They say not to coz the weight of ur uterus can put pressure on the artery that carry's blood back to heart causing dizzyness etc.. But I think ur body will let u know. Im 31+6 and still sleep on my back alot without a problem!
I try and sleep on my back doesn't wrk for me I'm 29+5 each person is different ur body will let u kno
I'm 36+5 and absolutely cannot breathe when sleeping on my back but that may just be me
My friend is a midwife and she went off her nut when she found out I had been sleeping on my back. Apparently this is the most common cause for still bring babies. As said above the babies weight can crush the blood flow to itself. I kept waking up on my back so I bought a maternity pillow. Best thing I ever did. They are longer than your body and support you al the way along your side, and help support your belly.