I’m in third trimester and it’s quite uncomfortable after lying on left side for a long time. I try to go for toilet breaks whenever it’s uncomfortable or I swap to lying on my right side for a while until the pain goes away then I turn back to left side.
i actually sleep elevated on my back throughout my pregnancy and my gynae said its safe.. as long as you are not flat on the bed and there is an angle. just make sure your pillows are positioned properly.