Hi sarah
i mentioned something about that on another post.
This is what I have in my book "what to expect when you're expecting"
"the back position, though more comfortable, rests the entire weight of your pregnant uterus on your back, your intestines, and the inferior vena cava (the vein responsible for returning blood from the lower body to the heart). This can aggravate backaches and hemorrhoids, inhibit digestive function, interfere with breathing and circulation, and possibly cause hypotension, or low blood pressure.
This doesn't mean you have to sleep standing up. Curling up or stretching out on your side-preferably the left side- with one leg crossed over the other and with a pillow between them, is best for both you and your fetus. It not only allows maximum flow of blood and nutrients to the placenta but also enhances efficient kidney function, which means better elimination of waste products and fluids and less edema (swelling) of ankles, feet, and hands."
hope it helped and answered some questions. Everyone has their own opinion though.