(also, my appointment isn't for another two weeks so I can't really ask my midwife about it till then... I'd ask your doctor though to see what is best for you)
It deprives the baby of Oxygen and nutrients.
My midwife told me to sleep on my side and my prenatal yoga instructor does not let us lie on our back for certain poses after 20 weeks. I didn't really understand why so I looked it up. I found this on webmd...
" When you lie on your back, the weight of the pregnant uterus slows the return of blood to your heart, which reduces blood flow to the fetus. That means the baby is getting less oxygen and fewer nutrients."
I go to sleep on my side but I have a bad habit of waking up on my back. When I do I turn over again, but it still bothers me. I like sleeping on my back... I just don't want to hurt the baby.
I posted about this too. People said your body usually alerts you if something is wrong. I would ask your Dr, I know I am next appt. But I'm thinking if I'm comfortable that's how I'll sleep whether its on my back, side, or stomach.
Sleeping on your back could cause pressure from your uterus and baby to press on a major vein (or artery I forget which) restricting blood flow which could lead to blood clots. I'm normally a side sleeper but for some reason i would always wake up on my back with my second baby. Both he and I are fine.