I'm 18 weeks and laid flat on my back earlier for around 40 minutes for an beauty appointment
I think I have started feeling little kicks and flutters recently. When on my back I am sure I felt the baby a few times which I thought was because I was lying down and everything was sort of nearer to the surface iyswim
I've since read that lying on back is contraindicated in pregnancy and now I'm worried baby was in distress? I've not felt anything since but if I am feeling baby it's usually sporadic anyway!
It's because the weight of the baby presses against one of your major blood vessels (I forget which one!) which makes you feel dizzy. There is nothing bad for the baby, just not ideal for you. You would definitely know about it very quickly if the blood vessel was cut off though, and would move pretty sharpish to change your position!
I was told by my midwife not to worry about this until about 26-28 weeks when baby starts weighing enough to put pressure on the blood flow etc
I had a massage at 37 weeks which meant i laid on my back for about 45 mins (i was stupid not to say anything to the woman) and when i got up i had a headache felt sick and lightheaded etc. However even then we were both fine! But i wouldn't suggest you do this when you are so close to term.
I'm 36 weeks and still wake up on my back. My midwife has said if baby is in distress they'll kick hard enough to move you. It's all about the weight on your blood supply etc, at 18 weeks I doubt the weight would be enough to cause distress. I felt my first movements when on my back, didn't feel any when on my side or standing until much later in my pregnancy, x,
Like others have said - it is about you rather than the baby as it may make you feel unwell. But it used to be thought that it cuts out blood supply to the baby - my understanding is that they no longer believe that is the case at all, and as others have said in any event if there was an issue your body would let you know! But that is why there is still some “old” and contradictory advice floating around out there. But if you are ever worried call the midwife line at your hospital - hearing it from them will no doubt be a lot more reassuring than from a bunch of Mumsnet people.