Why not try sleeping at an angle on your back if you can't sleep on your sides? Like add a couple of pillows on your upper back so you don't sleep fully on your back. At least you won't be fully horizontal. The idea is that your tummy is heavy so by sleeping on your back it may press and block blood circulation on a smaller vein on your back and affect you and your baby's blood oxygen level. I think different people will have different effect since different people have different body constitution, weight and blood pressure, so even if one has nothing bad happen, it may not apply to another. Personally, I don't think that a short while sleeping on your back or waking up on your back would be problematic.
Im a back sleeper too! I will try to sleep on my sides but always ended up on my back when i woke up. Also, i read that it's fine to be on your back for a short while (up to 1-2 hours). So at times when i really got bad shoulder pain or pelvis bone pain, i will turn to my back for a while.
my doc never asked me to not sleep on my back. He just said that since ur tummy is growing bigger sleeping on back will put more pressure and u will not be comfortable. So I think there is no harm sleeping on your back on and off for a couple of hours as long as u feel comfortable
When I told my mum I had problems sleeping cos gotta sleep sideways, she was like huh their generation no one cared about this lol 😂
if you start on the sides and end up on back it’s fine. just reposition yourself