Research now suggests that falling asleep and/or waking up on the left, both or either, is protective of baby’s wellbeing.
Meanwhile, balancing out what is too tight in pelvic muscles and ligaments will help baby move towards the left in several days or weeks of the daily exercises shown here, or may take professional help to accomplish in pregnancy. A well-working metabolism helps, and walking and movement with a whole-food, low-sugar (or no sugar) diet helps your metabolism.
I also highly recommend that when you lie on your side that you use a pillow between your knee AND your ankle. Don’t let your ankle “hang” without support as this twists the uterine ligaments.
Switch sides when it’s uncomfortable. Getting up to go to the bathroom is shown to be protective of baby’s well-being, so don’t get too frustrated with getting up at night.
If your doctor recommends you sleep on your left side because you have high blood pressure, that’s another matter. I’ll stay out of that discussion.
For fetal position, the balance is on the inside. Attend to that and which side you sleep on will be less important – and more comfortable.