Why do most women get their first period around age 12 when their bodies are usually not well developed enough to safely carry a baby to term?
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Real ELI5: Pokemon aren’t ready to do battle as soon as you catch them. They need to be trained. And then if you train them right, they evolve into their final form.
The same is true for human fertility. It takes time for all the bits to grow or change into their final form. Some bits, like the lining of the womb can develop pretty quickly once the right hormones are working. Other bits, like the size of the bones a baby has to pass through during birth grow and evolve much slower.
A girl getting her first period means that the mix of hormones in her body are now much closer to the adult mix than the child mix she used to have.
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