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PARIS – The size and shape of women's birth channels vary according to the region of the world in which they live, but most medical textbooks are based on a European body type, scientists said Wednesday, warning against health risks.
Differences in the depth and width of the pelvic canal determine the path taken by the newborn in the world and forcing a birth to conform to a single standard can be harmful for the mother and the child, they reported in the journal Proceedings of the Royal Society B.
"The training of an obstetrician is based on a basin model developed from European women," said senior author Lia Betti, a lecturer in anthropology of evolution at the University. from Roehampton to London.
"But the typical pelvic form and the typical type of childbirth may differ among populations," she told AFP. "An update seems necessary, especially in a multiethnic society."
Women in sub-Saharan Africa, for example, tend to have a deeper channel, while at the other extreme, Amerindian women generally have a wider one, she explained. .
European and Asian women are somewhere in between.
This is important because, at birth, a baby turns when moving in the canal, aligning the sides of the head and shoulders on the contours of the canal.
"If a woman's birth canal is substantially different from the model described in the textbooks, the baby's movement will also deviate from the expected pattern," Betti said.
She cited examples from the early to mid-twentieth century of "horrific consequences" when forceps were used to rotate babies during delivery, based on erroneous assumptions about the shape of the pelvis.
Radiography of pregnant women – a common practice until the 1950s to determine the position of babies – has helped to build a database highlighting differences in female anatomy.
According to the World Health Organization, some 300,000 women die each year during or shortly after childbirth, often as a result of "birth-related complications."
One of these "complications" is the fact that humans have a big brain locked in bones.
"Because of its tight fit, the fetus must perform a series of rotations to navigate successfully in the mother's birth canal," Betti explained.
The shape of the female pelvis has also been described as an evolutionary compromise between a short and compact body, suitable for walking on two limbs, and a spacious pelvic canal for big-brained newborns.
But none of this tells us why women in Namibia, Nanjing and Norway have different internal architectures to give birth to.
There are three possible explanations, none of them mutually exclusive, said the authors.
One of them is the cold environments, which may have led to the emergence of wider hips – and, at the same time, to a wider entrance to the genital canal – to reduce the losses. of heat by increasing body mass for isolation.
Some studies have suggested that natural selection may have contributed to the disappearance of body types unsuitable for delivery.
The proof of these two theories, however, is at best unequal.
According to Betti, the most likely cause of variation is more related to human migration.
Homo sapiens is native to Africa and has quickly dispersed to new continents 60,000 to 100,000 years ago – most recently in the case of North America.
Each founding population was small and formed what is called a genetic bottleneck.
The further away we are from Africa, the less migrant groups have genetic diversity. This means that whatever traits they share – fair hair and fair skin in Scandinavia, for example – they dominate locally.
The same is true for the shapes and contours of the basin, which remain much more diversified in sub-Saharan Africa than elsewhere.
"This clearly shows that birth canal variation has been shaped by the population's history," said Betti.
"The distance from Africa accounts for 43.5% of the diversity of channels within human populations."
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