Women develop wider pelvis during their childbearing years to allow for the birth of large-brained babies, a new study has found.
With the onset of puberty, the female pelvis expands and with the onset of menopause, it contracts again. In contrast, the male pelvis remains on the same developmental trajectory throughout a lifetime, researchers said.
The results suggest that the morphology of the female pelvis is influenced by hormonal changes in puberty and during menopause.
Women have wider hips than men because their pelves must allow for the birth of large-brained babies. Nevertheless, many female pelves are still not wide enough, which can result in difficult births.
Traditionally, the human pelvis has been considered an evolutionary compromise between birthing and walking upright; a wider pelvis would compromise efficient bipedal locomotion.
However, according to new studies, wide hips do not reduce locomotor efficiency.
Using computed tomographic data, researchers led by Marcia Ponce de Leon at the University of Zurich in Switzerland tracked pelvic development from birth to old age and found that until puberty, male and female pelves are similar in width.
With the onset of puberty, the male pelvis remains on the same developmental trajectory, while the female pelvis develops in an entirely new direction, becoming wider and reaching its full width around the age of 25-30 years.
From the age of 40 onward, the female pelvis then begins to narrow again.
The researchers hypothesise that these processes are steered by changes in hormone levels. With the onset of puberty, oestrogen concentration reaches high levels, which are maintained until menopause.
High oestrogen levels thus maintain high fertility and also guarantee that the female pelvis develops and maintains its obstetrically most favourable form.
“This implies that the female body can modulate its pelvic dimensions ‘on demand’, and is not dependent on genetically fixed developmental programmes,” said Ponce de Leon. At the same time, hormone levels also depend on environmental and nutritional factors.
“This suggests that difficult childbirths are not necessarily an evolutionary mis-step, but more a question of the balance between the hormones and the external factors influencing the size of the birth canal and the prenatal development of the child,” said Ponce de Leon.
The female pelvis contract again with the onset of menopause as a narrow pelvis is better suited to stabilising the pelvic floor and thus to withstanding the high pressures that are generated in the abdomen during bipedal walking, researchers said.