Objection 1. It would seem that we should not say that Christ was conceived of [de] the Holy Ghost. Because on Romans 11:36: "For of Him [ex ipso] and by Him, and in Him, are all things," the gloss of Augustine says: "Notice that he does not say, 'of Him' [de ipso], but 'of Him' [ex ipso]. For of Him [ex ipso], are heaven and earth, since He made them: but not of Him [de ipso], since they are not made of His substance." But the Holy Ghost did not form Christ's body of [de] His own substance. Therefore we should not say that Christ was conceived of [de] the Holy Ghost.
Objection 2. Further, the active principle of [de] which something is conceived is as the seed in generation. But the Holy Ghost did not take the place of seed in Christ's conception. For Jerome says (Expos. Cathol. Fidei) [Written by Pelagius]: "We do not say, as some wicked wretches hold, that the Holy Ghost took the place of seed: but we say that Christ's body was wrought," i.e. formed, "by the power and might of the Creator." Therefore we should not say that Christ's body was conceived of [de] the Holy Ghost.
Objection 3. Further, no one thing is made of two, except they be in some way mingled. But Christ's body was formed of [de] the Virgin Mary. If therefore we say that Christ was conceived of [de] the Holy Ghost, it seems that a mingling took place of the Holy Ghost with the matter supplied by the Virgin: and this is clearly false. Therefore we should not say that Christ was conceived of [de] the Holy Ghost.
On the contrary, It is written (Matthew 1:18): "Before they came together, she was found with child, of [de] the Holy Ghost."
I answer that, Conception is not attributed to Christ's body alone, but also to Christ Himself by reason of His body. Now, in the Holy Ghost we may observe a twofold habitude to Christ. For to the Son of God Himself, who is said to have been conceived, He has a habitude of consubstantiality: while to His body He has the habitude of efficient cause. And this preposition of [de] signifies both habitudes: thus we say that a certain man is "of [de] his father." And therefore we can fittingly say that Christ was conceived of the Holy Ghost in such a way that the efficiency of the Holy Ghost be referred to the body assumed, and the consubstantiality to the Person assuming.
Reply to Objection 1. Christ's body, through not being consubstantial with the Holy Ghost, cannot properly be said to be conceived "of" [de] the Holy Ghost, but rather "from [ex] the Holy Ghost," as Ambrose says (De Spir. Sanct. ii.): "What is from someone is either from his substance or from his power: from his substance, as the Son who is from the Father; from his power, as all things are from God, just as Mary conceived from the Holy Ghost."
Reply to Objection 2. It seems that on this point there is a difference of opinion between Jerome and certain other Doctors, who assert that the Holy Ghost took the place of seed in this conception. For Chrysostom says (Hom. i in Matth. [Opus Imperf., among the supposititious writings): "When God's Only-Begotten was about to enter into the Virgin, the Holy Ghost preceded Him; that by the previous entrance of the Holy Ghost, Christ might be born unto sanctification according to His body, the Godhead entering instead of the seed." And Damascene says (De Fide Orth. iii): "God's wisdom and power overshadowed her, like unto a Divine seed."
But these expressions are easily explained. Because Chrysostom and Damascene compare the Holy Ghost, or also the Son, who is the Power of the Most High, to seed, by reason of the active power therein; while Jerome denies that the Holy Ghost took the place of seed, considered as a corporeal substance which is transformed in conception.
Reply to Objection 3. As Augustine says (Enchiridion xl), Christ is said to be conceived or born of the Holy Ghost in one sense; of the Virgin Mary in another--of the Virgin Mary materially; of the Holy Ghost efficiently. Therefore there was no mingling here.
Source: Summa Theologica (New Advent)