Not sure exactly, but I know once you get into your 2nd tri you should not lie flat on your back. Check out google maybe, as I am sure there must be some restrictions. Good luck!
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Obviously your joints and muscles are getting ready for baby's entry into the world, so there may be some physiological changes to your body. If you have a separation of the abdominal muscles and do abdominal exercises it could make it worse. It's called diastasis. Ask your doctor if you have it.
I exercised throughout my pregnancy and adapted as I went. I used a Step bench with an extra riser to incline when I did abs, and eventually modified the exercises in class even more.
I actually just asked my doctor about abs exercises this morning. She no crunches, or anything that flexes your abs in that way. She said the best thing to do is find exercises (Pilates, Yoga) that strengthen your core without doing actual crunches. HTH!
Re: Abs