Is it safe to do sit-ups or abdominal crunches during pregnancy?
Sit-ups and crunches are fine in your first trimester. But by your second trimester, you'll need to avoid lying on your back because as your uterus grows, its weight can compress the blood vessels leading to your heart, potentially depriving your developing baby of oxygen. Of course, by that time in your pregnancy, you'll most likely find that sit-ups are almost impossible to do anyway.
That's fine. The only problem is that lying flat on your back tends to lower your blood pressure, which could inhibit the blood supply and oxygen to the fetus. About 6 to 10 percent of pregnant women experience rapid drops in blood pressure when they lie down — even if it's only for a few minutes. The problem can begin in early pregnancy, though it's more likely to occur after 20 weeks.
If you really want to work your abdominal muscles, a better exercise would be to get on the floor on your hands and knees and alternate arching your back up and relaxing it.