Yeah, I'm pretty sure your not supposed to lay on your back...haven't heard about not being able to lay on your right side, tho
When you get into your mid to late 2nd trimester you're not supposed to lay on your right ride because the baby puts too much pressure on your liver.
Lay on your left side or back instead. Having a body pillow helps a lot too!!
You're Not Supposed To Lay On Your Back It Could Possibly Reduce The Blood Flow To Your Baby , (NOT to scare you) Or Your Right Side There's Too Many Health Risk Plus If You Lay On You Left It'll Probably Feel Just a Little More Comfortable Than Laying On Your Right .