Diastasis is a separation of the two halves of the rectus abdominis muscle in the middle of our belly that sometimes occurs during pregnancy. You can check for it by lying on your back with your knees bent. (If you start to feel faint while on your back, roll to your left side; then use pillows under your shoulders to prop yourself up.) Place your fingertips 1 to 2 inches below your bellybutton, fingers pointing toward your feet. Lift your head as high as you can and see if you feel a ridge protruding from the midline of your abdomen - that's diastasis.
If you have it, take care to not exacerbate the separation when you do abdominal exercises. Try a modified ab crunch: If you are past your first trimester, prop yourself up with pillows so your shoulders are higher than your belly. Wrap a sheet or towel (folded lengthwise to about 8 inches wide) around your waist and criss-cross it in front. Don't knot it. Grasp and pull the ends up and outward at 45-degree angles as you contract your abdominal muscles, exhale and raise your head. Do not lift your shoulders. Diastasis often heals after childbirth. If yours does not, talk to your OB-GYN.