Objection 1. It would seem that the Blessed Virgin cannot be called Christ's Mother in respect of His temporal nativity. For, as stated above (Question 32, Article 4), the Blessed Virgin Mary did not cooperate actively in begetting Christ, but merely supplied the matter. But this does not seem sufficient to make her His Mother: otherwise wood might be called the mother of the bed or bench. Therefore it seems that the Blessed Virgin cannot be called the Mother of Christ.
Objection 2. Further, Christ was born miraculously of the Blessed Virgin. But a miraculous begetting does not suffice for motherhood or sonship: for we do not speak of Eve as being the daughter of Adam. Therefore neither should Christ be called the Son of the Blessed Virgin.
Objection 3. Further, motherhood seems to imply partial separation of the semen. But, as Damascene says (De Fide Orth. iii), "Christ's body was formed, not by a seminal process, but by the operation of the Holy Ghost." Therefore it seems that the Blessed Virgin should not be called the Mother of Christ.
On the contrary, It is written (Matthew 1:18): "The generation of Christ was in this wise. When His Mother Mary was espoused to Joseph," etc.
I answer that, The Blessed Virgin Mary is in truth and by nature the Mother of Christ. For, as we have said above (5, 2; 31, 5), Christ's body was not brought down from heaven, as the heretic Valentine maintained, but was taken from the Virgin Mother, and formed from her purest blood. And this is all that is required for motherhood, as has been made clear above (31, 5; 32, 4). Therefore the Blessed Virgin is truly Christ's Mother.
Reply to Objection 1. As stated above (Question 32, Article 3), not every generation implies fatherhood or motherhood and sonship, but only the generation of living things. Consequently when inanimate things are made from some matter, the relationship of motherhood and sonship does not follow from this, but only in the generation of living things, which is properly called nativity.
Reply to Objection 2. As Damascene says (De Fide Orth. iii): "The temporal nativity by which Christ was born for our salvation is, in a way, natural, since a Man was born of a woman, and after the due lapse of time from His conception: but it is also supernatural, because He was begotten, not of seed, but of the Holy Ghost and the Blessed Virgin, above the law of conception." Thus, then, on the part of the mother, this nativity was natural, but on the part of the operation of the Holy Ghost it was supernatural. Therefore the Blessed Virgin is the true and natural Mother of Christ.
Reply to Objection 3. As stated above (31, 5, ad 3; 32, 4), the resolution of the woman's semen is not necessary for conception; neither, therefore, is it required for motherhood.
Source: Summa Theologica (New Advent)