There are many advantages to giving birth in positons such as squatting, kneeling or getting on your hands and knees, rather than lying on your back. X-rays have shown that the pelvic outlet becomes wider when squatting or on your hands and knees, which helps to shorten labor.
The evidence largely suggests that lying on your back during birth prolongs labor, and slows contractions, yet the majority of women in the US give birth in this position.
So how did humans end up giving birth in this position in the first place? Well, the answer is basically because a perverted king enjoyed his wives giving birth in this position way back in 17th-century France. The position has only been widespread in Western culture for the past 200 years, and King Louis XIV is to blame.