Hey stranger,
I actually asked my OB about this in terms of sleeping on back & she said that there was no reason not to!
She said if you are comfortable, then do it and there was no danger to you or baby to do it, my GP said the same.
I have to admit I am still a bit nervous as ALL the books say not to!
Good on you for going to the gym 4 times a week!
I am down to twice.