Yes, as long as you follow certain guidelines it’s absolutely ok to do abdominal workouts while pregnant. It’s actually a good idea because it can prevent diastasis recti, separation of the abdominal muscles during pregnancy.
Standard exercises like sit ups and crunches are fine during the first trimester, but come the second and third you will want to avoid lying flat on your back, this will lower your blood pressure and therefore reduce blood flow to the fetus. Try using a stability ball to prop yourself up on during your abdominal routine.
A workout I love is what they call in yoga the “cat and cow” pose. Get on your hands and knees and alternate arching your back and then lowering it. Not only does it work you abdominal but it a great back stretch. For a better description and visual see the following link from Yoga Journal.
Planks are also a good option and a great workout for your abs. They are quite challenging however, so if you cannot get into full plank position, on your toes with arms extended or on your forearms, try it with your knees on the ground. There are many forms a plank exercise can take, side planks, reverse planks, etc… Livestrong.com has a good description of some different variations.
Finally, here’s a great video I found from Babyfit.com that goes through a complete abdominal workout you can do anytime during pregnancy.