Tretinoin cream for acne

Has anyone else used or currently uses this for acne? If so, did you or are you currently seeing results? I'm kind of worried because it says for some reason not to use while pregnant and even though we have been trying for a long time with no luck, I don't want it to affect anything should I become pregnant. Does anyone know why it isn't safe for pregnancy? My dermatologist put me on it knowing we are trying to conceive...its just my acne is so bad I can't take it anymore. Please let me know! :)