To Crunch, or not to Crunch

I am TTC...never really worked out in my life. I'm gaining some weight now, but don't want it to get out of control, so I was thinking of doing crunches just to keep the belly decent. Would it be a good idea to start doing that? Or just wait until I get pregnant and have a baby and do it after? I heard that exercising before getting pregnant will be good for the muscles of the abdomen...but I don't want to risk doing anything while TTC