Research shows from about 28 weeks pregnant, you should start to go to sleep on your side, either side is fine. If you wake up on your back in the middle of the night, don't worry- just roll over onto your side again! Falling asleep on your side means you stay on your side for the deepest and often longest part of your sleep, that's why we emphasise 'settling' to sleep on your side.
Either side is fine! As long as you're not sleeping flat on your back.