The belief is that lying on your back means your uterus is putting pressure on your vena cava - which is a major vein carrying blood to and from your legs, and baby
The research os based on questionnaires given to women who had still births, asking them to remember how the were sleeping in the days before the baby died. Dunno about you, but if my baby has just died, I don't think I'd be able to remember.
They did another study looking at a load of pregnant women and found it made fuck all difference, and none of the babies were still born.
Lying on my back makes me feel funny now at 32 weeks, and has since about 25 weeks. You are 'meant' to stop from the third trimester - like PP I'm sceptical about the stillbirth research, but I think for lots of women it gets uncomfortable anyway.
I slept on my left side for most of the pregnancy (as advised) and it's my natural sleeping position anyway. By my third trimester I had SPD and had to sleep on my back sometimes as I was so uncomfortable. Baby is fine. Don't worry
I my third trimester I used to feel faint/palpitations if I lay on my back for more than a few mins. I had to lie on my side/sit up when attending appt for scans, etc. Yes there are dangers and risks but, you will feel it way before there is any risk to baby.
It’s about the uterus partially blocking the artery that serves the legs, so only from 3rd trimester. But even when I was a student nurse in the 80 s I remember an obstetrician saying it probably only applied to women lying on hard surfaces, not beds. So I’m surprised it’s still a thing, especially as nobody mentioned it to me when I was pregnant 14 and11 years ago.