Once you reach the second trimester, it's recommended that you not do exercises in the supine position (on your back). If you are currently trying to conceive, I agree with the previous posters that you're fine to do any core workout just like anyone else.
"You may have a fresh start any moment you choose, for this thing that we call "failure" is not the falling down but the staying down." Mary Pickford
"No matter how slow you go, you are still lapping everybody on the couch."