The American College of Obstetricians and Gynecologists recommend that pregnant women not lie in the supine position after the first trimester. Lying on the back can cause hypotensive syndrome. This is where the combined weight of the baby and the uterus obstructs the venous return and decreases cardiac output. Some exercises to strengthen the core muscles are fine. Having a strong core is important for birth and the recovery post birth. Vertical and incline core exercises are great for pregnancy. Different variations of plank can also be done successfully while pregnant. Listen closely to your body and make sure the exercise isn’t putting stress on the back.