Imagine serving as a physician in the obstetrics
and gynecology department of a large hospital.
Two women arrive on the same day ready to
each give birth to their first child. Both
are twenty-five and experienced an uneventful
pregnancy. They are fit and ready for the
experience. Both planned for a vaginal delivery.
One did give birth vaginally, but the other
ultimately needed a cesarean section.
What happened? Why would two healthy women
of similar size, weight, and fitness experience
different childbirth outcomes?
My name is Florence Yuan, and this is Darwin,
MD.
[Darwin, MD jingle]
It’s no secret that giving birth is hard.
Each day, some 830 women die from preventable
causes related to pregnancy and childbirth.
The global mortality rate is 216 deaths per
100,000 live births.
Meanwhile, the global infant mortality rate
is trending downward. The United Nations Department
of Economic and Social Affairs reports 35
deaths per 1,000 live births between 2010
and 2015. That’s down from 142 deaths per
1,000 live births in the 1950s.
Why is it so difficult for humans to give
birth?
This seems to run counter to evolution, an
efficiency-seeking process designed to make
sure our genes pass smoothly between generations.
Our closest relatives, the great apes, have
a relatively easy time with giving birth and
will generally give birth alone to avoid infection
or stress from others. For humans, though,
it takes a village. Mothers will actively
seek out others to help during childbirth.
This illustrates the concept of trade-offs
discussed in the first episode. At some point
in human evolution, the advantages of human
company outweighed the disadvantages of having
other humans and all their bacteria around.
Nevertheless, childbirth is still difficult
even with lots of help. Returning to the trade-off
model, making childbirth easier may actually
work against gene transmission
from generation to generation.
The problem, which evolutionary medicine researchers
are still arguing about today, is what trade-off
makes human childbirth so difficult. Now,
there are probably many trade-offs which all
build upon each other to make childbirth hard.
(We’ll get into that more later.)
But, when scientists first started thinking
about this in the mid-twentieth century, they
developed an explanation to neatly justify
why childbirth is hard, meaning that medical
intervention would be a necessary part of
the process
to save the lives of mothers and children.
First, let’s discuss the factors that make
human development unique among primates.
Human babies are less developed than the babies
of most of our close relatives in several
interesting ways. Most primates are precocial:
that is, babies are born with some degree
of independence and mobility, able to open
their eyes and hold onto their mothers soon
after birth. On the other hand, human babies
are far more altricial than our primate relatives.
For example, infants’ motor skills are incredibly immature;
try getting a newborn human to hang from its mother the way baby chimpanzees do!
Brain development likewise needs time.
The brains of baby chimpanzees are 40 percent
of adult size. Humans, not so much;
infant brains are less than a third of the size of
adult brains.
Based on these characteristics of human development,
scientists thought of an explanation which
became, and arguably still is, the prevailing
explanation for why childbirth is hard.
It’s called the obstetrical dilemma.
The theory goes like this: Hominins, the evolutionary
ancestors of humans after they split from
other primates, began to walk upright on two
legs (known as bipedalism). This caused a
variety of changes to take place in the pelvis
and lower limbs. Most notably, the pelvis
became narrower as muscle groups changed their
structure and position. The birth canal also
changed in shape, so that the entrance into
the pelvic cavity was oriented differently
than the exit. Because of this, babies couldn’t
just slide straight out of the birth canal;
their heads and shoulders had to twist inside
the pelvis so that they could exit through
the widest parts of the birth canal.
After hominins became bipedal, a second challenge
presented itself. Hominins’ brains started
growing in size and complexity in response
to new environments and other factors,
a process known as encephalization.
This meant that newborns’ brains were also
larger, and these larger brains were enclosed
by larger skulls which needed to fit through
female hominins’ increasingly narrow pelvises.
Now we have all of the ingredients for a neat
and tidy explanation.
Childbirth is hard, the obstetrical dilemma
says, because widening the pelvis to make
birth easier would make it impossible for
female humans to walk properly on two legs.
So, babies are born with heads and brains
that only just fit through the female’s
narrow pelvis. Problems arise when the baby’s
head is too big to fit through, a complication
known as cephalopelvic disproportion. As for
why human babies are more altricial than those
of other primates, the obstetrical dilemma
says that babies are born developmentally
premature so that their heads are smaller
and more likely to fit through the birth canal.
Since the 1960s when the obstetrical dilemma
hypothesis first emerged, it’s been generally
accepted as *the* explanation for why childbirthis hard.
However, it is often the case that the truth resists simplicity.
Recent research has called into question
some of the fundamental
assumptions of the obstetrical dilemma,
making our job of answering
the question “Why is childbirth so difficult?”
much more, well, difficult.
For example, do we actually give birth earlier
and to more premature babies than other primates?
It turns out that no, we don’t.
Human pregnancies are both absolutely and
relatively longer than those of chimpanzees,
gorillas, and orangutans. In fact, in a 2012
paper, Holly Dunsworth and colleagues calculated
that human gestation length is 37 days longer
than expected for a primate of similar body mass.
Compared to other primates, we also
give birth to newborns weighing 50 percent
more than expected, and these newborns’
brains and bodies are huge compared to the
body size of their mothers. This means that
human babies are not born prematurely,
and in fact, human mothers may devote more of their resources to their babies
than other primate mothers do.
What about the pelvis? There’s no denying
that it’s a tight fit to get babies through
the narrow birth canal. But, is the female
pelvis uniquely restricted because of bipedalism?
Not necessarily.
A 2015 paper by Anna Warrener and colleagues
used a dynamic model of hip mechanics to compare
the efficiency of walking and running in male
and female subjects. They found that a wider
pelvis did not lead to less efficient walking
or running.
Technically, humans could have even wider
hips to make childbirth easier without compromising
their ability to walk, but for whatever reason,
there hasn’t been a need to evolve wider
hips. This means that the need to walk on
two legs is not what’s restricting the size
of the pelvis, as the obstetrical dilemma
suggests.
Additionally, the problem of a tight fit,
or cephalopelvic disproportion,
isn't even unique to humans.
Some primates with smaller bodies, such as
monkeys and gibbons, also have a close
correspondence between the size of the maternal birth canal and the size of the baby’s head.
Cephalopelvic disproportion does sometimes occur.
If this problem is due to the constraints
of bipedalism, then why does it also exist
in monkeys and gibbons? They don’t walk
on two legs like we do.
Plus, if the female pelvis really is so tightly
constrained by the trade-off between bipedalism
and encephalization, then female pelvises
should be similar in shape. That is,
stabilizing selection should be acting on the female pelvis to keep that delicate balance between
upright walking and large brains. However, that hasn’t been the case either.
A 2015 study found that the human female birth
canal varies even more in size and shape than
human limbs do. So our arms—our arms!—
may be more similar
in size and shape than our pelvises.
Obviously, our arms aren’t under
the constraints of this trade-off.
These recent papers are part of the growing
body of evidence that while the obstetrical
dilemma sounds neat and simple, it is
probably not the best explanation
for why childbirth is hard.
How can these contradictions be resolved?
Holly Dunsworth’s 2012 paper offers an alternative
hypothesis, which she calls the energetics
of gestation and growth, or EGG, hypothesis.
This hypothesis suggests that pregnancy ends
when the energy needs of the growing fetus
exceed the mother’s ability to meet those needs.
The basal metabolic rates of pregnant mothers
quickly increase to near the upper
limit for human metabolism as the fetus develops
and grows bigger. If pregnancy was even one
month longer, the mother would probably become
unable to provide enough resources to support
both herself and the fetus.
So, the trade-off that makes human childbirth
so difficult may not be the balance between
bipedalism and encephalization, as the
obstetrical dilemma hypothesis states.
Rather, the trade-off may be between fetal energy needs and maternal energy supply,
as it is in primates and other mammals.
Here’s the thing, though. Like we touched
on earlier, the truth resists simplicity.
It’s likely that both of these trade-offs
and more are at play
in the complicated process that is childbirth.
For example, nutrition likely plays a crucial
role in the size of both mother and baby,
which would play into both of the trade-offs
we’ve discussed. There is also recent evidence
that the female pelvis actually changes shape
during puberty to help facilitate childbirth
and reverts back during menopause when
childbirth is no longer a concern,
which further complicates the discussion.
The debate over what makes childbirth hard
is a prime example of how science and medicine
can change our understanding of ourselves
and the world. With the advent of cesarean
sections (also known as C-sections) and other
medical technologies, the role of the pelvis
in human childbirth is changing. More babies
are born alive today than in the past
due to evolutionary constraints
overcome by medical intervention.
Who knows how that might change
our evolutionary future?
Remember the two women who opened the episode?
Let’s determine why they took
different approaches to birthing.
The obstetrical dilemma would drive the conclusion
that the woman with the C-section probably
had a birth canal too small for her baby’s
head to squeeze through, a constraint caused
by her need to walk on two legs. The conclusion
would be different under the EGG hypothesis.
The baby, powered by the mother’s ample
investment in resources, grew
too large for a normal birth canal.
What can be both frustrating and fascinating
about evolutionary medicine questions is that
often, there is no clear-cut answer. Perhaps
the answer to this episode’s “why” question
is a combination of both explanations,
or includes
other factors that we haven't
even talked about.
For example, the woman who needed a C-section
may have had a condition called placenta previa,
where the placenta partially or completely
covers the cervix, which is where the baby
would exit the uterus. This makes a C-section
essentially required for a pregnancy which
might otherwise be completely healthy.
Ultimately, as in the cases of both cephalopelvic
disproportion or placenta previa, there are
circumstances when a cesarean section is
required to preserve the
life and health of both
mother and baby.
When do you need a C-section, though, and
when would a vaginal delivery be better?
And, no matter how you’re giving birth, what
kind of personal and medical support provides
the biggest benefits and what is unnecessary
or even harmful?
That’ll be the topic of our next episode of
Darwin, MD. See you then!
[credits]
