There is no soul entering the body, not piecemeal or otherwise. There is no “stage” where we are nothing (using your definition of humanity). There is simply a process of sperm entering the egg. There is a stage where it has not entered and a stage where the sperm and egg are fused and the DNA has been combined. We can also hypothesize various partial fusion stages.
Oh but the soul/spirit intellect does indeed enter the womb at conception. Each and every soul/spirit intellect created has the opportunity to take this flesh journey. Christ said just to 'see' the kingdom of God this flesh journey was the first step.
Now it is also Written that when the flesh body dies that soul/spirit intellect returns to the Maker that sent it. Thus every conception wherein a soul/spirit intellect enters the womb is not lost when the flesh dies, but back with the Maker.
However, the big business of murdering the child still in the womb is not ignored by the Creator.