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.

Summary

• Contrary to long-standing assumptions, human childbirth does not appear to be uniquely difficult or risky.
• Complication and mortality rates vary within and among species, with extensive overlap between humans and other mammals. Some species even show comparable complication rates to human populations, such as hunter-gatherer populations without medical intervention.
• Underlying causes and risk factors of birth difficulties are also shared among humans and other mammals.
• These patterns persist due to evolutionary trade-offs, where traits that improve offspring survival (e.g., higher birth weight) also increase birth risks and mortality.
• The findings place human childbirth in a broader evolutionary context and challenge the idea of human uniqueness.

About the University of Vienna:

At the University of Vienna, curiosity has been the core principle of academic life for more than 650 years. For over 650 years the University of Vienna has stood for education, research and innovation. Today, it is ranked among the top 100 and thus the top four per cent of all universities worldwide and is globally connected. With degree programmes covering over 180 disciplines, and more than 10,000 employees we are one of the largest academic institutions in Europe. Here, people from a broad spectrum of disciplines come together to carry out research at the highest level and develop solutions for current and future challenges. Its students and graduates develop reflected and sustainable solutions to complex challenges using innovative spirit and curiosity.