Human Female Pelvis: Epic Fail or Evolutionary Advantage?

 

“Most women can’t deliver a baby without medical assistance because the human head is so large” … Ouch

 I love science. Recently I got across a YouTube channel called BeSmart that explains science to kids in funny and engaging ways (and adults like it too). I was enjoying the episode on “Evolution FAILS in the Human Body” (I know) when the host, Dr. Joe Hanson, made that statement.

The whole thing is supposed to be kind of funny, but let’s hang in there for a few minutes and clarify what we are dealing with.

 

What Dr. Joe refers to in this statement is known as the “obstetrical dilemma”:

Because we humans evolved to walk upright on two legs, our pelvis had to get narrower for better stability and locomotion. But human females still had to deliver babies the same way as other mammals – via the pelvis (and its birth canal that is large enough). As a result, the human female pelvis is believed to be a compromise between designs most suitable for both childbirth and bipedal locomotion.

At the same time, human babies’ brains grew larger than other mammal babies’ brains at the time of delivery. They had to be born earlier, “uncooked”: by one estimation a human fetus would have to undergo a gestation period of 18 to 21 months instead of the usual nine to be born at a neurological and cognitive development stage comparable to that of a chimpanzee newborn.

Therefore, with smaller pelves and bigger heads, our births are more difficult than in mammals.

 

Reconstructed pelvises of the juvenile male (left) and adult female (right) _Australopithecus sediba_ specimens. Peter Schmid, courtesy of Lee Berger and the University of the Witwatersrand (source)

Here it is the belief about the necessity of medical help that we welcome to the stage:

  • Because our births are more difficult, we need help;

  • Therefore, we need medical help – typically meaning hospitals and doctors.

 

Yes, our births are trickier compared to cats and even chimpanzees. Yes, there are reasons why human mothers prefer to have company and support at birth.

But wait, how did we survive as a species, if the first instrument that really made it possible to save a baby’s life in complicated labour - the forceps – was introduced only in 1600?!(*) Which makes this tool available in 0.0001% of the timeline of human existence. I chose this moment as a threshold because this is when the birth territory started to be explored by men - barbers and doctors, instruments, charts and linear reasoning. That’s the earliest sign of the “medicalisation of birth”, however useful it might have been at that time and continues to be in some situations.

Till then, human females used the help of other women in labour, and there are two evolutionary hypotheses for this. One is that it was for social reasons. Two, that it was because a human baby comes out facing away from his/her mother, and the mother can’t see her baby’s face. Over time, women developed some serious knowledge base about help needed in birth, and therefore influenced the selection process, allowing even bigger heads to go through the tight birth canal. For centuries, this help was provided exclusively by female companions or midwives. You, me and Dr. Joe are here mostly because our female ancestors successfully birthed babies long before hospitals and doctors were the norm.

As anthropologist Dr. Anna Warrener points out in one of her interviews, the best evidence that human childbirth generally works quite well is the fact that there are about 7.9 billion people on the earth.

And the more we label childbirth “difficult”, the more we reinforce the idea that it is, especially in mothers giving birth for the first time. Can we please stop? Yes, it’s not exactly fun and leisure, but it works, and the overwhelming majority of us as totally fit for the job!

 

A male pelvis (left) compared to a female pelvis (right) (source)
Even when a female pelvis is described in textbooks, this knowledge is often based on the pelvis of a deceased person – because that’s how the study of anatomy has been done

 

The second belief is not directly stated but rather underlies medicalised thinking: the bony pelvis is what we see when we study anatomy: rigid and unadaptable.

For the biggest part of the history of anatomy, the object of study was a young white male person – or, rather, his skeleton. Most anatomical atlases use this model as a baseline of “normal” representation. And this brings in a whole load of assumptions. Even when a female pelvis is described in textbooks, this knowledge is often based on the pelvis of a deceased person – because that’s how the study of anatomy has been done. Till recently at least - but even with new technologies like MRI we still learn anatomy via the dissection of humans who are not alive.


  • In real life, your bones have a lot of variation.

  • In real life, you have ligaments. During pregnancy, the ligaments between joints in the pelvis become more relaxed, to allow the pelvic “cavity” to expand during labour. The hormone relaxin rises throughout the first trimester of pregnancy, then it will decline but will soar again in the hours before delivery.

  • In real life, your position will change the internal dimension of your pelvis: for example, it was measured that the hands-and-knees position can give you an extra 10-20mm, depending on the plane the measurement is taken for. That’s a lot of space for manoeuvre!

“Thus, measurements of pelvic size obtained on immobile skeletonized pelves and by X-rays, even X-rays taken during labour, may differ from the actual dimensions once the labouring person is away from the X-ray machine.”(*)

 

And still, we come across pregnancy advice such as this one:

“If you’re pregnant or planning to become pregnant and have concerns about how your pelvis shape might affect childbirth, speak with your doctor. They can examine your pelvis to help get an idea of how it’s structured”.

This advice is potentially harmful as your pelvic structure is not rigid but has the potential for change and accommodation. And your freedom of movement matters, including – and especially – the expulsion stage, when many providers still ask you to be on your back, with knees apart. Please don’t forget to claim the two extra 2 centimetres, they are yours to grab!

 

Let’s summarise these adaptations of your pelvis in labour we discussed so far:

  • our pelvis allows for movement between its three bones, and

  • the space between pelvic bones depends on your position and ability to move,

but also let me remind you that:

  • the baby’s head is moulded in the process AND moulds the maternal pelvis as well

  • human babies, unlike other mammals, rotate while navigating the pelvis – making this passage possible AND priming themselves for the rotational movement that will be crucial for motor development once outside the womb.

These changes are minimal, but they are precise in their coordination: like having the right key for the right lock.

Let’s talk more about rotation

I only briefly mentioned rotation in the birth canal so far, but the fact is this: There is no other mammal whose pelvis fits so tightly to their baby's head that babies have to rotate within the birth canal. There is no other mammal that developed a “twisted” birth canal to adapt to this need.

Watch the rotational movement of birth - starting from 0:49

But also, there is no other mammal whose babies come to walk completely upright and rotate around their axis.

Can you imagine a pirouetting chimpanzee? Surely the ability to move around one’s axis in any direction without spending extra time on prep was an important survival skill.

This kind of movement organisation is, by the way, Moshe Feldenkrais’s favourite explanation of a good posture – or “acture”, as he didn’t like the “posture” word.

“...posture relates to action, and not to the maintenance of any given position. Acture would perhaps be a better word for it.” -Moshe Feldenkrais, The Potent Self: A Study of Spontaneity and Compulsion

Could this ability require finer motor control and a need for more brain real estate? Could it contribute to the development of a brain and a nervous system as complex as ours?

 

I’ll add to the mix a statement made by Michelle Turner, the author of the Newborn Movement Assessment ™:

“One of the primary purposes of a vaginal delivery is to initiate rotation in a newborn’s central nervous system and also, to stimulate the brain’s ability to learn from movement and environmental stimulation”.

 

When I put these ideas together, what I wonder is this:

Could this tight relationship between the human female pelvis and human fetus head size be an evolutionary advantage, after all? Did it help us develop even better brains as a species?

Whether this suggestion is true or false – I humbly don’t know, but I am excited about this evolutionary possibility.

 

I hope I’ve convinced you in any case that our female pelvis is pretty cool. Especially if we don’t stand in its way and do everything to support the wonderful physiology of birth and let the pelvis adapt to do its job.

  

At the end of that BeSmart episode, Dr Joe says: “… surviving isn’t about being perfect, it’s about being good enough. It’s about being imperfect in the perfect way … Because you’re a survivor of a 4-billion-year story.” And with this, I couldn’t agree more: “A good enough pelvis” sounds like a great concept!


P.S.: Just another thought. Do you know who are the species that can easily give birth to babies up to 40% of their size? Bats! Because they spend a lot of time upside-down, their pelvic floors are not challenged as much as ours, and they can allow their pubic symphysis to open wide during birth. You can guess that in bats, this wouldn’t bring a risk of incontinence and prolapse. But if humans had chosen this evolutionary pathway, they wouldn’t be humans!

Imagine: while you are labouring and coping with labour challenges and discomforts, you are literally shaping and priming the human evolution with your pelvis!

And if you are a midwife - your help and assistance are an integral part of this evolutionary process, for which humanity is forever grateful.

Thoughts or questions? Let me know in the comments, I am curious :)


Sources and references:


 
Alina KomnatnayaComment