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.
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