So, I learned today some interesting things.

The reason we build a lining in our uterus for a pregnancy is because human embryos are super invasive and burrow deep into the uterus.

So basically, it's there to protect an embryo from damaging your uterus.

But yeah there's only a few other creatures on earth that build a uterine lining BEFORE they get pregnant. Humans, bats? Some primates and one other weird creature that I don't know how to spell.

But this is interesting is it not?