Objection 1. It would seem that Christ's body was not animated in the first instant of its conception. For Pope Leo says (Ep. ad Julian.): "Christ's flesh was not of another nature than ours: nor was the beginning of His animation different from that of other men." But the soul is not infused into other men at the first instant of their conception. Therefore neither should Christ's soul have been infused into His body in the first instant of its conception.
Objection 2. Further, the soul, like any natural form, requires determinate quantity in its matter. But in the first instant of its conception Christ's body was not of the same quantity as the bodies of other men when they are animated: otherwise, if afterwards its development had been continuous, either its birth would have occurred sooner, or at the time of birth He would have been a bigger child than others. The former alternative is contrary to what Augustine says (De Trin. iv), where he proves that Christ was in the Virgin's womb for the space of nine months: while the latter is contrary to what Pope Leo says (Serm. iv in Epiph.): "They found the child Jesus nowise differing from the generality of infants." Therefore Christ's body was not animated in the first instant of its conception.
Objection 3. Further, whenever there is "before" and "after" there must be several instants. But according to the Philosopher (De Gener. Animal. ii) in the generation of a man there must needs be "before" and "after": for he is first of all a living thing, and afterwards, an animal, and after that, a man. Therefore the animation of Christ could not be effected in the first instant of His conception.
On the contrary, Damascene says (De Fide Orth. iii): "At the very instant that there was flesh, it was the flesh of the Word of God, it was flesh animated with a rational and intellectual soul."
I answer that, For the conception to be attributed to the very Son of God, as we confess in the Creed, when we say, "who was conceived by the Holy Ghost," we must needs say that the body itself, in being conceived, was assumed by the Word of God. Now it has been shown above (6, 1,2) that the Word of God assumed the body by means of the soul, and the soul by means of the spirit, i.e. the intellect. Wherefore in the first instant of its conception Christ's body must needs have been animated by the rational soul.
Reply to Objection 1. The beginning of the infusion of the soul may be considered in two ways. First, in regard to the disposition of the body. And thus, the beginning of the infusion of the soul into Christ's body was the same as in other men's bodies: for just as the soul is infused into another man's body as soon as it is formed, so was it with Christ. Secondly, this beginning may be considered merely in regard to time. And thus, because Christ's body was perfectly formed in a shorter space of time, so after a shorter space of time was it animated.
Reply to Objection 2. The soul requires due quantity in the matter into which it is infused: but this quantity allows of a certain latitude because it is not fixed to a certain amount. Now the quantity that a body has when the soul is first infused into it is in proportion to the perfect quantity to which it will attain by development: that is to say, men of greater stature have greater bodies at the time of first animation. But Christ at the perfect age was of becoming and middle stature: in proportion to which was the quantity of His body at the time when other men's bodies are animated; though it was less than theirs at the first instant of His conception. Nevertheless that quantity was not too small to safeguard the nature of an animated body; since it would have sufficed for the animation of a small man's body.
Reply to Objection 3. What the Philosopher says is true in the generation of other men, because the body is successively formed and disposed for the soul: whence, first, as being imperfectly disposed, it receives an imperfect soul; and afterwards, when it is perfectly disposed, it receives a perfect soul. But Christ's body, on account of the infinite power of the agent, was perfectly disposed instantaneously. Wherefore, at once and in the first instant it received a perfect form, that is, the rational soul.
Source: Summa Theologica (New Advent)