You are still on the "ok" to sleep how ever you like.. Once baby gets bigger you should sleep on your left. Sleeping on your right will stress and damage your liver due to baby weight, sleeping supine(on your back) will not reach enough oxygen to baby. So the safest is sleeping on your left:) more blood flow and oxygen for baby.
It's recommended to sleep on your left side but you can sleep on your Right. I've been doing it my whole pregnancy and my baby is fine. But don't sleep on your back.