A new study about the female pelvis is casting it in a very interesting light: not only does it widen during puberty, but it also contracts come menopause.

The obstetric dilemma hypothesis has been a conundrum for several scientists but is also sometimes used to explain why women have difficulty giving birth. According to the hypothesis, in order for humans to walk upright, it requires a narrow pelvis, but giving birth to large-brained and large-bodied babies needs the opposite, or a wider pelvis.

A recent University of Zurich research delved into that and revealed that a female pelvis tends to adapt its width in anticipation of birthing large-brained babies: it widens during puberty before menopause and contracts after that.

Researchers created 3D models out of the pelvic CT scans of 124 women and 151 men whose ages ranged from infants to 95 years old to help see the changes during age progression.

Upon modeling, they learned that both men and women actually have almost the same pelvic width during the early years. That of women, however, becomes wider by 25 percent once they reach puberty and narrows by as much 80 percent by the time they are around 70 years old. The pelvises of men and women tend to become of the same width again around 40 years old.

"Our data suggest that estrogens have a strong influence on the development of the female pelvic morphology during puberty. At the same time, they imply a weak-to-absent influence of androgens on human male-specific pelvic development during puberty," explained the researchers.

Previous studies have already shown that, generally, estrogen levels are very high during puberty and the childbearing years to help the body become more fertile and then gradually decrease once the women hit the menopausal age.

The pelvic contraction during menopause may also be necessary to ensure that the pelvic floor is reinforced while it receives significant pressure from the abdomen during walking.

As for the possibility of evolution, "this [research] implies that the female body can modulate its pelvic dimensions 'on demand,' and is not dependent on genetically fixed developmental programs," said lead author Marcia Ponce de Leon.

In other words, the difficulty during childbirth may not necessarily be because of evolution, which is believed to have made the pelvis narrower, but rather on factors such as nutrition and environment, which can influence the hormone levels and thus the width of the pelvis.

The study is published in the Proceedings of the National Academy of Sciences.

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