I think it's 2nd Tri. Although I'm not a back sleeper, I found that in pregnancy, I'd find myself waking up in my back a ton! Can't really help it unless you put a pillow behind you.
I put pillow on both sides of me and it helps a ton! But yes 2nd trimester is when it can decrease circulation to the fetus. To make you feel better with my first pregnancy i kept waking up on my back and dr told me thats why your body is waking up so you can turn
I asked my doctor that question yesterday at my 12 week appointment. She said you can sleep in any position that's comfortable! (And had some words she would like to share with the folks at "What to Expect) and cited a study that was recently published on the topic. Your body will roll over/adjust if needed during sleep. I was so pleased to hear that news from her!
My doctor said the same thing. Before the book, before google, pregnant women slept in comfortable positions. That was the guideline. And look, here we are today
Re: Sleep positions
SammyD