Why do nulliparous teenagers have the highest rate of unassisted vaginal deliveries compared to their older counterparts?

What is it about?

For a first birth (primiparity) youngest women deliver better than there older counterparts, and it is a linear law from 14 years old to 40+ of ages. It is due to a physiological uterine contractility declining inexorably with ages

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Why is it important?

It shows that the human female has been "shaped" to deliver without medical help since puberty (like all other 4,300 mammals)