Last Updated on June 29, 2026 by Staff
For a time scientists thought that humans were the only ones who had a hard time giving birth. They believed that our big brains and narrow pelvises which help us walk on two legs made childbirth especially tough for humans.
However a new study in Nature Ecology & Evolution says that’s not true. Researchers from University College London found out that some small primates also have tight births. In fact some newborns have heads that are almost twice as big as their mothers pelvic opening. The findings suggest that difficult childbirth is not a human problem.
It has evolved in primate species.
Challenging Old Beliefs
The idea that humans have a time giving birth because of their big brains and narrow pelvises has been around for a long time. This theory said that human evolution created a problem between two things: big brains and walking on two legs. While bigger brains meant baby heads walking on two legs made the human pelvis narrower making childbirth harder.
The new research questions this idea by looking at childbirth in primates. Of just studying eight species the researchers looked at 29 different primate species. They used three-dimensional modeling techniques to do this. Their results showed that many small monkeys have birth problems or even worse than humans. This means that difficult childbirth is more common than scientists thought.
Better Measurements
One reason earlier studies didn’t find this out was the way they took measurements. Previous research used childbirth as the standard for measuring newborn head size.
Scientists measured from the forehead to the back of the skull assuming all babies are born crown-like most human babies. However this doesn’t apply to other primates. Some species, like gelada monkeys often give birth face- because of their long snouts. Measuring their skulls using methods gave wrong estimates of how tightly babies fit through the birth canal. To fix this the researchers took measurements that reflected each species unique birth position and body.
These measurements helped them create 3D models of childbirth in many primates. The improved approach showed that several species have tighter births than previously thought.
Natures Solutions
Although many small primates have births, evolution has also given them special adaptations to make delivery easier.
For example female rhesus macaques have bones that fuse together later than males. This delay helps their pelvis stay flexible during their years creating extra space for childbirth.
Bushbabies have a more unusual solution.
In females the pelvic bones never fully fuse together allowing the pelvis to expand during birth and fit the baby’s head.
These adaptations show that evolution has found ways to deal with childbirth challenges in different primate species.
A New Understanding
The researchers think these findings change our understanding of childbirth evolution. Thinking that humans are the only ones with hard births, scientists now see that several primates face similar problems. The study highlights that evolutionary pressures affecting pregnancy and delivery have acted across primate species.
This broader perspective may help scientists understand how anatomy, evolution and reproduction interact, in humans and other animals. The research also shows the importance of studying species using methods tailored to their biology.
By expanding the number of species examined and using accurate measurements the researchers have shown that the story of childbirth evolution is more diverse than previously thought.
As future studies investigate primates scientists hope to find additional evolutionary adaptations that have helped different species overcome the challenges of giving birth.