[525]

I agree that it should definitely be a choice.

That being said, a significant portion of the pain is from giving birth on a hospital bed - or just lying flat in general. It is said (as a man, I have never given birth) that it is less painful, and therefore less in need of an epidural, if the birthing is done in a upright or crouching position. Like how an orangutan does it : http://youtu.be/JfVnFJDjUyQ?t=30s

The very idea of lying down for a birth dates back to the 18th century, when some French king wanted to see the birth of his heir. Of course men were not allowed in the birthing room, so he had to peek through a hole in the wall - or something like that. And the queen had to be in a position that allowed the king to see, so she was lying down facing so the king could see what was going on.

[mcv]

You don't have to lie on a bed in the hospital. You can still use a birthing stool or something (assuming the hospital has them, but they're cheap, so there's no reason not to).

[ryanhuff]

Having witnessed the process, something tells me that a significant portion of the pain is not caused by laying down. So, citation please.

[lotsofmangos]

I would like to cite Newton here. Specifically his work on gravity as it applies to motion.

[viraptor]

I believe parent meant that position doesn't change size. It may be faster, but one way or another the pain does come from pushing the baby out and potentially making lots of damage to someone's crotch in the process.