The way to be safe sleeping on your back is to put a pillow down by your hips, i sleep that way most of the night because sleeping on my sides isn't comfortable. I slept on my stomach until 16 weeks. It's all about what works for you and baby.
The gravid uterus presses on the vena cava (large vessel that returns deoxygenated blood to the heart for reoxygenation) impeding the flow of blood. Which means not only is your blood flow decreased but so is blood flow to the uterus.
I slept for a long time on my stomach,than on my back
And as ^^ said,just listen to your body
Now that i m 22weeks i wake up at night if i lay on my stomach becaise the baby starts kicking and wakes me up to move
As long as you're comfortable on either your back or your stomach, it's no problem. And discomfort will wake you up if you roll in your sleep
You won't be able to fully control how you relax and sleep each night, but you body will naturally move into comfortable spot each night as you grow. I know I would wake up thinking how long have I been on my back and realize my body was telling me to move. Just listen to your body as you go it will adjust you just have to learn to listen to it. Trust me it's hard with your first.