Sleeping on your back: when to stop

Natalie • 💜 👫Sept. 4, 1999 • 💍 Feb. 19, 2013 • 👰🏻 Sept. 5, 2014 • 👶🏻👶🏻 Nov. 17, 2016 💕
I'm trying to figure out when I'm supposed to stop sleeping on my back, and getting mixed feedback. Is it when it's no longer comfortable, or at a certain gestation? I'm 16 weeks with twins now, some reading suggests at this point I should be sleeping on my side. Which I do mostly, but just trying to play things safer. Thanks!!!