Jonah Goldberg has opened up the topic of ensoulment with respect to abortion, and an email that he published concerning the Christian view doesn’t take its conclusions quite far enough to be entirely relevant to his broader stance on abortion:

What Christianity actually teaches is that man—and man alone—is a psychosomatic entity consisting of a body and a soul. Both together comprise the human person. Animals are pure body—even to the extent that they have intellect, they do not have immortal souls; angels on the other hand, are pure spirit, and thus have only tenuous links to the material world. Man alone participates in the entire “kosmos” created by God, who made all things visible and invisible (which formulation in the patristic mind implied material and immaterial). Man therefore has a unique place in God’s plan as mediator of creation. The patristic understanding of the Second Coming, therefore, is not the obliteration of the material universe so that man can live an airy-fairy existence in some immaterial heaven (white robe and harp optional), but the restoration of this world to the state it had before the fall of Adam. To Christians, as to Jews, the resurrection of the dead means specifically the reuniting of the soul and the body in a restored humanity no longer subject to death and corruption.

A previous emailer had suggested that God “puts a body around our soul,” but Christian doctrine is clear that each human is a created being, that only Jesus (“begotten, not made”) is co-eternal backwards in history with God, and that God forms us in the womb. If that “us” is to be taken as including both body and soul, it follows that the soul is formed there, too, and no reason exists to suppose that our souls do not develop in a way similar to our bodies, with the main difference being that nature does not impose such a rigid trajectory on our spirits.
Upon conception, the progress of both facets of the unique human being begins, with the nascent soul definable mainly in terms of its volition to develop. At various stages in youth, the person becomes aware of existence, aware of his or her unique existence, and aware of his or her subordination to the rest of nature. These are milestones, not steps. Upon death — although I’ll have to get back to you many decades hence (God willing) to speak with the confidence of experience — the trajectory can continue sharply toward God (Heaven), gradually toward Him (Purgatory), or gradually or sharply away from Him (Hell).
I offer my afterlife speculation only to present a full, if approximate, picture of the human being’s existence in total, which is how the Christian believes God to see it. Because we’ve only hints and clues with respect to life after death, when Christians speak against morally illicit killing — whether the murder of adults or abortion — they tend to emphasize the insult to God and the deleterious effects on the killer. I submit that it strains a God-centered philosophy to suppose that there are distinctions to be made, on either of these counts, along the human lifespan. As for the effects on the killed, we would certainly like to believe that a person murdered in utero would instantly be saved, but it seems just as likely to me that he or she faces only a slow, purgatorial advancement toward Heaven.
Whatever the case, the point is that there is no such thing as “ensoulment,” except inasmuch as it is synonymous with the creation of the individual (aka conception). This line of reasoning may ultimately leave us no less certain whether the preborn have souls, but it requires that the question be not whether they have received them yet, but whether souls actually exist. In most usages, “soul” is shorthand for “the thing that makes us each uniquely valuable,” and if it is not real, then human life is devalued no matter its development.