It'a amazing how much different info there is on this. When I was pg we had to lie on our backs for 30 mins+ at the antenatal classes, but I've seen all sorts of horror stories elsewhere.

Anyway I remember reading that lying on your back is OK if your knees are raised, so you are OK at night if you put a cushion under your knees. I have just had a look on Google though and can't find any reference to that anywhere. Instead, I found this, which is almost as good:

"Avoid exercises that require you to lie on your back after 3 to 4 months of pregnancy. If you do lie on your back, use a small pillow as a wedge under your right hip to help move the weight of your baby off important blood vessels, which lie behind your uterus. This helps maintain good circulation (blood flow) to your baby."

I'd give you the page refernce but it's only a boring thing about how important exercise is.

I wouldn't be too paranoid about it, Pie. The advice about lying on your back is relatively new - and women have been having babies forever without this advice. I would avoid intentionally lying on your back - and the pillow thing sounds good if it doesn't twist your spine and make your back problems worse - but I wouldn't worry if you find you have rolled during the night. Just roll back again.

I have the opposite problem - I find it very difficult to sleep unless I am lying on my tummy - which at 19 weeks is beginning to get slightly uncomfy :/

I also cannot believe it causes real harm as I'm sure women have laid on their backs while pregnant since time immemorial. If it killed babies hardly any would survive. BTW, lack of sleep is much more harmful - to YOU! After all, you count too. Give yourself a break and stop worrying.

If you are very pregnant you are likely to find it uncomfortable anyway... there is also an isue with blood pressure and some women feel dizzy on their back.
As for restricting oxygen to the baby... basically when you lye on your back with a heavy uterus you compress the vena cava which brings oxygen to the placenta.... so taht is why they say long lengths of time is not a good idea.

Pie - get on all fours and extend the leg (not in the air) that has the sciatica and make that leg all floppy, wiggle it gently... that does wonder for sciatica - I do that in my antenatal yoga class.
You can do this as often as need be.

Pie, I am 24 wks pg and often wake up in the night on my back without a clue how long I ve been like that for. Try not to get too freaked out about it. I did actually ask my doctor about this some weeks ago, as it was something concerning me as well. He assured me that the uterus pressing on the vena cava couldnt harm the baby in any way. Instead it could make you feel dizzy and he said something about it making it more likely that you would get 'twitchy' legs lying like this (which I do get a lot at the mo). Apparently it is best to lie on your left side because the vena cave runs down your right!!

Pie, I can corroburate rainbowface's info. I too asked and was told that nothing would harm the baby but it might make me a little light headed.

Does it harm the baby if you lie continually on your right side? I have heard that you're supposed to lie on your left side but I always sleep on my right. Can't help it.

I found lying on my side with dd1 really uncomfortable and spent entire pregnancy sleeping on my back - no problems at all. Maybe your personal build and physique plays a part in it. To be honest, getting any decent sleepat all is my main criteria for this pregnancy and if I am comfortable that will have to do. Don?t wish to sound unfeeling for unborn baby, but I just don?t believe the risk is great enough to worry about it.

Pie - do not worry about lying on your back at this stage. This advice is more for the third trimester of pregnancy where the weight of the uterus can cause supine hypotension ( low blood pressure when lying). You will know if this is happening because you feel light headed and faint. It does not matter either what side you lie on. The left side is good for placental blood flow, but that really is more useful in labour if the CTG monitoring does not show a very good tracing. Some babies are better when their mothers are on the right side.
Basically your body will indicate if you are lying in a poor position, either by you feeling faint or the baby squirming in annoyance