients and oxygen to be transported to the fetus more efficiently. However, with the growth of the womb, the flow of blood in the body can be disrupted and lightheadedness can occur. While exercising, blood flow shifts away from your internal organs so that your muscles, lungs and heart get a larger share of oxygen.