I am a big fan of working out, but my doctor advised me to stay away from ab work outs and twisting.
You want to stay away from exercises that directly work out your abs..so no sit ups
I'd save the sit ups for after you have the baby. ;) I've been doing prenatal yoga and love it!
Ive always done whatever workouts I did before prior to being pregant up until about the 2nd trimester, as long as I had the energy for it. Strong abs really help during labor. But you shouldnt be laying on your back after around the 2nd trimester anyway. Some suggestions are on the exercise ball or plank holds because it strengthenes your core. But you have to keep it mild, no straining. Anytime you feel strained, stop. Those ab ligaments are easy to pull while you have the relaxin hormones and closer to the end you definitely want to lay off the ab workouts. On the plus side, you can start again right after a natural birth, but not a c section.
There are certain modified ab workouts you can do during pregnancy that are good to do, working out your core is not a bad thing, and you will be using those muscles for labor. You can look up core exercises while pregnant and find them. It is important to ok it with your doctor too though. Sit ups are not recomended.