you're not supposed to lie on your back after the first trimester. it makes the uterus lie on one of your blood vessels and can restrict the flow of blood to your baby.
i tried to avoid lying on my back from about week 10. if i turned over from one side to the other, i got nervous. i didn't like the weight going backwards, even for that fraction of a second.
ironic. that it will never have made a difference to my baby.
every time i lie on my back now - to read in bed, lying on the sofa to watch tv - i remember.