Theres huge debate about this but doctors will usually tell you to listen to your body and if anything is uncomfortable and causes pain then to stop. Any abdominal crunches are not advisable but most other exercises are fine just make sure you drink lots of water and dont get over heated!!!!
I dont think ur supposed to do crunches or anything that involves u lying on ur back(which cuts bloodflow to the placena) which is why were not supposed to sleep on our back either...As long as u were excersicing before, everything should be fine...Like running and stuff like that, but if not, dont do it! Start off slow and dont do any weight training stuff most def!