Women’s pelvises are shrinking – how is that changing childbirth?
Over the past 150 years, the rise in Caesarean sections and changes in diet could have led to smaller pelvises among women – which may make vaginal birth more difficult but could also reduce common conditions associated with childbirth
By Michael Marshall
24 June 2025
Medical advances have changed childbirth – potentially enough to impact human evolution
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Women’s pelvises have become narrower over the past 150 years, according to a study of over 8000 people from three countries. There are many factors at play, but whatever the ultimate cause, it is the latest piece of evidence leading researchers to rethink the “obstetrical dilemma”, a description of the competing evolutionary pressures on pelvis size: the need to accommodate babies’ large heads drives pelvises to widen, but the need to walk bipedally pushes them to narrow.
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The biggest coincidence in human evolution
We don’t know exactly what is driving this change, or all the ways it will affect people’s health. But if pelvises continue shrinking at this pace, it could make Caesarean sections more likely out of necessity – which could have a host of knock-on effects.
Maciej Henneberg at the University of Adelaide in Australia and his colleagues reanalysed an existing dataset of 1247 Australian women, born between 1900 and 1984, and found that pelvic width decreased by 0.42 millimetres per year. Likewise, among 3486 Polish women, pelvis widths decreased by 0.47 mm per year between 1880 and 1970, and among 320 Mexican women, pelvis width shrank by 0.42 mm per year between 1900 and 1970. In the same time periods, average height increased and shoulder width either held steady or increased.
“Given that in these different regions, it evolved in the same direction, even though body height increased, I personally find this convincing,” says Philipp Mitteroecker at the University of Vienna in Austria.
“The dataset is fantastic,” says Lia Betti at University College London.
