Q: Whether Christ's Body Was Formed in the First Instant of Its Conception?

A: On the contrary,
Gregory says (Moral. xviii): "As soon as the angel announced it, as soon as the Spirit came down, the Word was in the womb, within the womb the Word was made flesh."

I answer that,
In the conception of Christ's body three points may be considered: first, the local movement of the blood to the place of generation; secondly, the formation of the body from that matter; thirdly, the development whereby it was brought to perfection of quantity. Of these, the second is the conception itself; the first is a preamble; the third, a result of the conception.

Now, the first could not be instantaneous: since this would be contrary to the very nature of the local movement of any body whatever, the parts of which come into a place successively. The third also requires a succession of time: both because there is no increase without local movement, and because increase is effected by the power of the soul already informing the body, the operation of which power is subject to time.

But the body's very formation, in which conception principally consists, was instantaneous, for two reasons. First, because of the infinite power of the agent, viz. the Holy Ghost, by whom Christ's body was formed, as stated above (Q. 32, A. 1). For the greater the power of an agent, the more quickly can it dispose matter; and, consequently, an agent of infinite power can dispose matter instantaneously to its due form. Secondly, on the part of the Person of the Son, whose body was being formed. For it was unbecoming that He should take to Himself a body as yet unformed. While, if the conception had been going on for any time before the perfect formation of the body, the whole conception could not be attributed to the Son of God, since it is not attributed to Him except by reason of the assumption of that body. Therefore in the first instant in which the various parts of the matter were united together in the place of generation, Christ's body was both perfectly formed and assumed. And thus is the Son of God said to have been conceived; nor could it be said otherwise.

Reply Obj. 1: Neither quotation from Augustine refers to formation alone of Christ's body, but to its formation, together with a fixed development up to the time of His birth. Wherefore in the aforesaid number are foreshadowed the number of months during which Christ was in the Virgin's womb.

Reply Obj. 2: This local movement is not comprised within the conception itself, but is a preamble thereto.

Reply Obj. 3: It is not possible to fix the last instant in which that matter was blood: but it is possible to fix the last period of time which continued without any interval up to the first instant in which Christ's body was formed. And this instant was the terminus of the time occupied by the local movement of the matter towards the place of generation.

Reply Obj. 4: Increase is caused by the augmentative power of that which is the subject of increase: but the formation of the body is caused by the generative power, not of that which is generated, but of the father generating from seed, in which the formative power derived from the father's soul has its operation. But Christ's body was not formed by the seed of man, as stated above (Q. 31, A. 5, ad 3), but by the operation of the Holy Ghost. Therefore the formation thereof should be such as to be worthy of the Holy Ghost. But the development of Christ's body was the effect of the augmentative power in Christ's soul: and since this was of the same species as ours, it behooved His body to develop in the same way as the bodies of other men, so as to prove the reality of His human nature.