Objection 1. It would seem that the flesh of Christ was not conceived of the Virgin's purest blood: For it is said in the collect (Feast of the Annunciation) that God "willed that His Word should take flesh from a Virgin." But flesh differs from blood. Therefore Christ's body was not taken from the Virgin's blood.
Objection 2. Further, as the woman was miraculously formed from the man, so Christ's body was formed miraculously from the Virgin. But the woman is not said to have been formed from the man's blood, but rather from his flesh and bones, according to Genesis 2:23: "This now is bone of my bones, and flesh of my flesh." It seems therefore that neither should Christ's body have been formed from the Virgin's blood, but from her flesh and bones.
Objection 3. Further, Christ's body was of the same species as other men's bodies. But other men's bodies are not formed from the purest blood but from the semen and the menstrual blood. Therefore it seems that neither was Christ's body conceived of the purest blood of the Virgin.
On the contrary, Damascene says (De Fide Orth. iii) that "the Son of God, from the Virgin's purest blood, formed Himself flesh, animated with a rational soul."
I answer that, As stated above (Article 4), in Christ's conception His being born of a woman was in accordance with the laws of nature, but that He was born of a virgin was above the laws of nature. Now, such is the law of nature that in the generation of an animal the female supplies the matter, while the male is the active principle of generation; as the Philosopher proves (De Gener. Animal. i). But a woman who conceives of a man is not a virgin. And consequently it belongs to the supernatural mode of Christ's generation, that the active principle of generation was the supernatural power of God: but it belongs to the natural mode of His generation, that the matter from which His body was conceived is similar to the matter which other women supply for the conception of their offspring. Now, this matter, according to the Philosopher (De Gener. Animal.), is the woman's blood, not any of her blood, but brought to a more perfect stage of secretion by the mother's generative power, so as to be apt for conception. And therefore of such matter was Christ's body conceived.
Reply to Objection 1. Since the Blessed Virgin was of the same nature as other women, it follows that she had flesh and bones of the same nature as theirs. Now, flesh and bones in other women are actual parts of the body, the integrity of which results therefrom: and consequently they cannot be taken from the body without its being corrupted or diminished. But as Christ came to heal what was corrupt, it was not fitting that He should bring corruption or diminution to the integrity of His Mother. Therefore it was becoming that Christ's body should be formed not from the flesh or bones of the Virgin, but from her blood, which as yet is not actually a part, but is potentially the whole, as stated in De Gener. Animal. i. Hence He is said to have taken flesh from the Virgin, not that the matter from which His body was formed was actual flesh, but blood, which is flesh potentially.
Reply to Objection 2. As stated in I, 92, 3, ad 2, Adam, through being established as a kind of principle of human nature, had in his body a certain proportion of flesh and bone, which belonged to him, not as an integral part of his personality, but in regard to his state as a principle of human nature. And from this was the woman formed, without detriment to the man. But in the Virgin's body there was nothing of this sort, from which Christ's body could be formed without detriment to His Mother's body.
Reply to Objection 3. Woman's semen is not apt for generation, but is something imperfect in the seminal order, which, on account of the imperfection of the female power, it has not been possible to bring to complete seminal perfection. Consequently this semen is not the necessary matter of conception; as the Philosopher says (De Gener. Animal. i): wherefore there was none such in Christ's conception: all the more since, though it is imperfect in the seminal order, a certain concupiscence accompanies its emission, as also that of the male semen: whereas in that virginal conception there could be no concupiscence. Wherefore Damascene says (De Fide Orth. iii) that Christ's body was not conceived "seminally." But the menstrual blood, the flow of which is subject to monthly periods, has a certain natural impurity of corruption: like other superfluities, which nature does not heed, and therefore expels. Of such menstrual blood infected with corruption and repudiated by nature, the conception is not formed; but from a certain secretion of the pure blood which by a process of elimination is prepared for conception, being, as it were, more pure and more perfect than the rest of the blood. Nevertheless, it is tainted with the impurity of lust in the conception of other men: inasmuch as by sexual intercourse this blood is drawn to a place apt for conception. This, however, did not take place in Christ's conception: because this blood was brought together in the Virgin's womb and fashioned into a child by the operation of the Holy Ghost. Therefore is Christ's body said to be "formed of the most chaste and purest blood of the Virgin."
Source: Summa Theologica (New Advent)