Researchers Think They’ve Discovered Why Human Birth Canals Are So Damn Twisted

by Kristen Mae
Originally Published: 

One of the stories I remember well from my education at a private Baptist elementary school is The Fall of Mankind. The story, laid out in the first book of the Bible, tells about how Eve ate the forbidden fruit in the Garden of Eden and then convinced Adam to have some too. As punishment for their disobedience, God cursed Adam to always toil and labor to get the earth to bear fruit. For Eve, and, according to what I was taught, for all women forever and ever, God’s curse was that we would endure long, excruciating childbirths. The verse from the New American Standard Bible reads: “To the woman He said, ‘I will greatly multiply Your pain in childbirth, In pain you shall deliver children; Yet your desire will be for your husband, And he shall rule over you.'”

Hope that pomegranate was worth it, Eve!

Those early verses underscored the expectation of steadfast obedience, lest you incur retribution not only on yourself, but on every human being to come after you, for all eternity. It didn’t seem fair, but the lesson was clear: Fuck around, and you will find out. Also, don’t forget that God loves you!

Later, I attended public school and there began the slow unpacking of my indoctrination. But it wasn’t until I took a college anthropology class that I began to think again about why it is that humans have such difficult births compared to other mammals. The theory at the time was that our narrower birth canal must be attributed to our bipedalism. It was a logical hypothesis, since it was the primary difference between humans and other primates who experienced much simpler births. The explanation was that a narrower pelvis contributed to the efficiency of bipedal locomotion — walking, in other words. Walking was more evolutionarily advantageous than a simple, pain-free birth, so that’s the direction our DNA traveled.

Now there’s a new explanation. A research team from the University of Vienna’s Unit for Theoretical Biology wanted to know exactly why humans have such a twisty birth canal, making birth incredibly long and painful compared to other mammals, and even other primates. Is it really just because we need narrow hips to facilitate walking? And even if that is the case, why so twisty? Why do human fetuses have to literally twist 90 degrees as they descend through the birth canal? Why is the birth canal itself not a uniform shape? Why is birth so fucking hard for us?

The specific question the research team posed was, “Why is the longest dimension of the lower birth canal not aligned with the longest dimension of the inlet, thus requiring the foetus to perform a complex rotation of the head and shoulders to pass through the birth canal? What is the advantage of a ‘twisted’ birth canal?”

If we think about the shape of a human baby, it makes sense that the baby has to twist as it comes out. The head is widest from front to back. The shoulders are widest from side to side. If the last part of the birth canal is elongated, or elliptical, it stands to reason that a 90 degree twist would be necessary to free the shoulders. But there’s a whole lot of twists and turns in the canal itself compared to other primates, and the researchers wanted to know exactly why.

And what they found is that the twisty shape of the human birth canal is not so much so we can walk more easily, but so we can stand erect. A straighter birth canal would require a deeper curvature of the lower spine, making standing up more difficult and creating health issues with the spine.

To be fair, standing erect on two legs is a requirement of bipedalism. So the original theory was definitely on track.

Additionally, the researchers noted that the narrower elliptical shape of the human birth canal prevents pelvic floor issues. Their models suggest that a wider (rounder) lower birth canal is more likely to cause issues with the pelvic floor in response to the physical pressure of childbirth — yes, like peeing when sneezing. Seriously, can we ever catch a fucking break?

Note that I said, “their models suggest.” Yes, that means this study was performed with modeling and not on human subjects. Researchers used a three-dimensional modeling and testing program called Finite Element Analysis, or FEA — the same complicated mathematical computer modeling used to simulate real-world scenarios for testing auto parts, airplanes, and bridges — to simulate the human birth canal.

So there is definitely more room for research, which the authors of the study point out. An article in Science Alert reminds us that evolutionary studies on Neanderthals have shown that this closest of human relatives had a birth canal more like a chimpanzee’s. This is a clue that our twisty, painful birthing process is, evolutionarily speaking, likely a more recent development.

It’s all very interesting, and it would be nice if any of these findings could somehow lead to easing the pain of childbirth. Mostly, I’m just relieved that my 36-hour labor with my first-born is likely not due to some poor ancient ancestor of mine taking a bite of an apple.

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