Among my old order mennonite women childbirth is NEVER discussed, so how they give birth is never passed down. This was what I saw in Lancaster, PA and what I see in Central NY. I'm surprised that sisters don't even know each others' due dates, and the thought of how they give birth would RARELY arise in conversation. When these women marry, they are not even sure about the logistics of having sex--it is never discussed. The example I gave about a sister discussing the hands & knees position is very rare in my area. This is a very progressive family. When a new baby arrives in a family, the younger children are told Jesus brought the baby. Even when the children are old enough to see Mom's belly getting big, the reality is never discussed. I have my own theory about how lying supine started. When women gave birth by themselves they had to squat because that is the only way she could catch her own baby. When labor attendants started helping, women could "relax" more to give birth and then layed down on their side or back after a long, exhausting labor--not considering that gravity might help the situation. I really think relaxing the pelvic floor muscles helps more in the birth of a baby than gravity. Lay down and relax, your body will do the rest.