After the first trimester, pregnant women are often warned against exercising while they're flat on their back. The reason: Lying in this position compresses the vein that carries blood back to the heart and can interfere with blood flow to the placenta.
But that doesn't mean you shouldn't gently strengthen your stomach muscles, especially the transverse abdominals. It will keep your lower back and spine strong, which can help you push better during labor.
A good exercise to try, from Julie Tupler, author of Maternal Fitness:
Sit cross-legged on the floor with your back to the wall. Breathe through your nose, and as you exhale, bring your belly button in toward your spine, keeping it there for 30 seconds.
Repeat five times, working up to ten repetitions.