Here are some answers which you are free to use to further develop your own, or use as is. We'll add to this page from time to time. In fact, human birth is a dangerous, hit and miss procedure.
Chickens do it a lot better.
Obviously, this either isn't a designed system, or it was designed incredibly badly. Let the religionists pick. That one doesn't need an answer, or, to look at it another way, the stare is the answer. A good way to begin this answer is to explain it with God. The body is subject to any number of genetic errors, and immediately breeds to failure when family genes intermingle, thus punishing the child for the alleged sins of the parents.