Why do humans menstruate? It's unusual for animals, so why?
In terms of public policy, this directly bears upon abortion access. Most other animals can spontaneously abort to protect the mother, but not humans, so we developed other methods to protect ourselves.
The anti-abortion ideology hinges on the idea that a soul enters the body at conception. The invasive placenta shows it's really a biological defect. If we were designed, then their "designer" is incompetent.