agree with others. you can tone up gently doing squats at home, not too low, half squats. I do series of different ones with small handweights..
I found the cross trainer was good in preg, but you mustn't overdo it. 10-15 minutes slowly. Now I'm 38 weeks I go outside and just go up and down on a step for 10 minutes, not too fast.
your aim is to gently strengthen yourself, keep supple and flexible and get some low level cardio, like walking, stepping. I found the pilates in pregnancy dvd quite good for stretches. you really need to do gentle stretching and flexibility exercises to ward off aches and pains.
also, as someone who had very nice toned thighs, bum and tummy,(and got it back again except for the tummy due to stretching) you just have to let that go until after the birth. I never had an ounce of fat on my thighs, but while pregnant and breastfeeding I slowly gained fat and everything went wobbly. You need that fat for nurturing and feeding your child. It goes away again with exercise after the birth, but for that period of your life, you need it. You can always be a superfit mum, don't worry about looking wobbly and weight gain in preg, there is nothing you can do about it, your body is very clever and wise and is going through these changes because you are nourishing a baby. Don't try to control your body too much during this time, there's plenty of time to get back in shape later.
given the placenta praevia, I would maybe even not risk going on a cross trainer, just do walking, gentle stepping or swimming, but squats and gentle toning is OK.