It is not only at the moment of her birth into the world that God thought of exempting the most Blessed Virgin Mary from all sin and making of her the Immaculate Conception.
From all eternity, before the creation of the world, the Blessed Virgin was conceived in the mind of God. The church applies this passage of Scripture to the Blessed Virgin: “From eternity… I was already conceived” (Prov. 8:23-24). God was already thinking of her, and he willed to shower her with all His graces and to give her that extraordinary privilege of the Immaculate Conception, exempting her from all fault: “You are all beautiful, O Mary, and there is no stain in you” (Hymn Tota pulchra es). So already, from all eternity, God was thinking of this admirable creature, the first after our Lord Jesus Christ.
God revealed little by little the place which He intended to give to the Blessed Virgin in the plan of salvation by announcing her Himself and by prefiguring her through illustrious women of the Old Testament. Throughout the Old Testament, we see that God was thinking of the Blessed Virgin. Already, immediately after the sin of Adam and Eve, God said to the serpent, “I will put enmities between thee and the woman; … she shall crush thy head” (Gen. 3:15). So the Virgin Mary was already present in the mind of God.
And if she was filled with the Holy Ghost from her birth, she was so even more, if possible, in the instant when the angel Gabriel came to announce that she would be the Mother of the Savior. “Hail, full of grace” (Lk. 1:28), that is what the angel Gabriel said to the most Blessed Virgin Mary; “the Holy Ghost shall come upon thee, and the power of the most High shall overshadow thee” (Lk. 1:35).