Indeed. Only a small minority of fertilized zygotes are capable of becoming a fetus. Even after implanting, about a third of them don’t continue to grow.
Given that God is rational, it would be more reasonable to say that a the spirit joins the body as early as 5 days after conception when the blastocyst attaches to the endometrium.
The entire process is a wonder of Biblical proportions when you think about it. That the zygote contains all the genetic material necessary to create a human body is phenomenal. The idea that the blastocyst is so designed as to create its own placenta just boggles the mind.
That a biological process of trial and error shaped by environment can lead to such complexity seems unnatural to me. The order I see around me, particularly the behavior of my fellow human beings, made in the image and likeness of God, all points to a creator. For some, evolution is that “creator”. It seems plausible to me only under the precondition that God got the ball rolling as part of the Plan.
Take a read of that book if you get the chance. I’d love to hear your thoughts and insights. I appreciate your comments here.