I am not sure that a fertilized egg that is not implanted becomes a human being. I do accept that it has the potential to become a human being.
I believe God knows which fertilized eggs will be implanted and which will not. Therefore, I think it is possible, God infuses a soul at fertilization only in the eggs that will be implanted and not in those that will not be implanted.
I also believe it is possible God infuses a soul at implantation, or at some other point later, but not much later during pregnancy.
I may be exaggerating, but you seem to express a view common to many American Indian tribes of the northeast at the time of the arrival of the Europeans. With them, a stillborn child would be buried next to a footpath, to facilaite the soul of the stillborn baby finding its way to a newly-fertilized egg within a passing woman. This way, the soul of that stillborn baby would not wander forever in limbo, but would have the opportunity to choose good over evil, and be able to join with God and all the spirits in heaven.
I don’t know God’s plan of salvation for each person, including stillborn and aborted babies. I believe that God has a plan for each and every one, and that the arm of the Lor is not so short that he cannot save anyone. We now know that it is entirely possible for a late term but still not delivered baby to make some choices. Perhaps God presents the choice of good or evil to them.
I would also like to say this about praying for the immortal souls of those who have gone on before us. We pray to a transcendent God. He exists all through time and independently of time. All of history, past and future, is “now” to him. So, if we pray for the salvation of a stillborn or aborted baby, or anybody about whom we have a doubt concerning their salvation, God can respond to our prayer.
What a mighty God is our God!
I misspoke when I said “fertilized egg,” I should have said “implanted fertilized egg.”