I am 7 months pregnant and I would like to confirm if it is correct to stand for salaah and perform all arkaans as normal and only sit on a chair for sujood n jalsa and at-tayyaat and salaam? Or if 1 position is done sitting all should be sitting?
The rule is whatever a person is able to do then he/she must do, but whatever they are unable to do then it falls from them and they are excused.
So whatever you can do sister from the salat without sitting then do it, but if you cannot do the sujood without bringing on your self harm then you must sit, and it does not matter if one position is done sitting and not all, based on the mentioned rule above of ability and inability.
For more information, this Arabic source is available.