I've stumble upon something I haven't heard of... Brachial Plexus Injuries. Wikipedia explains what this is:
The brachial plexus is a network of nerves that conducts signals from the spine to the shoulder, arm and hand. Brachial plexus injuries are caused by damage to those nerves. Symptoms may include a limp or paralyzed arm, lack of muscle control in the arm, hand, or wrist, and lack of feeling or sensation in the arm or hand. Although injuries can occur at any time, many brachial plexus injuries happen during birth: the baby's shoulders may become impacted during the birth process causing the brachial plexus nerves to stretch or tear.
I've learned of the commonality of this injury at birth from a website called The United Brachial Plexus Network. They have a web page called "Brachial Plexus Injury Prevention Program", Positioning for Prevention. Here is a quote from that page:
The obvious problem is that by lying on one's back or by sitting semi-reclined on one's tailbone, the space of the pelvic outlet (birth canal) may be reduced by up to 30%. By adopting a traditional hospital position that is convenient for birthing professionals, one unknowingly reduces the space the baby has to enter this world.
There is no medically sound reason to give birth on one's back, and there is every reason to give birth in a safer position that works with female anatomy and physiology, in a way that women’s bodies were designed.
It is so easy to open up the birthing canal to help ensure a safer passage for the baby, and to reduce the risk of injury to the mother's body as well.
By simply rolling over to one's side, which can be easily accomplished, even with an epidural, allows the sacrum the freedom to move back as the baby is passing through.
What is most important to remember is to get the mother off her back!
Some of the risks this web page lists are: broken collarbone/clavicle (that actually happened to my firstborn); compressed umbilical cord; disruption of the baby's oxygen supply. Please visit the website listed above for a complete reading.
I have birthed in a squatting/kneeling position. Having done this, I simply don't understand how I could have birthed in a supine position the first two times! Nearing delivery time, my pelvic region feels like "a bag of bones". They are flexible and "loose". I know it's my body preparing for the passage of my baby. What a shame not to take advantage of nature's provision and instead take a much more painful route. Even more shameful is that practitioners prefer this position because it works for them!