Head vs Pelvis

While reading Birth: The Surpising History of How We Are Born by Tina Cassidy I came across an interesting discussion of the evolution of the mechanics of human birth.

It starts out by stating that human babies are becoming larger over time as a byproduct of better nutrition and overall quality of life, and human brains appear to have grown larger over the last 2 million years.

She continues by describing the very specific way the human baby has to travel through the pelvis in order for birth to work succesfully. It is best for the baby to enter the pelvis chin tucked and facing to the side, and upon moving through baby needs to twist and and turn face down. Once the head is born, the baby must turn one more time to let the shoulders through. All of this twisting and turning evolved to accommodate larger head sizes with marginally larger pelvic openings. There is not a lot of room for deviation from this prescription of movement, but there is some, i.e. not all successful vaginal births happen this way.

The most interesting conclusion from this is that as humans evolved larger heads, human females did not evolve larger pelvises in the same proportion. Essentially, humans had to start giving birth to their babies before they were "ready" to leave the womb. This is one reason why human infants are one of the most helpless creatures compared to their counterparts in the wild. We all have seen the image of the newborn foal awkwardly standing and then running just minutes or hours after being born. I think I read, as far as mammals go, Panda and human infants rank as the most helpless. The first few months out of the womb is sometimes called the 4th trimester.  As we evolve further and likely brain sizes grow, thus head sizes, will humans eventually get to a point where all births are cesarean sections and vaginal birth is not even possible?

For multiples, this issue is less a factor because they are almost always smaller than singles and thus have an easier time traversing the pelvis. This is one reason I feel so fortunate we are having twins! Of course there are two births as opposed to one and that presents different challenges, but I am confident that K will  have an easier time helping each one of our little ones out because there are two of them.