hairymcleary - because of additional blood flow and demands on the body, many people find they get dizzy. I think it's quite a cautious view myself, as many people find it no problem at all but given the injuries you'd get from dropping a dumbell on your head then it's probably safest!
I usually get round this with clients by getting them seated on a swiss ball (for abs/balance) and using one hand at a time. This makes you use your abs more than if you use two hands by putting you off balance and means you're far less likely to feel dizzy.
hope that helps.
fracesrivis - advice from country to country does differ, also it has changed back and forth in the last few years. The theory is that you increase risk of pressure on major blood vessels by lying on your back for a long time. That said, many people don't find it uncomfortable.
but.....IME crunches are not a great exercise to do whilst pregnant, potentially damaging and not really beneficial. There are plenty of other abs exercises which I think would work much better, give you a better shape. If you like the feeling of crunches and it feels good there's a good modification which you can do using the swiss ball, can explain if you like!
Rowing machine I wouldn't bother with while pregnant, does nothing for your posture - unless you're a rower with great technique.
Steppers are fine unless you have SPD or problems with your hips, and you should watch your balance which obviously changes as you get bigger knockers and a big bump!