15 If the Man Jesus Christ began His bodily life with the same beginning as our body and soul, if He were not, as God, the immediate Author of His own body and soul alike, when He was fashioned in the likeness and form of man, and born as man, then we may suppose that He felt the pain of our body; since by His beginning, a conception like ours, He had a body animated with a soul like our own. But if through His own act He took to Himself flesh from the Virgin, and likewise by His own act joined a soul to the body thus conceived, then the nature of His suffering must have corresponded with the nature of His body and soul.
For when He emptied Himself of the form of God and received the form of a servant when the Son of God was born also Son of Man, without losing His own self and power, God the Word formed the perfect living Man. For how was the Son of God born Son of Man, how did He receive the form of a servant, still remaining in the form of God, unless (God the Word being able of Himself to take flesh from the Virgin and to give that flesh a soul, for the redemption of our soul and body), the Man Christ Jesus was born perfect, and made in the form of a servant by the assumption of the body, which the Virgin conceived?
For the Virgin conceived, what she conceived, from the Holy Ghost alone, and though for His birth in the flesh she supplied from herself that element, which women always contribute to the seed planted in them, still Jesus Christ was not formed by an ordinary human conception. In His birth, the cause of which was transmitted solely by the Holy Ghost, His mother performed the same part as in all human conceptions: but by virtue of His origin He never ceased to be God.
Source: On the Trinity (New Advent)