You'd think people would be smart enough to realize gravity assists birth. The laying on your back thing is actually a left over Catholic thing from the late middle ages. It was commonly thought that a mother should suffer the pains of childbirth in silence as a form of pennance and she would be bound to the birthing bed to keep her from "Harming" the baby or being immodest by allowing her body to be exposed in any way. Most babies were actually born under the blankets with a midwife guiding the process solely by touch. To scream in childbirth was considered shameful, as well. The binding happened right around the time that the birthing stool fell out of favor