Objection 1. It would seem that Christ's Mother was not a virgin in His Birth. For Ambrose says on Luke 2:23: "He who sanctified a strange womb, for the birth of a prophet, He it is who opened His Mother's womb, that He might go forth unspotted." But opening of the womb excludes virginity. Therefore Christ's Mother was not a virgin in His Birth.
Objection 2. Further, nothing should have taken place in the mystery of Christ, which would make His body to seem unreal. Now it seems to pertain not to a true but to an unreal body, to be able to go through a closed passage; since two bodies cannot be in one place at the same time. It was therefore unfitting that Christ's body should come forth from His Mother's closed womb: and consequently that she should remain a virgin in giving birth to Him.
Objection 3. Further, as Gregory says in the Homily for the octave of Easter [xxvi in Evang., that by entering after His Resurrection where the disciples were gathered, the doors being shut, our Lord "showed that His body was the same in nature but differed in glory": so that it seems that to go through a closed passage pertains to a glorified body. But Christ's body was not glorified in its conception, but was passible, having "the likeness of sinful flesh," as the Apostle says (Romans 8:3). Therefore He did not come forth through the closed womb of the Virgin.
On the contrary, In a sermon of the Council of Ephesus (P. III, Cap. ix) it is said: "After giving birth, nature knows not a virgin: but grace enhances her fruitfulness, and effects her motherhood, while in no way does it injure her virginity." Therefore Christ's Mother was a virgin also in giving birth to Him.
I answer that, Without any doubt whatever we must assert that the Mother of Christ was a virgin even in His Birth: for the prophet says not only: "Behold a virgin shall conceive," but adds: "and shall bear a son." This indeed was befitting for three reasons. First, because this was in keeping with a property of Him whose Birth is in question, for He is the Word of God. For the word is not only conceived in the mind without corruption, but also proceeds from the mind without corruption. Wherefore in order to show that body to be the body of the very Word of God, it was fitting that it should be born of a virgin incorrupt. Whence in the sermon of the Council of Ephesus (quoted above) we read: "Whosoever brings forth mere flesh, ceases to be a virgin. But since she gave birth to the Word made flesh, God safeguarded her virginity so as to manifest His Word, by which Word He thus manifested Himself: for neither does our word, when brought forth, corrupt the mind; nor does God, the substantial Word, deigning to be born, destroy virginity."
Secondly, this is fitting as regards the effect of Christ's Incarnation: since He came for this purpose, that He might take away our corruption. Wherefore it is unfitting that in His Birth He should corrupt His Mother's virginity. Thus Augustine says in a sermon on the Nativity of Our Lord: "It was not right that He who came to heal corruption, should by His advent violate integrity."
Thirdly, it was fitting that He Who commanded us to honor our father and mother should not in His Birth lessen the honor due to His Mother.
Reply to Objection 1. Ambrose says this in expounding the evangelist's quotation from the Law: "Every male opening the womb shall be called holy to the Lord." This, says Bede, "is said in regard to the wonted manner of birth; not that we are to believe that our Lord in coming forth violated the abode of her sacred womb, which His entrance therein had hallowed." Wherefore the opening here spoken of does not imply the unlocking of the enclosure of virginal purity; but the mere coming forth of the infant from the maternal womb.
Reply to Objection 2. Christ wished so to show the reality of His body, as to manifest His Godhead at the same time. For this reason He mingled wondrous with lowly things. Wherefore, to show that His body was real, He was born of a woman. But in order to manifest His Godhead, He was born of a virgin, for "such a Birth befits a God," as Ambrose says in the Christmas hymn.
Reply to Objection 3. Some have held that Christ, in His Birth, assumed the gift of "subtlety," when He came forth from the closed womb of a virgin; and that He assumed the gift of "agility" when with dry feet He walked on the sea. But this is not consistent with what has been decided above (Article 14). For these gifts of a glorified body result from an overflow of the soul's glory on to the body, as we shall explain further on, in treating of glorified bodies (XP, 82): and it has been said above (13, 3, ad 1; 16, 1, ad 2) that before His Passion Christ "allowed His flesh to do and to suffer what was proper to it" (Damascene, De Fide Orth. iii): nor was there such an overflow of glory from His soul on to His body.
We must therefore say that all these things took place miraculously by Divine power. Whence Augustine says (Sup. Joan. Tract. 121): "To the substance of a body in which was the Godhead closed doors were no obstacle. For truly He had power to enter in by doors not open, in Whose Birth His Mother's virginity remained inviolate." And Dionysius says in an epistle (Ad Caium iv) that "Christ excelled man in doing that which is proper to man: this is shown in His supernatural conception, of a virgin, and in the unstable waters bearing the weight of earthly feet."
Source: Summa Theologica (New Advent)