I ahvnt really done much during the pregnancy tbh, i go swimming every now and then, and i like to try and walk her out atm, but apart from that i've only done the usual amount. However i found that things d oget harder as you get bigger, i used to be fine being at work on my feet all sat and sun but before i left i was coming home SO tired!
I wouldnt over do it but i'd say second tri is the time to start taking light exercise if you intend to, its still relatively easy to pick up and you'll hopefully pick up a bit of your fitness level before you get too big