Human Birth Canals Are Seriously Twisted. Researchers Think They've Figured Out Why

Human physiology is unique in that it has a strange twist that makes giving birth more difficult for our species. This mystery has been solved by a biomechanical model of gait and posture.
Because of its narrowness, the human birth canal is kinked in the inlet. This means that contractions must be made by the mother to rotate the baby's large brain and wide shoulders almost 90 degrees so they can fit into the pelvis.

Imagine your foot sliding into a tight boot with an awkward entrance. The baby can get stuck and it can cause serious injury to both mother and baby. This is a common problem in up to 6 percent of all births around the world.

What's the benefit? Surprisingly enough, even though this is a critical element in the reproduction and survival of our species, we are still trying to figure it out.

(Stansfield et al., BMC Biology, 2021)

Above: The rotational births of humans. A) The head is shown turning 90 degrees to fit in the largest dimension of a pelvic plane. B) The layers of the birth canal.

Some of the most important parts of human pregnancy remain a mystery today. For example, we don't know why our species has to endure such dangerous labors than other mammals.

It is believed that the human pelvis was designed to facilitate walking. Evolutionarily speaking the benefits of bipedal movement on daily basis were well worth the additional risks associated with small hips and large-brained babies.

The new study reveals that extensive biomechanical models of our pelvic floor show that the shape of our birth canal does not help us to walk, but rather helps us stand.

Philipp Mitteroecker was also part of this study. "We believe that the transverse inlet lengthening of the pelvic inlet evolved because of the limits to the front-to back diameter in humans imposed balancing upright posture rather than the efficiency of bipedal locomotion," he said.

A baby might be able to pass through the inlet at the womb without fuss if it was a larger oval than the birth canal.

However, in humans, this would mean that the pelvis tilts at a greater degree than it does now, which would result in a steeper curve to the lower back.

The new models indicate that an extra curve could compromise the stability and health our spines. This is why the birth canal inlet has a new shape.

Other primates like chimpanzees can have a deeper pelvis inlet because they tend to be on all fours and don't put a lot of weight on the hips. Chimpanzees' young need to only twist their heads slightly to reach the birth canal.

To fit through the narrow ellipsoid, a human baby must move their body almost 90 degrees.

It's not an easy transition into the real world, even after such a complicated maneuver. Also, the outlet of the human birth channel is slightly different from that of primates. The baby must turn again to lift its shoulders, which are wider on a different axis than the head.

Researchers have created models that suggest that the outlet of the birth channel is shaped in this way to support the pelvic floor.

The results show that if the lower birth canal had a wider outlet, it would increase pelvic floor stability. However, it would make childbirth more risky. The last twist would make it too difficult for the shoulders and head to move through.

The authors conclude that "our results provide an evolutionary explanation for the unusual shape of the human's birth canal."

This is a fascinating idea, and it's based on a well-thought-out model. However, real-world research will be required to find out if this is why humans were born with a twist or a shout.

For example, evolutionary studies have shown that female Neanderthals had birth channels more like chimpanzees. This suggests that twisting may be a unique evolutionary development.

It is interesting to see how ancient Neanderthals walked and stood on two feet. This would help us understand why our pelvis is so different.

The study was published by BMC Biology.