I answer that, The whole Trinity effected the conception of Christ's body: nevertheless, this is attributed to the Holy Ghost, for three reasons. First, because this is befitting to the cause of the Incarnation, considered on the part of God. For the Holy Ghost is the love of Father and Son, as stated in the FP, Question [37], Article [1]. Now, that the Son of God took to Himself flesh from the Virgin's womb was due to the exceeding love of God: wherefore it is said (Jn. 3:16): "God so loved the world as to give His only-begotten Son."
Secondly, this is befitting to the cause of the Incarnation, on the part of the nature assumed. Because we are thus given to understand that human nature was assumed by the Son of God into the unity of Person, not by reason of its merits, but through grace alone; which is attributed to the Holy Ghost, according to 1 Cor. 12:4: "There are diversities of graces, but the same Spirit." Wherefore Augustine says (Enchiridion xl): "The manner in which Christ was born of the Holy Ghost . . . suggests to us the grace of God, whereby man, without any merits going before, in the very beginning of his nature when he began to exist was joined to God the Word, into so great unity of Person, that He Himself should be the Son of God."
Thirdly, because this is befitting the term of the Incarnation. For the term of the Incarnation was that that man, who was being conceived, should be the Holy one and the Son of God. Now, both of these are attributed to the Holy Ghost. For by Him men are made to be sons of God, according to Gal. 4:6: "Because you are sons, God hath sent the Spirit of His Son into your [Vulg.: 'our'] hearts, crying: Abba, Father." Again, He is the "Spirit of sanctification," according to Rm. 1:4. Therefore, just as other men are sanctified spiritually by the Holy Ghost; so as to be the adopted sons of God, so was Christ conceived in sanctity by the Holy Ghost, so as to be the natural Son of God. Hence, according to a gloss on Rm. 1:4, the words, "Who was predestinated the Son of God, in power," are explained by what immediately follows: "According to the Spirit of sanctification, i.e. through being conceived of the Holy Ghost." And the Angel of the Annunciation himself, after saying, "The Holy Ghost shall come upon thee," draws the conclusion: "Therefore also the Holy which shall be born of thee shall be called the Son of God."