I've blogged about this before but it bears repeating. In the Bible it affirms that God states that he "knew" the person before he or she was born. That is true. Before they were born. Not before they were conceived.
Souls are not floating in heaven waiting for the next greased chute into life. Souls are created by God at precisely the same stage of life for each and every person. That is when the egg is fertilized by the sperm, becomes an embryo, attaches to the inner womb of the woman, and "quickens" (makes its first cell division of growth that it is alive). It is at that precise moment that God creates the soul to match that embryo as it makes its first pulse of life attached in the mother's womb. There is no question about this at all.
As usual you have to take my word for it, but you can really prove this through logic, if you choose to do so. We all know that every human has a guardian angel. Jesus affirms this in the Gospel, so Christians are fully aware of the guardian angel truth of their faith. Now, when would God send the guardian angel? Would he wait until the baby pops out of the womb? I mean, who needs a guardian angel during the first nine months of life, right? Of course not. God gives the guardian angel to the child at the embryo's first pulse of life. He obviously doesn't have guardian angels hanging around inside the womb or willie just waiting in case a baby comes along, LOL. So the guardian angels are not there "in advance" of the baby's existence as a embryo. Likewise, they don't drop in on the baby after the mother's been pregnant, ignoring the pre-born baby as it's mom takes crack, or decides to have an abortion, or if there is a risky pregnancy, and, obviously, through the every day miracle of a good and blessed gestation of a much loved and wanted pre-born child. Logic dictates that the guardian angel is sent to the baby at the baby's first inception as a living individual being, when it takes its first cellular growth step of life, which is the archaic term "quickening."
Anyone who has had a miscarriage or an abortion and who believes in God comforts themselves that the baby is in heaven. What exactly is in heaven if the baby did not have its soul from the first cellular growth of life? So obviously the baby receives its soul and its guardian angel from God the moment that it is viable and quickening in the mother's womb. The soul and the guardian angel are each created and unique to the embryo at that first moment. God does not have a "last in first out" or "holding pattern" "inventory" of "new or used" souls.
The Bible is very clear that the body is the temple of the soul. So it's not like any old soul can get stuffed into whatever body comes along for it. That's pagan thinking and it's wrong and destructive, and anti-God and anti-life. When a body is created, a new temple, as an embryo is in truth, a new soul that is suited for that temple is created by God and sent to that body. Period. It is that soul that returns to God for judgment and dispensation when life ends, either pre-born (when obviously no judgment is needed) or at any point in that person's body and life when it comes to an end through death. The temple then is gone but the soul returns to God, who created it for that body temple specifically and exclusively. Thus God knows the soul before the person was born, because he created that soul for the pre-born embryo.
Here's a common sense exercise. Think of a famous saint, let's say John the Baptist. Is it not obvious that God created the soul of the person who would be John the Baptist, and sent the soul to the womb that would carry John the Baptist? I mean, it's not like God just grabbed some soul out of thin air that was hanging around in the waiting room and the soul turns out to have done very well for himself and for the salvation history of the world by becoming John the Baptist. God doesn't say, "Hm, let's grab this soul that's been hanging around and send it down the greased chute to earth and see how he turns out. Wow, look at that, he turned out to be John the Baptist!" God could kind of chuckle to himself and say, "Yee ha! Good thing that soul didn't take a wrong turn and end up in some reindeer herder in Iceland!" Um, obviously not. God creates the soul that will occupy the temple of the body that was just created when an embryo is formed and implanted. Build the temple/create and send the soul. Build the temple/create and send the soul. That's what happens millions of times a year. God can handle it. He's God. He created every uncountable particle in the universe, and all the unmeasurable space in between. God can handle creating a new and individual soul that is perfect for the temple that has just been conceived through the creation of conception.
That's why I am SO SICK of New Age reincarnation idiots. Again, this is not a slam on traditional faiths who believe in reincarnation, such as Buddhists. Their error is understandable because they do not historically believe in an all powerful directive God. Their faith sought to explain a code of conduct and relied on earthly "karma" to reward/punish good people and evil doers. I have no problem with people who are born into this tradition. But I have a real problem with "modern thinkers" who decide all on their own that God is part of a sausage machine where the handle is cranked and souls are recycled. So who would John the Baptist have been in his "past life?" The guy who had the best cave? Someone who "lost his head" in a previous life and therefore needs to "lose his head again in the next life, but this time for a 'good cause'"? Yeah, have another puff on that spiritual cigarette that you smoke.