Why Is Human Childbirth So Hard?

A chimpanzee mother typically gives birth alone, often quickly, and can reach down to catch her baby. A human mother, in almost every culture ever studied, has help. Assisted birth is close to universal, though a few societies value giving birth alone.

The reason is written into human anatomy. A newborn’s head only just fits through the mother’s pelvis, and the passage is not straight. For a hundred years, researchers have offered an explanation for why evolution left us with such a dangerous system.

The tight squeeze

The human birth canal is not a simple tube. At the entrance it is widest from side to side; at the exit it is widest from front to back. The baby has to rotate as it descends, usually entering sideways and leaving facing the mother’s back, so that the widest part of its head follows the widest part of the canal. Because the baby faces away, a mother cannot easily guide it out or clear its airway. That is one reason why assisted birth is so common.

The anthropologists Wenda Trevathan and Karen Rosenberg argued that this awkward arrangement selected for help at birth: relatives, or eventually trained midwives, who could reach the baby, clear its airway and support the mother. On this view, human birth has been a social event for a very long time, perhaps hundreds of thousands of years.

The classic explanation: the obstetric dilemma

In 1960 the anthropologist Sherwood Washburn proposed what became known as the obstetrical dilemma. Walking on two legs favoured a narrow pelvis, because a wide one would supposedly make walking less efficient. Big brains, meanwhile, meant bigger baby heads. Natural selection had to compromise: a pelvis wide enough to permit birth but not so wide that walking suffered, and babies born early, with only a fraction of their brain growth complete, so that their heads could still fit through.

It neatly explains why human babies are born so helpless. Humans are sometimes said to be born “too early”, with the brain finishing much of its growth in an extra year outside the womb, an idea related to the “expensive brain” story.

The cracks in the theory

The obstetric dilemma has been tested rigorously in the last two decades, and parts of it have not held up.

A wide pelvis does not slow walking

In 2015 Anna Warrener and colleagues measured the energy cost of walking and running in men and women with different pelvic widths. A wider pelvis did not make locomotion noticeably more expensive. If a wide pelvis costs little, evolution had less reason to keep it narrow.

The energetics theory

In 2012 Holly Dunsworth and colleagues proposed a different explanation: the energetics of gestation and growth. A pregnant mother can sustain a fetus only up to a metabolic ceiling of roughly twice her resting metabolic rate. As the fetus grows, demands rise until they approach that limit, and birth happens then, regardless of the pelvis. The timing of birth, on this view, is set by the mother’s energy budget, and the tight fit is a consequence rather than the cause.

Not all groups face the same risk

Jonathan Wells and colleagues have argued that the risk of obstructed labour varies a great deal between populations and depends on nutrition, growth and the environment. In many populations today, mothers stunted by poor childhood nutrition eat richer diets in pregnancy and have larger babies, and gestational diabetes can increase fetal size further — making mismatches more likely. In other words, the difficulty may partly be a modern, avoidable problem layered on top of an ancient compromise.

Most researchers now see the answer as a combination: pelvic constraints, energy limits and modern conditions all contribute.

What fossil pelvises say

Pelvic bones are rare but informative.

• Australopiths such as Lucy had a wide, flat pelvis, and newborn brains were small — only slightly larger than a chimpanzee’s. Birth was probably easier and needed less rotation.
• Homo erectus. A broad female pelvis from Gona, Ethiopia, about 1.2 million years old, described by Scott Simpson and colleagues in 2008, indicates a newborn brain around 30% of adult size, close to the modern human pattern. Difficult birth and helpless infancy may have begun by then.
• Neanderthals. Marcia Ponce de León and colleagues reconstructed the skull of a Neanderthal newborn from Mezmaiskaya cave in 2008 and estimated a brain size at birth close to 400 cubic centimetres, similar to a modern human baby. Neanderthal mothers faced a challenge very like ours.

So the tight fit is not a consequence of walking upright, since australopiths walked upright with easy births. It is more closely tied to the growth of brains in the genus Homo. (See why humans started walking upright for the earlier story.)

Are we still changing?

Caesarean section now rescues mothers and babies who would once have died. In 2016 Philipp Mitteroecker and colleagues modelled the effect: because babies with large heads relative to the pelvis now survive, selection against obstructed labour has relaxed, and they estimated that the rate of these mismatches may have risen by 10–20% in recent decades. It is a small effect, but a striking example of medicine shaping the genes of the next generation.

Why it matters

Globally, roughly a quarter of a million women still die each year from pregnancy and childbirth, most of them in countries with limited medical care. The difficulty of human birth is both a legacy of our evolution and one of the clearest examples of how the past shapes health today — and of how culture, in the form of help at birth, has long compensated for biology.