Final answer:
Pregnant women in their second or third trimester should avoid exercises that involve lying flat on their back. Instead, exercises that can be done in a side-lying or upright position are recommended.
Step-by-step explanation:
Pregnant women in their second or third trimester should avoid exercises that involve lying flat on their back for extended periods of time. This position can put pressure on the vena cava, a major vein that helps blood flow back to the heart. When the vena cava is compressed, it can reduce blood flow to the uterus and the baby.
Instead, pregnant women should opt for exercises that can be done in a side-lying position or in an upright position. Examples of exercises that are safe for pregnant women include walking, swimming, stationary cycling, and prenatal yoga.