I did BodyPump right up till about 33 weeks (although the uprigt rows were getting a little more like reverse curls!) I DID lower the weights of course. I don't think that exercise in the TWW or even the first tri for that matter is going to hurt much - maybe just drop the intensity a little if you are used to going hardcore - I stopped running and used the bike and cross trainer instead and tried to keep my HR down to 1o 140BPM which is what they suggest you keep it to during pg.

In any case, if it takes you a while to conceive (it took us almost a year) stopping or toning down your regular routine each TWW can be hugely frustrating - I found it quite damaging to my self-esteem and my general mood.