She goes through the whole process by which human fetuses operate as parasites on their mothers (as opposed to in other mammals where the mother's control the process), thanks to the "ravenous hemochorial placenta." Therefore, she continues, mother's body needs more control over whether she gets pregnant, so has created an endometrium as a "lethal testing-ground which only the toughest embryos survive. The longer the female can delay that placenta reaching her bloodstream, the longer she has to decide if she wants to dispose of this embryo without significant cost."
The solution, for higher primates, was to slough off the whole superficial endometrium – dying embryos and all – after every ovulation that didn't result in a healthy pregnancy. It's not exactly brilliant, but it works, and most importantly, it's easily achieved by making some alterations to a chemical pathway normally used by the fetus during pregnancy. In other words, it's just the kind of effect natural selection is renowned for: odd, hackish solutions that work to solve proximate problems.And like most hacks, my friend N. said, trying to deal with the kludge has caused so many more problems than if we had just fixed it in the first place!
So much for intelligent design.