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Why Women Deliver With Difficulties by thinkmoney(m): 2:07pm On Dec 22, 2016 |
By Colin Barras
22 December 2016
Giving birth can be a long and painful process. It
can also be deadly. The World Health
Organization estimates that about 830 women
die every day because of complications during
pregnancy and childbirth – and that statistic is
actually a 44% reduction on the 1990 level.
"The figures are just horrifying," says Jonathan
Wells , who studies childhood nutrition at
University College London in the UK. "It's
extremely rare for mammalian mothers to pay
such a high price for offspring production."
So why exactly is childbirth so risky for humans?
And is there anything we can do to further reduce
those death rates?
Scientists first began thinking about the problem
of human childbirth in the middle of the 20th
Century. They soon came up with an idea that
seemed to explain what was going on. The
trouble began, they said, with the earliest
members of our evolutionary lineage – the
hominins.
From an early date in our prehistory,
hominin babies may have had to twist and
turn to pass through the birth canal
The oldest hominin fossils so far found date back
about seven million years. They belong to
animals that shared very few of our features,
except perhaps one: some researchers think that,
even at this early stage, hominins were walking
upright on two legs .
To walk on two legs efficiently, the hominin
skeleton had to be pushed and pulled into a new
configuration, and that affected the pelvis.
In most primates the birth canal in the pelvis is
relatively straight. In hominins, it soon began to
look very different. Hips became relatively narrow
and the birth canal became distorted – a cylinder
that varied in size and shape along its length.
So from an early date in our prehistory, hominin
babies may have had to twist and turn to pass
through the birth canal. This would have made
birth a far more difficult task than it had been
previously.
Then things got even worse.
About two million years ago, our hominin
ancestors began to change again. They lost their
more ape-like features such as a relatively short
body, long arms and small brain. Instead they
began to gain more human-like ones, like taller
bodies, shorter arms and bigger brains.
That last trait in particular was bad news for
female hominins.
I was going to find evidence that
supported the obstetric dilemma, but very
soon everything came crashing down
Big-brained adults start out life as big-brained
babies, so evolution came into conflict with itself.
On the one hand, female hominins had to
maintain a narrow pelvis with a constricted birth
canal in order to walk efficiently on two legs. But
at the same time the foetuses they carried were
evolving to have larger heads, which were a
tighter and tighter fit through those narrow
pelvises.
Childbirth became a distressingly painful and
potentially lethal business, and it remains so to
this day.
In 1960, an anthropologist called Sherwood
Washburn gave this idea a name: the obstetrical
dilemma. It is now often called the "obstetric
dilemma". Scientists thought it explained the
problem of human childbirth perfectly. Many still
think it does.
But some, including Wells, are no longer happy
with this standard explanation. In the last five
years, Wells and several other researchers have
begun to push against the classic story of the
obstetric dilemma.
They think Washburn's idea is too simplistic, and
that all sorts of other factors also contribute to
the problem of childbirth.
Holly Dunsworth of the University of Rhode
Island, Kingston, was drawn to the obstetric
dilemma while she was still a grad student. "I
thought it was so exciting, I was going to find
evidence that supported the obstetric dilemma,"
she says. "But very soon everything came
crashing down."
We have bigger babies and longer
pregnancies than you would expect
The problem was with the predictions Washburn
made. "When Washburn wrote his article, he was
actually saying that the obstetric dilemma was
solved by giving birth to babies at a relatively
early stage in their development," says Wells.
Go back to that moment two million years ago
when human brains began to grow larger.
Washburn suggested that humans found a
solution of sorts: shortening the length of the
human pregnancy. Human babies were forced out
into the world earlier than they really should be,
so that they were still relatively small, with
diminutive, underdeveloped brains.
Washburn's explanation seems logical. Anyone
who has held a newborn can appreciate how
underdeveloped and vulnerable they are. The
standard view is that other primates hold onto
their pregnancies for longer and give birth to
babies that are more developmentally advanced.
But, says Dunsworth, it is simply not true.
"We have bigger babies and longer pregnancies
than you would expect," she says.
Women give birth to babies with larger
brains than we would expect
In an absolute sense human pregnancies are
long. They typically last 38-40 weeks, whereas a
chimpanzee pregnancy is 32 weeks long, and
gorillas and orang-utans give birth after about 37
weeks.
As Dunsworth and her colleagues explained in a
2012 paper , this remains true even if we adjust
the pregnancy durations to take into account
differences in body mass. Human pregnancies
last 37 days longer than they should do for an
ape our size.
The same thing applies for brain size. Women
give birth to babies with larger brains than we
would expect of a primate with the average
woman's body mass. This means that a key
prediction of Washburn's obstetric dilemma is
incorrect.
There are other problems with Washburn's idea
too.
A central assumption of the obstetric dilemma is
that the size and shape of the human pelvis –
and the female pelvis in particular – is highly
constrained by our habit of walking upright on
two legs. After all, if evolution could have
"solved" the problem of human childbirth by
simply making women's hips a little wider and
the birth canal a little larger, it surely would have
done so by now.
The birth canal is extraordinarily variable
in size and shape
In 2015, Anna Warrener at Harvard University in
Cambridge, Massachusetts, and her colleagues
questioned this assumption.
The researchers collected metabolic data from
male and female volunteers who were walking
and running in the lab. Volunteers with wider
hips were no more inefficient at walking and
running than their narrow-hipped peers. From
purely energetic considerations, at least, there
does not seem to be anything stopping humans
evolving wider hips that would make childbirth
easier.
"The basic premise of the obstetric dilemma –
that having a small or narrow pelvis is best
for biomechanical efficiency – is likely not
correct," says Helen Kurki of the University of
Victoria in British Columbia, Canada.
Kurki was not involved with Warrener's study, but
her own research has identified yet more
problems for the traditional obstetric dilemma
hypothesis.
If the female pelvis really is tightly governed by
two opposing forces – the need to be narrow for
walking and the need to be wide for giving birth
– the shape of the birth canal should vary little
between women. It should be "stabilised" by
natural selection.
Pregnant women sometimes joke that
their developing foetus feels like an
energy-sapping parasite
But after analysing hundreds of human skeletons,
Kurki reported in 2015 that the birth canal is
extraordinarily variable in size and shape . It
varies even more than the size and shape of
human arms, a trait that is known to vary
between individuals.
"I think my findings do support shifting attitudes
to the obstetric dilemma," says Kurki.
Washburn's tidy narrative does not seem quite as
satisfying as it once did. There has to be
something else going on.
Dunsworth thinks she has identified one
important missing piece in the puzzle: energy.
"We max out toward the end of pregnancy," says
Dunsworth, herself a mother. "Those last weeks
and months of pregnancy are tiring. They are
pushing right against the possible sustainable
metabolic rates in humans. It has to end at some
point."
Evolution could, in principle, make the
pelvis larger – but it has not had to
Pregnant women sometimes joke that their
developing foetus feels like an energy-sapping
parasite. In a sense it really is, and its energy
demands grow with every passing day.
In particular, human brains have an almost
insatiable appetite for energy. Growing a second,
tiny brain inside the womb can push a pregnant
woman close to the edge, metabolically speaking.
Dunsworth calls this idea the energetics of
gestation and growth (EGG) hypothesis. It
suggests the timing of childbirth is governed by
the difficulties of continuing to nourish a
developing foetus beyond 39 weeks – not by the
difficulties of squeezing the baby out through the
birth canal.
Dunsworth thinks people obsess too much about
the tight fit between a baby's head and its
mother's birth canal. It might seem too much of
a coincidence that the two are so closely size-
matched, but she says the pelvis has simply
evolved to be the size it needs to be. Evolution
could, in principle, make the pelvis larger – but it
has not had to.
For most of human evolution, childbirth
might have been quite a lot easier
By and large, Kurki shares this view. "The
obstetric canal is big enough, the majority of the
time, for the foetus to pass through," she says.
This is true. But even so, take another look at
the maternal mortality figures: 830 deaths every
day. Even among women who do not lose their
lives during childbirth, some studies say the
process leads to life-changing but non-lethal
injuries in as many as 40% of cases . The price
women pay for childbirth seems extraordinarily
high.
Wells agrees. "It's impossible to imagine the
problem has been this bad over the long term."
Perhaps it has not. In 2012, Wells and his
colleagues took a look at the prehistory of
childbirth, and came to a surprising conclusion.
For most of human evolution, childbirth might
have been quite a lot easier.
The prehistory of childbirth is a difficult subject
to study. The hominin pelvis is rarely preserved
in the fossil record, and newborn skulls are even
thinner on the ground. But from the meagre
evidence available it seems that some earlier
species of human, including Homo erectus and
even some Neanderthals, had a relatively easy
time of it when it came to giving birth .
A shift to farming may have led to
developmental changes that made
childbirth far more difficult
In fact, Wells and his colleagues suspect
childbirth might even have been a relatively
minor problem in our species – at least to begin
with. There are very few newborn baby skeletons
among the human remains from early hunter-
gatherer groups, which might hint that death
rates among newborns were relatively low.
This situation changed a few thousand years
ago. People began farming, and newborn baby
skeletons became a far more common feature of
the archaeological record, at least in some
places.
If there was a rise in newborn death rates at the
dawn of farming, there were almost certainly
several factors involved.
For instance, early farmers began living in
relatively dense settlements, so transmissible
disease probably became a far greater problem .
Newborns are often particularly vulnerable when
an infection is going around a community.
But Wells and his colleagues suspect a shift to
farming also led to developmental changes that
made childbirth far more difficult. A rise in infant
mortality at the dawn of farming might be due in
part to a raised risk of death during childbirth.
Human childbirth suddenly became more
difficult about 10,000 years ago
There is one striking feature archaeologists have
noticed when comparing the skeletons of early
farmers with their hunter-gatherer ancestors. The
farmers were noticeably shorter in stature,
probably because their carbohydrate-rich diet
was not particularly nutritious compared to the
protein-rich hunter-gatherer diet.
This is a telling observation for those who study
childbirth, says Wells, because there is evidence
of a link between a woman's height and the size
and shape of her pelvis. In general, the shorter a
woman, the narrower her hips. In other words,
the shift to farming almost certainly made
childbirth a little bit more challenging.
On top of that, the carbohydrate-rich diets that
became more common with farming can cause a
developing foetus to grow larger and fatter . That
makes the baby harder to deliver.
Combine these two factors and human childbirth
– which might have been relatively easy for
millions of years – suddenly became more
difficult about 10,000 years ago.
Something rather like this "farming revolution
effect" replays whenever human diets become
poorly nutritious – particularly if those diets also
contain a lot of carbohydrates and sugars, which
encourage foetal growth.
"We can make a simple prediction that the
nutritional status of mothers should be
associated with a local prevalence of maternal
mortality and difficulties with giving birth," says
Wells. The statistics clearly follow such a
pattern , suggesting that improving nutrition
might be a fairly easy way to reduce maternal
mortality.
Pregnant women have adapted to nourish
their foetus for as long as they can
Both Dunsworth and Kurki think that Wells has
identified something significant in his work –
something that perhaps would only be evident to
a researcher with the right background in
nutrition and development.
"I'm so lucky that Jonathan is describing these
complex issues from his perspective of human
health," says Dunsworth. "At the same time I'm
approaching the problem from my perspective of
human evolution."
So we now have a new explanation for the
difficulties of human childbirth. Pregnant women
have adapted to nourish their foetus for as long
as they can before it grows too large to feed
internally. The female pelvis has adapted to be
just the right size to allow this maximally-
nourished foetus to travel through safely. And
dietary changes in the last few thousand years
have upset this fine balance, making childbirth
risky – particularly for mothers who have a poor
diet.
However, Dunsworth says that is probably not the
end of the story.
Washburn's ideas made good intuitive sense for
decades, until Dunsworth, Wells, Kurki and others
began to pick them apart. "What if the EGG
perspective is too good to be true?" asks
Dunsworth. "We have to keep searching and keep
collecting evidence."
This is exactly what other researchers are doing.
For instance, in 2015 Barbara Fischer of the
Konrad Lorenz Institute for Evolution and
Cognition Research in Klosterneuburg, Austria
and Philipp Mitteroecker of the University of
Vienna, Austria took another look at the female
pelvis.
A woman's pelvis takes on a shape more
conducive to childbirth in her late teens –
when she reaches peak fertility
It seemed to them that Dunsworth's EGG
hypothesis – compelling though it is – could
actually be seen as complementary to
Washburn's ideas, rather than disproving them
entirely. Dunsworth agrees: she thinks many
factors are involved in the evolution of modern
childbirth.
Fischer and Mitteroecker investigated whether
there is any correlation between female head size
and pelvis size. Head size is heritable, at least to
some extent, so women would benefit during
childbirth if those with larger heads also naturally
had a wider pelvis.
The researchers' analysis of 99 skeletons
suggested such a link does indeed exist . They
concluded that a woman's head size and her
pelvic dimensions must somehow be linked at
the genetic level.
"This does not mean that the [problem of
childbirth] has been resolved," says Fischer. But
the problem would be even worse if there was no
link between head size and pelvis width.
And there is another complication: women's
bodies change as they get older.
A May 2016 study led by Marcia Ponce de León
and Christoph Zollikofer at the University of
Zurich, Switzerland examined pelvic data from
275 people – male and female – of all ages. The
researchers concluded that the pelvis changes
dimensions during the course of a woman's
lifetime.
Many babies are now born by Caesarean
section
Their data suggested that a woman's pelvis takes
on a shape more conducive to childbirth in her
late teens – when she reaches peak fertility. It
then stays that way until around her 40th
birthday, when it then gradually changes shape to
become less suitable for childbirth, ready for the
menopause.
The scientists suggest these changes make
childbirth a little easier than it otherwise would
be. They call this idea the "developmental
obstetric dilemma" (DOD).
"The DOD hypothesis provides a developmental
explanation for the variation in pelvic obstetric
dimensions," says Ponce de León.
If all these evolutionary pressures are acting on
childbirth, is the process still changing and
evolving even now?
In December 2016, Fischer and Mitteroecker
made headlines with a theoretical paper that
addressed this question.
Earlier studies had suggested that larger babies
have a better chance of survival and that size at
birth is at least somewhat heritable . Together,
these factors might lead the average human
foetus to push up against the size limit imposed
by the female pelvis, even though it can be fatal
to push too far.
We all either did or didn't arrive in the
world through a pelvis
But many babies are now born by Caesarean
section, an operation in which the baby is taken
out of the mother's abdomen without ever
entering the birth canal. Fischer and Mitteroecker
suggested that, in societies where C-sections
have become more common, foetuses can now
grow "too large" and still have a reasonable
chance of survival.
In theory, as a consequence the number of
women giving birth to babies that are too big to
fit through their pelvis might have risen by 10 or
20% in just a few decades, at least in some parts
of the world. Or, to put it in cruder terms, people
in these societies might be evolving to have
larger babies .
For now this is only an idea and there is no hard
evidence that it is really happening. But it is an
intriguing thought.
"We all either did or didn't arrive in the world
through a pelvis," says Wells. "If we did, that
pelvis mattered. And if we didn't, that in itself is
interesting."
Ever since live birth evolved, babies have been
constrained to some degree by the size of the
birth canal. But maybe, for some babies at least,
that is no longer true.Join over six million BBC Earth fans by liking us
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