Difficult childbirth is not unique to humans, study finds

The tight fit of a baby's head through a mother's birth canal, which causes great difficulty in childbirth, is not unique to humans, as previously understood.

Instead, some small-bodied primate babies have heads almost twice as large as their mothers' pelvic space, a new study led by UCL researchers has found.

The findings, published in Nature Ecology & Evolution, challenge the theory of an exclusively human 'obstetrical dilemma' - the idea that our large heads and narrow pelvises (adapted for upright walking), have made childbirth uniquely difficult for our species.

Researchers at UCL have revisited the evidence and found that although constricted birth is not experienced by other apes, it is common among many small-bodied primates, particularly American monkeys like bushbabies and squirrel monkeys. For example, the heads of newborn squirrel monkeys can be almost twice the size of the mother's pelvic space.

Co-corresponding author Dr Nicole Torres Tamayo (UCL Anthropology) said: "Much of the data that informed earlier studies was flawed. It had been collected in a human-centric way that failed to consider the anatomy of other species.

"As well as greatly expanding the number of species considered, we collected measurements that took into account the specific anatomy of different species. This data then informed our 3D modeling.

"In the past, the measurement for the newborns' heads were from the forehead to the back of the skull. This assumed that all babies are born crown-first, as most humans are. But species like the gelada monkey, with their pronounced snouts, are often birthed face-first.  We took this positioning into account."

Using advanced 3D modelling techniques and by greatly expanding the number of species studied (from eight to 29), the research team found that tight fits at birth were especially common among proportionally smaller species.

Interestingly, we found that some of the small-bodied primates that experience a constrained fit during childbirth have developed clever adaptations to make the process less difficult. The pelvic bones of female rhesus macaques fuse together later than in males, during their reproductive years, and in bushbabies they never fuse, allowing the pelvis to expand during birth to accommodate the neonatal head.

The findings of our study reshape previously held assumptions about how unique human childbirth is, revealing a diversity of obstetrical dilemmas and adaptations across primates."

Dr. Lia Betti, Co-corresponding author, UCL Anthropology 

This study was funded by a Sasakawa Foundation Butterfield Award, a Leverhulme Trust Project and two Kyoto University Cooperative Research Programme Awards. Data collection of numerous newborn primates was funded by National Science Foundation grants.