Objection 1. It would seem that the flesh of Christ was assumed by the Word before being united to the soul. For Augustine [Fulgentius] says (De Fide ad Petrum xviii): "Most firmly hold, and nowise doubt that the flesh of Christ was not conceived in the womb of the Virgin without the Godhead before it was assumed by the Word." But the flesh of Christ would seem to have been conceived before being united to the rational soul, because matter or disposition is prior to the completive form in order of generation. Therefore the flesh of Christ was assumed before being united to the soul.
Objection 2. Further, as the soul is a part of human nature, so is the body. But the human soul in Christ had no other principle of being than in other men, as is clear from the authority of Pope Leo, quoted above (3). Therefore it would seem that the body of Christ had no other principle of being than we have. But in us the body is begotten before the rational soul comes to it. Therefore it was the same in Christ; and thus the flesh was assumed by the Word before being united to the soul.
Objection 3. Further, as is said (De Causis), the first cause excels the second in bringing about the effect, and precedes it in its union with the effect. But the soul of Christ is compared to the Word as a second cause to a first. Hence the Word was united to the flesh before it was to the soul.
On the contrary, Damascene says (De Fide Orth. iii, 2): "At the same time the Word of God was made flesh, and flesh was united to a rational and intellectual soul." Therefore the union of the Word with the flesh did not precede the union with the soul.
I answer that, The human flesh is assumable by the Word on account of the order which it has to the rational soul as to its proper form. Now it has not this order before the rational soul comes to it, because when any matter becomes proper to any form, at the same time it receives that form; hence the alteration is terminated at the same instant in which the substantial form is introduced. And hence it is that the flesh ought not to have been assumed before it was human flesh; and this happened when the rational soul came to it. Therefore since the soul was not assumed before the flesh, inasmuch as it is against the nature of the soul to be before it is united to the body, so likewise the flesh ought not to have been assumed before the soul, since it is not human flesh before it has a rational soul.
Reply to Objection 1. Human flesh depends upon the soul for its being; and hence, before the coming of the soul, there is no human flesh, but there may be a disposition towards human flesh. Yet in the conception of Christ, the Holy Ghost, Who is an agent of infinite might, disposed the matter and brought it to its perfection at the same time.
Reply to Objection 2. The form actually gives the species; but the matter in itself is in potentiality to the species. And hence it would be against the nature of a form to exist before the specific nature. And therefore the dissimilarity between our origin and Christ's origin, inasmuch as we are conceived before being animated, and Christ's flesh is not, is by reason of what precedes the perfection of the nature, viz. that we are conceived from the seed of man, and Christ is not. But a difference which would be with reference to the origin of the soul, would bespeak a diversity of nature.
Reply to Objection 3. The Word of God is understood to be united to the flesh before the soul by the common mode whereby He is in the rest of creatures by essence, power, and presence. Yet I say "before," not in time, but in nature; for the flesh is understood as a being, which it has from the Word, before it is understood as animated, which it has from the soul. But by the personal union we understand the flesh as united to the soul before it is united to the Word, for it is from its union with the soul that it is capable of being united to the Word in Person; especially since a person is found only in the rational nature
Source: Summa Theologica (New Advent)