To work on your core the best exercise is a pelvic tilt against the wall. Stand flat against the wall then contract your tummy muscles flattening your lower back against the wall. Try four reps then try holding the fifth one for ten seconds, release then do it all again. Each day try to do more. But like everyone else said, don't over do it if the doctor has told you to take it easy.
I would stay away from sit ups and other core exercises. If your doctor says take it easy, then you need to take it easy! Whatever you gain now, you can lose after you have the baby. For now, try to eat healthy to maintain steady weight gain.
ur belly getting big cuz the babys pushing everything up and eventually he/she will pretty much take over ur body
Walking maybe some.sit ups and squats .. don't do to many try 10 a day walk for 20 min..