Sleeping on back?

Lisa • Pregnant with our first baby boy 💙 due sept. 14, 2015. 👼
Hi ladies, 
I forget when are we supposed to stop lying on our backs? I have already started training myself to sleep on my side and have a big pillow however I have a 2 hour dentist appointment where I will be laying on my back and I'm nervous about that. I'm 17 weeks. My dentist knows I'm