Thanks ladies.

I tend to sleep mostly on my left side, but sometimes I just get so tired of sleeping that way that my arm starts to hurt a little and I get uncomfortable.

It's true, we have a big vein (inferior vena cava) that brings blood back to the heart in the back of our abdominal organs. Blood flow might be impeded by the growing uterus that's why it is not advisable to sleep on our back or to lie on our back for a long period of time by the second trimester. You can place a pillow under your right hip to prevent you from rolling onto your back while you sleep. Hope this answers your question. :)

Sometime during the second trimester, the baby and uterus get big enough to put pressure on one of your major veins or arteries when you are on your back. It can cause complications. I prop a pillow behind my back to keep from rolling over onto it.

I'm only 12 weeks and I already can't sleep on my back or stomach. I sleep side to side lol

Yes correct no laying on back you have w better blood flow with the left side

After a certain point yes it is bad to sleep on your back. Plus for me at least when I get big and lay on my back I almost black out from it cutting on blood flow. As for your right side it is best to sleep on your left but it's still fine to lay on your right.