One idea is that it takes less energy and resources like food for women only to be fertile part of the time. So the lining on a woman’s uterus builds up and gets shed on a regular basis.
People born with male bits don’t have a uterus, so no need for a period.
Hi TheBrainyPotatoes,
Periods are the lining of the uterus, being broken down so that a fresh new lining can be made, this happens in time with the release of an egg from the ovaries. Its important to have a fresh lining to the uterus for the fertilised egg to embed into so that a woman can get pregnant.
So I guess that we have periods because women are the ones who carry the baby. That isn’t always the case in nature, in sea horses the father carries the embryos in a special pouch until he gives birth.
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