After you develop a baby bump, ab crunches become impossible. By the same token, if you suffer from back pain or hip pain, both of which are not that uncommon for expectant women, you will obviously not do exercises that hurt you and seem to make those conditions worse for you. Most exercises are just fine for pregnant women though. Swimming, prenatal yoga, pilates, or walking are all great exercises for pregnant women. But perhaps simply being active throughout your pregnancy is even more beneficial than specific pregnancy work outs that you decide to do.
During my pregnancies, I remained extremely active and besides going to prenatal yoga classes for a few hours twice a week (that's not really a lot of time let's face it) I simply carried on doing what I did before I became pregnant. That meant lots of walking, traveling internationally, and doing home improvements. I am convinced that my nine months of fitness contributed to the fact that I did not have to worry about postpartum exercises at all. I returned to my pre-pregnancy size immediately after birth (no, actually that's a lie I looked better after I gave birth than before getting pregnant) and I am sure that my active pregnancies contributed to that. Barring medical problems, you can do the same! The key to staying healthy is doing as much as you feel up to, and listening to your body. Also see: Is it safe to exercise during pregnancy?