I am actually trying to lose while trying to conceive. Yes, you can do crunches. My doctor told me I could continue all regular exercise through the first trimester and then things like crunches and anything else that strains your stomach muscles needs to stop. I would say crunch away and talk to your doctor once you get that positive pregnancy test.
I have to say though that your weight for your height sounds absolutely heavenly! I am two inches shorter than you and don't think I have ever seen my scale that low.