The matter of ensoulment is, of course, not a matter observable by biological science. Medieval theologians had no knowledge of embryology. Since the soul (philosophically speaking) is the life force, or form, of the body, it was assumed the soul had to be present when the body became human.
Not knowing embryology, medieval theologians thought that the body was “unformed” for a period of time after the seed was planted.
Later knowledge of genetics corrected this notion: the new individual is genetically complete when the sperm and egg unite to form a new human individual in the form of the zygote. From that point on, the new individual is genetically complete, individual, and human. All that remains is development of the body.
It should be noted that in spite of their belief in delayed ensoulment, these theologians nevertheless unqualifiedly condemned the practice of abortion, whether before or after the alleged ensoulment.
Moderns who use the ensoulment argument in favor of abortion are being somewhat disingenuous, since most of them don’t even believe in the existence of the soul.