I've read a lot of posts from people who are worried about lying or sleeping on their back in pregnancy - here's something I found that seems to explain things and definitely reassured me. The website it comes from is here.
"Sometimes women are concerned that sleeping on one's back is dangerous in pregnancy, either because it may deprive the baby of oxygen, or because it may encourage posterior presentation. Here are some comments from independent midwife, Virginia Howes, of Kent Midwifery Practice:
"One thing I do come across often is the idea that sleeping on your back is bad when you are pregnant. Women should sleep in whatever way they are comfortable. The important thing is a good night's sleep and women do not need to feel guilty when they wake up on their back. This is a myth that has come about through a misunderstanding of the facts and unfortunately is constantly perpetuated.
"When epidurals were first introduced into childbirth the dose of drugs used was considerably higher than it is now and the paralysing block very dense. Women were being left on their back and unable to move even if they wanted to. The heavy uterus would press onto the big oxygen-carrying blood vessels in the lower back and cause a decrease in a woman´s oxygen levels and consequently the oxygen reaching the baby. Thus came about the information that women should not be left to lie on their backs when they have an epidural. Quite correct information for that group of women.
"However if a woman without an epidural lies on her back and her oxygen levels are compromised, the first thing that will happen is she will become short of breath. That will happen prior to the baby being compromised and of course the woman will move off of her back or wake up and move."
Virginia Howes - Kent Midwifery Practice