Fwiw, our recent American culture is one of the very few in the world that encourages women to lie on their back when pushing a baby out. In most other countries and cultures, as well as our own pre-hospital days, women labored and pushed more upright, squatting/on all fours/standing while holding someone for support, sitting in a birthing chair...because that is how the body works best, working with gravity, and to open up the pelvis.
I read a whole book on the history of labor and birth in our country, and one thing I took away from it is many things (like on your back, feet in stirrups pushing) evolved out of convenience sake, not because they were backed by science or necessity. A recent study showed this once again.
http://www.ncbi.nlm.nih.gov/pubmed/21826038
"One third of the recommendations put forth by the [American College of Obstetricians and Gynecologists] in its practice bulletins are based on good and consistent scientific evidence."
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