The Human Baby as an External Foetus

by Deb on September 17, 2009

I originally studied human anatomy and development, so this is a pet interest of mine.  And I think we’re all fascinated by babies and development!  However I owe a great deal of this post to the amazing, unfortunately late, Stephen Jay Gould.  He was one of the great science writers of the 20th century and explained part of this beautifully.  If you are interested in history, natural or human, read his books.  This is a topic that can be dealt with over hundreds of pages and I’m doing it in a blog post, so I’m whizzing over it.  Please ask questions or comment, I’d love to explore this further!

It sounds weird, but there are several things that lead to the idea that human babies aren’t really meant to be here yet.

  • Our pregnancies are way too short for our lifespans.
  • Our babies are far more helpless than other primate babies.
  • The way babies grow is like other animals do when they are still inside, not once they are born.
  • Our pelvis’ are a compromise – birthing is much harder for humans than other animals.


Very roughly, an animal’s lifespan is related to it’s ‘metabolic age.’  So mice and elephants have around the same number of heartbeats and take the same number of breaths but mice live much quicker and therefore shorter lives.  If you look at our metabolism, we live far too long.  We should really only last around 40-50 years.  Comparing our life history to other primates, the proportions are out of whack too.  We have this weird gap in the middle called being a teenager, and we live far too long after menopause.  But our pregnancies are relatively short – to fit into the normal primate pattern they should be 18-21 months long.

Helpless Babies

We all know it, human babies are completely helpless and dependent on their parents.  There are two main types of mammal babies, ones that are tiny and helpless and left in a nest, like mice, and ones that are well developed and go with their parents, like horses.  Humans don’t really fit into either camp.  Our babies are more like the mice, but they don’t come in litters and we don’t leave them while we go foraging.  But they certainly aren’t ready to run with the herd, or even hold on to Mum.  It’s like they are meant to be big, single, well-developed babies, but they came too soon.

Growth Patterns

There are distinct growth patterns at different ages.  All placental mammals grow very quickly as foetus’ then slow down once they are born.  This is especially true in brains, in most animals they only grow a little bit after they are born.  Except of course in humans.  Baby growth patterns and particularly brain development are like other animals in utero, most more than triple their birth weight in their first year.  Their growth patterns don’t match other animals until they around 9 months old.

Starting to notice a pattern?

It seems that 9-12 month old babies are more like newborn primates than our newborns are.  So why don’t we have 18-21 month pregnancies?  Elephants manage it.  There’s not a simple answer, there are all sorts of advantages in terms of energy and learning stimulating brain development.  But a big part of the answer is because we walk upright.  To balance upright and walk on two legs the muscles have to pull in different directions than if you are using all four legs. So the bones they are attached to have to change shape as well.  And an important one in terms of birth is the pelvis.

Pelvis during birth

From Lovejoy, 2005

On the left is a chimp’s pelvis and baby’s head during birth.  In the middle is a possible reconstruction of a possible human ancestor, and on the right is a human.  You can see straight away there is always room around the chimp baby’s skull, it doesn’t have to turn and the fontanelle (gap between the skull plates) is always open.  The human baby  has no extra room in the pelvis, has to twist to get through at different levels, and the fontanelle is squashed closed when they are right in the pelvis.  In the reconstruction it seems that there are already some adaptations during birth – the baby turns but doesn’t have to turn back and does not close the fontanelle.  This ancestor appears to walk fully upright, but had a head only a little bigger than a chimp – an upright ape.

Obviously humans can still give birth safely most of the time – otherwise we wouldn’t be here.  But it is more dangerous than for many other animals and our pelvis is very much a compromise between being able to walk and being able to give birth.  Our babies have had to adapt the way they birth to get out, they have also had to adapt the timing.  If we had the 18-21 month pregnancy we ‘should’ there is no way the head would fit through the birth canal – can you imagine birthing a 9-12 month old?


This change in timing has had huge implications for humans as a species.  Rather than having relatively robust babies who can hold on to us, we have helpless external foetuses.  Think of the social impact – they aren’t attached to a placenta but need to be nourished almost around the clock.  They need to be carried and held, which severely restricts whoever’s doing the holding.  Forget fire, I’d be prepared to bet that the earliest human invention was some form of baby carrier!  There are hints that AL288-1 above could still grasp with their feet, and (male) researchers have speculated on climbing ability.  Personally I wonder if it was useful for babies to hold on.  Mothers holding and feeding a newborn cannot forage as well for themselves but need help from others.  And then there’s the impact on the baby’s developing brain.  Most animals grow this most amazing organ while the world is filtered through their mother.  Our babies develop their mental connections while having direct sensory and social stimulation.  Our large brains made early birth necessary, but did early birth then give them the opportunity to develop even further?

Our babies’ first year can be daunting, confusing and frustrating.  But when it all gets a bit much, maybe it will help to remember they aren’t even supposed to be here yet. 

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{ 7 comments… read them below or add one }

Capricious September 17, 2009 at 10:35 am

My gosh, this actually makes total sense. And it makes it so much easier to deal with a rough night and a needy day knowing it!


Anni Taylor September 19, 2009 at 6:58 pm

I haven’t heard the term ‘external foetus’ before but for some reason I like it. It does describe the intricate care we must give babies in order for them to survive.

I’ve seen a program where the shape of a woman’s pelvis was being discussed in relation to the difficulty of birthing babies, and it does seem true that the ‘design’ could do with a few tweaks!


Annie September 21, 2009 at 8:11 am

I was talking to my mum about this the other day, trying to explain why I wear my babies as much as possible, sleep with them, feed them on demand etc – its because they really do need to still be with their mums for the first 9-12 months. So cool to see some more information on this, thank you!


Deb September 27, 2009 at 9:32 pm

You’re welcome, it’s a pet theory – I love it when you get lots of different lines of evidence that back something up. I’d really like to look at some of these old fossils from the point of view of baby carrying – having had fairly clingy michelin babies myself I just don’t see how it would have been possible without the baby being able to hang on or a carrier!


Sandi November 21, 2009 at 1:03 pm

I often refer to this as the 4th trimester.


ngyuntju March 12, 2011 at 9:22 pm

I am in science nerd heaven. Great blog, thanks!


daisy swadesh June 23, 2012 at 5:57 am

I once found this illustration in a book, and have been trying to find it on the web every since. This one shows the differences very clearly.
The change in the configuration of the pelvis to adapt for bipedalism made the birth canal a bottleneck. The only possible solution to permit increased brain size is greater post-natal brain development.
But once increased post-natal brain development began our brains could grow much bigger. A human baby’s brain doubles in volume in the first year and continues growing at a slower pace; a chimpanzee’s brain doubles in volume in 5-6 years.


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