Final answer:
Exercise in the supine position and standing for long periods of time are not recommended during pregnancy to prevent reduced blood flow to the uterus.
Step-by-step explanation:
Exercise in the supine position and standing for long periods of time are not recommended during pregnancy in order to prevent reduced blood flow to the uterus. This is because when lying on the back, the weight of the baby can compress the vena cava, a large vein that carries blood back to the heart. This compression can obstruct blood flow and lead to reduced oxygen supply to the uterus. Similarly, standing for long periods of time can cause blood to accumulate in the legs, reducing blood flow to the uterus.