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.
Extreme exercise can cause too large of a shift of oxygen away from the uterus.
That's why reducing the length of time that you exercise strenuously and monitoring your heart rate may prevent problems with your pregnancy. During exercise the brain releases more norepinephrine, a neurotransmitter that can reduce depression and influence blood flow to your heart and kidneys. Norepinephrine also increases smooth muscle contraction and can cause increased uterine activity in the form of painless contractions. These contractions have not been shown to cause labor, but because of the possible influence on contractions, women who have risks for preterm labor should be on mild exercise programs. Elevated Body Temperature Exercise causes an increase in core body temperature.