Objection 1. It would seem that, by her sanctification in the womb, the Blessed Virgin did not receive the fulness or perfection of grace. For this seems to be Christ's privilege, according to John 1:14: "We saw Him [Vulgate: 'His glory'] as the Only-Begotten [Vulgate: 'as it were of the Only-Begotten'] full of grace and truth." But what is proper to Christ ought not to be ascribed to some one else. Therefore the Blessed Virgin did not receive the fulness of grace at the time of her sanctification.
Objection 2. Further, nothing remains to be added to that which is full and perfect: for "the perfect is that which lacks nothing," as is said Phys. iii. But the Blessed Virgin received additional grace afterwards when she conceived Christ; for to her was it said (Luke 1:35): "The Holy Ghost shall come upon thee: and again, when she was assumed into glory." Therefore it seems that she did not receive the fulness of grace at the time of her first sanctification.
Objection 3. Further, "God does nothing useless," as is said De Coelo et Mundo i. But it would have been useless for her to have certain graces, for she would never have put them to use: since we do not read that she taught which is the act of wisdom; or that she worked miracles, which is the act of one of the gratuitous graces. Therefore she had not the fulness of grace.
On the contrary, The angel said to her: "Hail, full of grace" (Luke 1:28); which words Jerome expounds as follows, in a sermon on the Assumption (cf. Ep. ad Paul. et Eustoch.): "Full indeed of grace: for to others it is given in portions; whereas on Mary the fulness of grace was showered all at once."
I answer that, In every genus, the nearer a thing is to the principle, the greater the part which it has in the effect of that principle, whence Dionysius says (Coel. Hier. iv) that angels, being nearer to God, have a greater share than men, in the effects of the Divine goodness. Now Christ is the principle of grace, authoritatively as to His Godhead, instrumentally as to His humanity: whence (John 1:17) it is written: "Grace and truth came by Jesus Christ." But the Blessed Virgin Mary was nearest to Christ in His humanity: because He received His human nature from her. Therefore it was due to her to receive a greater fulness of grace than others.
Reply to Objection 1. God gives to each one according to the purpose for which He has chosen him. And since Christ as man was predestinated and chosen to be "predestinated the Son of God in power... of sanctification" (Romans 1:4), it was proper to Him to have such a fulness of grace that it overflowed from Him into all, according to John 1:16: "Of His fulness we have all received." Whereas the Blessed Virgin Mary received such a fulness of grace that she was nearest of all to the Author of grace; so that she received within her Him Who is full of all grace; and by bringing Him forth, she, in a manner, dispensed grace to all.
Reply to Objection 2. In natural things at first there is perfection of disposition, for instance when matter is perfectly disposed for the form. Secondly, there is the perfection of the form; and this is the more excellent, for the heat that proceeds from the form of fire is more perfect than that which disposed to the form of fire. Thirdly, there is the perfection of the end: for instance when fire has its qualities in the most perfect degree, having mounted to its own place.
In like manner there was a threefold perfection of grace in the Blessed Virgin. The first was a kind of disposition, by which she was made worthy to be the mother of Christ: and this was the perfection of her sanctification. The second perfection of grace in the Blessed Virgin was through the presence of the Son of God Incarnate in her womb. The third perfection of the end is that which she has in glory.
That the second perfection excels the first, and the third the second, appears (1) from the point of view of deliverance from evil. For at first in her sanctification she was delivered from original sin: afterwards, in the conception of the Son of God, she was entirely cleansed from the fomes: lastly, in her glorification she was also delivered from all affliction whatever. It appears (2) from the point of view of ordering to good. For at first in her sanctification she received grace inclining her to good: in the conception of the Son of God she received consummate grace confirming her in good; and in her glorification her grace was further consummated so as to perfect her in the enjoyment of all good.
Reply to Objection 3. There is no doubt that the Blessed Virgin received in a high degree both the gift of wisdom and the grace of miracles and even of prophecy, just as Christ had them. But she did not so receive them, as to put them and such like graces to every use, as did Christ: but accordingly as it befitted her condition of life. For she had the use of wisdom in contemplation, according to Luke 2:19: "But Mary kept all these words, pondering them in her heart." But she had not the use of wisdom as to teaching: since this befitted not the female sex, according to 1 Timothy 2:12: "But I suffer not a woman to teach." The use of miracles did not become her while she lived: because at that time the Teaching of Christ was to be confirmed by miracles, and therefore it was befitting that Christ alone, and His disciples who were the bearers of His doctrine, should work miracles. Hence of John the Baptist it is written (John 10:41) that he "did no sign"; that is, in order that all might fix their attention on Christ. As to the use of prophecy, it is clear that she had it, from the canticle spoken by her: "My soul doth magnify the Lord" (Luke 1:46, etc.).
Source: Summa Theologica (New Advent)