Abdominal exercises done while lying on your back aren't recommended after the first trimester. The reason for this is that when you lie on your back, your enlarged uterus presses on one of your major blood vessels, reducing the amount of blood pumped around the body and to the baby. However, you can strengthen abdominal muscles without lying on your back by using gravity and your body to strengthen and tone the torso. In pregnancy, you can tone your abdomen with exercises where you adopt an all-fours position or an upright sitting position. Try to do these exercises around 2-3 times a week.