Lying on your back- 2nd tri and beyond

What has your doctor recommended about lying on your back during your 2nd trimester and beyond? I'm entering my 12th week and my doctor said I can continue laying in my back until it gets uncomfortable. I keep reading though that the weight if my uterus can reduce blood flow to uterus. I've been trying to lie on my side when I sleep but I always end up on my back. What does your doctor say about this?