Is it safe to do sit-ups or stomach crunches in the early stages of pregnancy? When should you stop?
It can be safe all the way through pregnancy (albeit more and more difficult towards the end)
My wife asked the doctor this when we discovered she was pregnant with our first, as she was then doing upwards of 400 sit-ups every evening, and the comment back was that it was much better to be doing that than not exercising.
The comment was just to avoid doing it if it hurt, and when she was much bigger she found that full crunches were just not possible any more, so had to settle for a month of gentle crunches. The doctor also said to just be aware of joints loosening towards the end of pregnancy and to take care not to do twisting sit ups at that pont - but they weren't possible then anyway.
A lady at the gym I use worked out regularly throughout her pregnancy, including situps/crunches/leg lifts. I asked her about it and she said her doctor told her no worries, just pay attention and avoid exercises that hurt differently than before pregnancy. She stopped running because it became uncomfortable but walked and used an elliptical machine.
She had her baby a few weeks ago and just came back this morning. She looks great. Apparently, her labor went less than an hour, which she attributes to being fit.