Did anyone else hear it was dangerous to lie on your back as from middle pregnancy?

I am finding it hard to stay comfortable at night - as many pregnant women do, but lyeing on my sides all night makes my back and my hips hurt (I do put a pillow between my knees) Sometimes the only postition that I find comfortable is on my back but I am wary as I have heard from a few different sources that it is dangerous as it can do something to a major blood vessel?? Other people I have asked have never heard of this, what do you lot think? h x