Hi Steve. Welcome aboard!
I believe that reincarnation is a fact. As for the mechanics of the thing, not completely sure. However, having read Ian Stevenson and some other good (evidential) accounts, I have formed an opinion. Some of the accounts describe the disembodied state and how the disembodied "soul" perceived - and even selected! - future parents.
It seems that the soul sometimes hovers around the earth plane and observes couples, feels an attachment of some kind then either elects to, or is compelled to, reincarnate as the couple's child. Apparently this can happen at some stage in the womb, at the time of birth or even a little time after birth; again, according to accounts.
NDEs also sometimes report observing souls preparing for reincarnation.
My own opinion is that when there is a miscarriage there may not have been a soul associated with the fetus or the soul, for whatever reason, reconsidered the decision to incarnate. On the other hand, sometimes things just go wrong.
Once we get past the idea that there is an all powerful, all knowing God micromanaging the minutia of our lives, we can be spiritual and yet accept that sometimes wires get crossed and accidents happen.