Objection 1. It would seem that Christ's flesh was first of all conceived, and afterwards assumed. Because what is not cannot be assumed. But Christ's flesh began to exist when it was conceived. Therefore it seems that it was assumed by the Word of God after it was conceived.
Objection 2. Further, Christ's flesh was assumed by the Word of God, by means of the rational soul. But it received the rational soul at the term of the conception. Therefore it was assumed at the term of the conception. But at the term of the conception it was already conceived. Therefore it was first of all conceived and afterwards assumed.
Objection 3. Further, in everything generated, that which is imperfect precedes in time that which is perfect: which is made clear by the Philosopher (Metaph. ix). But Christ's body is something generated. Therefore it did not attain to its ultimate perfection, which consisted in the union with the Word of God, at the first instant of its conception; but, first of all, the flesh was conceived and afterwards assumed.
On the contrary, Augustine says (De Fide ad Petrum xviii [Written by Fulgentius): "Hold steadfastly, and doubt not for a moment that Christ's flesh was not conceived in the Virgin's womb, before being assumed by the Word."
I answer that, As stated above, we may say properly that "God was made man," but not that "man was made God": because God took to Himself that which belongs to man--and that which belongs to man did not pre-exist, as subsisting in itself, before being assumed by the Word. But if Christ's flesh had been conceived before being assumed by the Word, it would have had at some time an hypostasis other than that of the Word of God. And this is against the very nature of Incarnation, which we hold to consist in this, that the Word of God was united to human nature and to all its parts in the unity of hypostasis: nor was it becoming that the Word of God should, by assuming human nature, destroy a pre-existing hypostasis of human nature or of any part thereof. It is consequently contrary to faith to assert that Christ's flesh was first of all conceived and afterwards assumed by the Word of God.
Reply to Objection 1. If Christ's flesh had been formed or conceived, not instantaneously, but successively, one of two things would follow: either that what was assumed was not yet flesh, or that the flesh was conceived before it was assumed. But since we hold that the conception was effected instantaneously, it follows that in that flesh the beginning and the completion of its conception were in the same instant. So that, as Augustine [Fulgentius, De Fide ad Petrum xviii] says: "We say that the very Word of God was conceived in taking flesh, and that His very flesh was conceived by the Word taking flesh."
From the above the reply to the Second Objection is clear. For in the same moment that this flesh began to be conceived, its conception and animation were completed.
Reply to Objection 3. The mystery of Incarnation is not to be looked upon as an ascent, as it were, of a man already existing and mounting up to the dignity of the Union: as the heretic Photinus maintained. Rather is it to be considered as a descent, by reason of the perfect Word of God taking unto Himself the imperfection of our nature; according to John 6:38: "I came down from heaven."
Source: Summa Theologica (New Advent)