The Holy Spirit will come upon you, and the power of the Most High will overshadow you. So the Holy One to be born will be called the Son of God. [Luke 1:35]
The key point in Gabriel's explanation is that what is about to happen to Mary will be the result of the direct intervention of God. The Holy Spirit is the agent of the Virgin Birth; overshadowing is the means of the Virgin Birth; the Son of God is the result of the Virgin Birth.
This suggests something that is often denied - even in evangelical circles. It is often suggested that the Virgin Birth was not necessary even though it really happened, i.e., God could have brought Jesus into the world in some other way. Gabriel's words seem to indicate the opposite. The whole point of verse 35 is that the Virgin Birth produces the Holy One of God. The "so" is very crucial. Without the virginal conception by the Holy Spirit, the Holy One of God will not be born.
That suggests that, in reality, there really was no other way for Jesus to be born. Gabriel's words imply that the Virgin Birth was not just another Christmas miracle that God could have dispensed with had he so chosen. Without the Virgin Birth, there would be no Christmas at all.
If someone inquires into the biology of the virginal conception of Jesus, we have only this verse to give them. The Greek word translated "overshadow" (episkiazo) was used of God's visible presence in the Old Testament tabernacle. It pictures the God of light personally dwelling with his people. We might also think of the Spirit of God hovering above the waters in Genesis 1:2. "God's powerful presence will rest upon Mary, so that she will bear a child who will be the Son of God." (Marshall, Luke, p. 71).