It is fine. My doctor suggested it to me since sleeping on my side causes sciatica and doesn't help with acid reflux.
I'm pretty sure it's safe. laying on your back is not good but sitting up is different. tons of pregnant women have to sleep like that and they're okay! ask your doc if you're too worried.
I sleep like that now if i lay down i get chest burns so i use three pillows to keep me up right....
I sleep like that also its the most comfy position I have horrible back pain where I'm at the point of walking and it hurts I can't lay on my back or its the worst thing in the world and I can't get up for like 15 mins struggling it ***** I'm 30 weeks
I would always slept like that with both my pregnancies , it was the only comfortable position to sleep in for me but I slept like that on my bed not the couch.