In general we can say one can train during pregnancy. But there are several important moments. Firstly, you should be accustomed to such loads, i.e. train before pregnancy. Secondly, pregnancy should pass normally. That is why you should definitely consult a doctor before training.
If doctor allows you physical loads, then during first trimester you can use all kinds of exercisers. At your will, you can reduce load, if you feel heightened fatigability. Remember, you need only to keep your physical shape, not trying outdo yourself.
During second trimester you should avoid exercisers requiring lying on back, and also any other exercises when you need to lie on back, like abdominal exercises. However, you still can work at shoulder, breast and arm muscles exercising in sitting position.
As your belly grows and breast increases, you can feel uncomfortable exercising in sitting position. In such case you should switch to a free form of exercising. Third trimester is not meant for exercisers.
Do not forget consulting your doctor as for physical load in each trimester. Every time you visit a doctor, ask him this question. During exercising watch your sensations, they should be habitual and comfortable.
