You’ll have been told about the ‘optimal’ baby position, head down, spine facing the front of the mother’s tummy and slightly to her left (Left Occiput Anterior (LOA).
If the baby is in any other position that’s absolutely fine … let’s explore this a little further … 
Babies can turn posterior (back to back) during labour even if they were anterior (spine facing forward towards the mother’s front) when the mother began her labour.
Babies change position and move throughout labour because they are constantly trying to find the best way out.
Lower back pain when the baby is descending through the pelvis –it’s a misconception that if there’s back pain the baby must be back-to-back. Labour can be long or quick regardless of what position YOUR baby is in.
Understanding how the baby needs to navigate the pelvis is really helpful for you to understand which positions would be supportive and helpful.
Remember the Physiology of the pelvis video:
Biomechanics
Links:
Molly O’Brien – Link
Moving for an easier birth video: Link
Resources that compliment with Ripple – Link
Forward Leaning Inversion
Sifting
Toffee Wrapper
Breech Tilt
Use of the rebozo – these resources can be found here