Human childbirth is commonly viewed as uniquely difficult and dangerous. The reason: the combination of bipedalism and large brains creates a tight fit between the baby and the birth canal. Research at the University of Vienna has now shown that many other mammals – from domestic livestock to wild species – face similar birth problems and mortality. In some species, these complications even occur as often as in some human populations, such as hunter-gatherers without modern medical care. The findings suggest that difficult childbirth is not uniquely human. The study was published in Biological Reviews.
Human childbirth has long been considered exceptionally difficult. A common explanation is the so-called “obstetrical Dilemma”: humans walk upright and have large brains, leading to a tight fit between the baby and the mother’s pelvis. This is thought to make birth especially risky in our species. But this idea has rarely been tested against data on birth outcomes from other mammals.
Looking beyond humans
In the new study, Nicole Grunstra from the Department of Evolutionary Biology at the University of Vienna reviewed a wide range of scientific literature and compiled data on birth complications across mammals. The analysis included domestic animals such as cows and sheep, as well as wild species living under natural conditions such as seals and deer.
The goal was to find out whether humans are truly exceptional, or whether difficult births are widespread in the animal kingdom.
Birth complications across mammals
The results show that birth difficulties are not unique to humans. They occur across many placental mammal species, including those in the wild where natural selection might be expected to eliminate such risks. Even whales and dolphins can have calves get stuck during birth, despite lacking a bony pelvis.
In some species, such as deer and antelope, rates of birth complications and female mortality are similar to those seen in human populations, including hunter-gatherer populations without modern medical care. The types of birth complications and underlying causes are also comparable. A tight fit between the fetus and the birth canal, for example, is common in species that give birth to large, well-developed offspring, such as monkeys, ungulates, and elephants. And overnutrition can cause the fetus to grow bigger in humans, other primates, and rodents.
Why evolution does not eliminate the problem
If birth complications can lead to death of the mother and the offspring, why has evolution not removed it? The study suggests that this is due to a trade-off. Larger offspring often have better chances of survival after birth, but they are also harder to deliver. This creates a small margin of error: too small and the newborn may die soon after birth (e.g. from disease), too large and it could die during birth.
In species that give birth to multiple, usually small, offspring such as dogs or pigs, another trade-off appears. Both very small and very large litters can increase the chance of obstruction during birth. Small litters produce larger pups that risk getting stuck, while large litters contain many small fetuses that can be mispositioned and block the birth canal.
These patterns help explain why birth difficulties persist, even in natural populations.
A new perspective on human childbirth
The findings place human childbirth in a broader evolutionary context. Rather than being uniquely difficult, it appears to follow a biological pattern shared with other mammals. In humans, the tight fit arises from the unique combination of a large brain and a pelvis adapted to bipedalism, while other species face their own challenges. Cows, horses and deer, for example, must deliver offspring with head and forelimbs at the same time through a rather inflexible pelvis.
The study challenges long-held assumptions and highlights the value of comparing humans with other species. It also shifts the focus from seeing human childbirth as an exception towards understanding it as part of a wider evolutionary landscape in which birth is riskier for mammalian mothers and their offspring than often assumed.