Thursday, May 7, 2009

Why Are Human Infants Born So Prematurely?

Anyone who has held a floppy headed, red faced newborn baby in their arms must have wondered at some time why human babies are born so helpless, whilst other mammals are up on their feet within minutes of birth. So why don't humans gestate their babies for longer? Wouldn't a newborn be better equipped for survival if it had had a little bit longer inside mum?

I had long accepted the simple logistical explanation for this. Human childbirth is very difficult, painful and dangerous, for two reasons. First of all, we walk on two feet and we have evolved very narrow pelvises (or should I say pelves) to facilitate this movement. So the female pelvis is a compromise between narrowness for walking, and width for birthing children. You can see some interesting pictures of male and female pelvises by clicking here.

The second reason for the difficulty of human childbirth is the sheer size of our heads, which must accommodate our highly developed brains. In order to make childbirth work at all, we have evolved a complex method of birthing that requires the baby to rotate during delivery to fit different bits of its body through the pelvis. It is no wonder that so many things can go wrong. Neither is it surprising that we have developed a tradition of having assistants present during birth, a practice which is unseen in the rest of the animal kingdom (as far as I am aware).

So one explanation for the relative immaturity of human newborns is that gestation has to be short so that a baby's head does not grow too large to fit through its mother's pelvis. This explanation is very logical, but it does not explain why chimpanzees, gorillas and orangutans also limit their gestations to eight or nine months. They may have large heads like us, but without the limits on their pelvis size, why don't they take the opportunity to cook their babies for just a little longer? These great ape babies are born just as helpless as human babies - just like humans, they cannot even raise their own heads (although they do acquire skills like clinging on to their mothers' fur quite quickly).

I found a possible answer in the book "Why Love Matters" by Sue Gerhardt. She suggests that a human baby receives extra time outside the womb so that its brain can develop appropriately for the social environment that it will live in. Since human beings live in a such a variety of different ways across the world, perhaps it is necessary to expose the human baby to its own society at a very early stage. She also suggests that the increased activity that a brain experiences outside of the womb may be necessary to help it to grow quickly enough. She explains this much more elegantly than I can:
"Towards the end of its first year, the preparatory stage of infancy comes to an end. In some ways, the human baby now reaches the level of development that other animals achieve inside the womb. But by doing it outside the womb, human brain building is more open to social influence. This extended human dependency outside the womb enables an intense social bond between caregiver and child to develop. This generates the biochemicals that facilitate a high level of neural connections and brain growth which will never be as rapid again."

The advantage of this explanation is that it is also applicable to primates like chimpazees and gorillas - they also have complex societies, and different cultures within different groups, and they may also require the extra time outside the womb to socialise their babies and to grow their large brains.

So perhaps our relatively short gestation is not just a compromise between small pelvises and big heads. Perhaps our babies really do need to meet us at that helpless early stage so that we can begin our lfetime's work of teaching them how to be human.

By the way, "Why Love Matters" is absolutely fascinating, especially if you have an interest in psychology or biology, and it is giving me so many eureka moments as I read it. And I still have half of the book to go!


Jessica - This Is Worthwhile said...

That's so interesting! The pelvis site talks about "subluxation" - I TOTALLY had that!! My midwife told me it was the cartilage in my pubis stretching out, but she wasn't as scientific. OMG, it was SO not a fun feeling, it hurt ALL the damn time!! I remember it starting about a week or two before my shower and that was 8/18 (my son was born 10/9). My friend who was there, who was 3 weeks more preggers than me, wasn't feeling a thing. I never saw her again after to find out if hers ever hurt.

This whole birth thing is pretty amazing. I was at the book store the other day and saw that Desmond Morris (of The Naked Ape fame) has written a baby book about development, etc. I think I'm going to check it out at the library some time.

Liz said...

I also found Why Love Matters fascinating, with many eureka moments, and have been recommending it ever since!

docwitch said...

This really is fascinating! It does make sense that there are complex interactions between the physiological and psycho-social involved.

I must look for that book, and for responses to it.

Loukia said...

Wow, this was the most interesting post I have read in a very long time... very thought provoking! Totally fascinating. But imagine we were pregnant as long as elephants - 22 months? I think we'd all end up only having 1 child, if that were the case! ;) But very interesting post... thanks.

Anonymous said...

I read long ago (can't remember where) that humans give birth so much earlier than other animals because of our flawed evolution.

When we were on all fours, humans gestated for a year or more, and baby came out much more developed than they do now.

However, when we began to walk upright, the babies needed to come out earlier because the new position and narrowness of our pelvis necessitated an earlier delivery/smaller child. This all seemed/seems to make sense to me.

Mon said...

Fascinating stuff.

I can't say I'm entirely convinced on her theory though. Saying we all live differently doesn't expalin early humans and their very simple and varied existence free of 'society'. And what about other animals with relatively complex social structures?

So I think for me the whole thing about human babies being born more helpless and creating a high amount of new brain-connections during their first years, is more about the 'what now' than the 'how come'. She's saying ,'this is why it happens', I say, 'this is what happens, what can we do with it?'

In other words - how can we best support them? You know?

Anyway, do share more snippets of that book, sounds really interesting.

Cave Mother said...

I know, it doesn't quite make sense - elephants seem to have pretty complex societies too, but they have long gestations. And whales. I didn't put this in the post because I wanted to present a coherent argument! But I thought it was a very interesting theory. Perhaps it is all down to bipedalism.

cartside said...

Definitely a new theory to me. It seems to make some sense because brain develops a the fastest rate during the first year, but I guess it's a chicken and egg thing - do we have short gestations to have brain development or does the brain develop so massively in the first year because the baby is out of the womb.

flowers said...

Oh so interesting. Thanks for sharing.