Objection 1. It would seem that Christ's conception was natural. For Christ is called the Son of Man by reason of His conception in the flesh. But He is a true and natural Son of Man: as also is He the true and natural Son of God. Therefore His conception was natural.
Objection 2. Further, no creature can be the cause of a miraculous effect. But Christ's conception is attributed to the Blessed Virgin, who is a mere creature: for we say that the Virgin conceived Christ. Therefore it seems that His conception was not miraculous, but natural.
Objection 3. Further, for a transformation to be natural, it is enough that the passive principle be natural, as stated above (Question 32, Article 4). But in Christ's conception the passive principle on the part of His Mother was natural, as we have shown (32, 4). Therefore Christ's conception was natural.
On the contrary, Dionysius says (Ep. ad Caium Monach.): "Christ does in a superhuman way those things that pertain to man: this is shown in the miraculous virginal conception."
I answer that, As Ambrose says (De Incarn. vi): "In this mystery thou shalt find many things that are natural, and many that are supernatural." For if we consider in this conception anything connected with the matter thereof, which was supplied by the mother, it was in all such things natural. But if we consider it on the part of the active power, thus it was entirely miraculous. And since judgment of a thing should be pronounced in respect of its form rather than of its matter: and likewise in respect of its activity rather than of its passiveness: therefore is it that Christ's conception should be described simply as miraculous and supernatural, although in a certain respect it was natural.
Reply to Objection 1. Christ is said to be a natural Son of Man, by reason of His having a true human nature, through which He is a Son of Man, although He had it miraculously; thus, too, the blind man to whom sight has been restored sees naturally by sight miraculously received.
Reply to Objection 2. The conception is attributed to the Blessed Virgin, not as the active principle thereof, but because she supplied the matter, and because the conception took place in her womb.
Reply to Objection 3. A natural passive principle suffices for a transformation to be natural, when it is moved by its proper active principle in a natural and wonted way. But this is not so in the case in point. Therefore this conception cannot be called simply natural.
Source: Summa Theologica (New Advent)