What is the use of menstruation? Apart from humans, primates and a few other mammals, no other animal has periods. Yet they all are able to have babies. Apparently a monthly bleeding is no prerequisite for reproduction.

Bas Defize, a molecular biologist specialised in developmental biology at the University College Utrecht in the Netherlands, has previously answered this question. To summarise: humans and primates menstruate because they produce a thicker endometrial lining than most other mammals. This lining is too thick to be fully absorbed by the body, so part of it is shed through the vagina.

What is the evolutionary advantage? 

So what is the use of menstruation? Defize makes one thing clear immediately: there will never be a 100% fitting answer to this question. ‘Some structures just arise during the development of species. The question is: does it bother the species a lot? This isn’t the case with menstruation. Although building such a thick endometrial lining does cost a lot of energy. Energetically, this isn’t very smart. Therefore you need to ask yourself: what is the evolutionary advantage of having a thick endometrial lining?

Some interesting theories exist about this. The first one is that the implantation of an embryo is actually a terrible thing, like a war. In this case a thicker endometrial lining offers more protection for the womb. The second one is that a thicker lining helps with selecting the embryo. If an embryo isn’t good, it isn’t worth investing effort in. A human endometrial lining is better suited to measure the quality of the embryo. However, we’ll never know for sure what the exact use of menstruation is. The ‘why’ questions are always the hardest ones to answer.’

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