Being on your back for brief periods is fine, your body will tell you if it's not (https://health.clevelandclinic.org/exactly-how-bad-is-it-to-sleep-on-your-back-when-youre-pregnant/). You're not going to hurt the baby, being on your back will hurt you before it hurts the baby, as long as you're not sleeping flat on your back all night long without changing positions. If you get lightheaded, nauseous, don't feel well for whatever reason, then change positions. Otherwise, do whatever is comfortable. As you get further along, it gets harder and harder to find something that works with the belly bump in the way. I'm at 35 weeks now, and there are definitely times when I'll be flat on my back plenty long enough before needing to change positions.
Re: Missionary after 20 weeks