Why do most women give birth (in the United States, anyway) while lying on their backs? Is it the most comfortable way to do it? Nope.
It goes back to Louis the XIVth and his thing about watching his mistresses deliver his offspring. He didn’t like having anything obstruct his view, so he insisted they lie on their backs.
It came to be the standard in the U.S. around 1900. Many women had started delivering babies in hospitals attended by doctors, and the supine position was more manageable for doctors than other positions.
This is a good article about some of the history: The Bizarre Reason Why Women Started Giving Birth on Their Backs.
I already wished I could have had a midwife and a doula to help when my daughter was born. This knowledge makes it that much more of a wish!