Q: Why shouldn't I do exercise while lying on my back after a certain point?
A: After your first trimester, lying supine (on your back) can cause your enlarged uterus and baby to compress your vena cava, the major vessel that returns blood to your heart. This reduces the amount of blood your heart has to pump back out, which can lower your blood pressure and reduce blood flow to the placenta, Shashoua explains. It also can cause you to feel dizzy, lightheaded or nauseated.
"For every supine exercise, there's an equally good alternative that's not done lying on your back," says Lenita Anthony, M.S., an exercise physiologist in San Diego and author of Pre- and Post-Natal Fitness (ACE Books, 2002). For examples, check out Anthony's book.