Hi Emma27. I am a back sleeper and am now 37 weeks. Even if I wanted to I couldn't back sleep now, it is waaaaaaaaaaayyyyyyyy too uncomfortable. If you are naturally a back sleeper you will probably find that, even if you force yourself to sleep on your side, that you will wake up on your back.
I sleep on my side now with a pillow for my belly to rest on. My Husband tries to thieve it.
I also sleep sort of on my back but with a pillow shoved under it so I'm sort of on a 45 degree angle. You might find that a good compromise.
Just for the record, I was sleeping on my back at 16 weeks and beyond. Apparently there's an artery in your back you can squash but I never worried about it.