I have been told (by Ob and Osteo) that as long as you are comfortable then it is ok to sleep or lie on your back - but do not exercise while lying on your back, the pressure will be too much. The weight of the baby/fluid/uterus can press on the major blood vessel running down your spine and this is where the issue is. I am now 36 weeks pregnant and can still lie quite comfortably on my back, and did throughout my last pregnancy too. But if you feel faint or nauseous then you should definitely avoid it.

The best side to lie on is your left side, as there is less chance of organs being squashed, but you will still find that you need to move positions in the middle of the night before anything goes numb.

I think your body will let you know what you need to do, so if you want to lie on your back, go for it, you will probably find that you will use your sides more as you get further along.

I still slept on my tummy until I was about 24 weeks, and after that I still rotate more on my tummy with my top knee brought up to take some of the weight off my tum. For a while I used a pillow under my tummy for support, but now I think it is too big and I find the pillow makes me uncomfy!! Sometimes I use a pillow between my legs, sometimes not. But I just go with what my body thinks is comfortable and I have never had an issue with any faintness while sleeping or lying down.