Humans not the only species with difficult births, new study finds 

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Small-bodied primate babies can have heads nearly twice as large as their mothers’ pelvic space, suggesting that the tight fit of a baby’s head through the birth canal is not unique to humans, as previously thought, a new study has found.

Findings published in the journal Nature Ecology and 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 said.

Co-corresponding author Nicole Torres Tamayo from the University College London’s department of 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.”

The researchers 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, they said.

“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 modelling,” Tamayo said.

“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,” Tamayo said.

The researchers used 3D modelling techniques and expanded the number of species studied from eight to 29, and found 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,” co-corresponding author Lia Betti, from the department of anthropology at University College London, said.

Betti explained that 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,” the author said.

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