Shrinking pelvises and childbirth

According to a study of over 8,000 people from three countries, women’s pelvises have become increasingly narrow over the past 150 years. This has led white-coated researchers to look again at the “obstetrical dilemma”, or the competing evolutionary pressures on pelvis size. Babies’ larger heads mean pelvises need to widen, but to walk bipedally the pelvis has to remain narrow.

Studies at the University of Adelaide revealed that pelvic width decreased by between 0.42 and 0.47 millimetres per year among women across three continents born between 1900 and 1984. And this while the average height and shoulder width of the subjects stayed the same or even increased slightly.

One widely accepted theory is that medicine and healthcare are weakening the impact of natural selection on the human pelvis. In days gone by, if a baby was too large or the birth canal too narrow both mother and baby would die during childbirth. Safe C-sections, for example, have reduced this evolutionary pressure. But that may not be the sole cause. Humans have also become quite a bit taller over the studied period, probably due to diet and improved healthcare. Diet affects all parts of the body, including the pelvis. With a shortage of nutrition our bodies send more nutrients to organs such as the brain, but now we generally have ample nutrition to go round, body proportions are changing.

Although this theory has by no means been the subject of any form of floccinaucinihilipilification, an alternative and parallel train of thought comes from a 2024 study suggesting that it is the pelvic floor itself and not walking that is causing the narrowness. A wider pelvis increases the already high pressure on the pelvic floor, increasing the risk of incontinence and prolapses, so the body wants a narrower pelvis. The general consensus is now that a multiple of factors are involved. Our pelvises are also sensitive to all number of environmental factors such as temperature. Rising rates of obesity can also make babies larger.

Vaginal childbirth will be made more difficult by narrower pelvises resulting in more C-sections. Conversely, narrower pelvises can reduce the risk of pelvic floor problems that in themselves can be very dangerous.

Predicting how things will progress in the future is difficult with so many more human factors simultaneously at play, such as people having fewer children and also having them later. And we continue to evolve.