I wonder what the Fathers and the ancient ones understood by conception. They obviously could not see spermatozoa nor eggs, much less the fact that they joined to become one (yes)moving cell with completely different DNA from the mother.
I'm no specialist in ancients views of conception, but I'm under the impression they saw it in agricultural terms, therefore the name "semen" which stands for seed. That's why some people thought woman had no participation whatsoever in the flesh of the new person, because it was seen as a plant: men were the only ones who could produce other human beings, like a tree that produces seeds. Women were unperfect men, necessary though, to be the "earth" where the seed had to be planted.
If that's the case, the question the ancients asked about conception were rather different from ours. They were asking themselves: when does a seed becomes a tree? That's where we see that "movement" would make sense as an argument, for when things start moving in the seed it has already started to become a tree. You could say it is already a tree in growth.
And the seed, for them, was just men's semen. Today, we know there are *two* "seeds", from the man and from the woman. Once they join, they obviously have ceased to be "seeds" and have become someone entirely new.
Some Fathers believed each soul was directly created by God and joined with the body on the 40th day after conception. I think St. Jerome says the point at which the unborn child begins to move in the womb is the point at which it had received a soul and become a living human person. Others believed the soul, like the body, was taken from the parents and was therefore joined to the body at conception.
The Church seems to have more or less universally accepted the latter opinion.
I understand that soulless bodies are dead, therefore, as long as there is life, there is a soul. So, since conception until natural or unnatural death.
From conception, the embryo is kept alive by the mother, so - just for the sake of argument - one could argue for a soulless embryo until the 40th day.