Christian scholar Blessed Duns Scotus (d. 1308) gives a name to this: preservative redemption. Preservative redemption addresses this question of Mary's redemption taking place before her Son Jesus was even born. Mary's conception and sanctification were simultaneous.

Theologically, it is this:  the unlimited God, the Creator of time, is also Lord over time. In other words, God can work outside of time. And God applies His graces throughout history (time) as He deems fit. Therefore, God, in His divine plan of salvation, willed that Mary would be saved first, in her humanity, by the application of the graces won for humanity by her Son, Jesus on the Cross.

Mary's lifetime of preparation, living with a sinless, loving heart, equipped her to make the perfect response to the Angel when he announced God's call on her life. Furthermore, Mary's Immaculate Conception provided in her body a perfectly pure temple, in a sense, for the Holy Spirit to later "overshadow."

The Son of God would take on flesh in Mary's sinless womb -- to use biblical imagery -- in a perfect tabernacle . . . the Holy of holies (Heb. 9:3) . . . the ark of the covenant (Heb. 9:4).

The splendor of an entirely unique holiness by which Mary is enriched from the first instant of her conception comes wholly from Christ: she is redeemed, in a more exalted fashion, by reason of the merits of her Son.The Father blessed Mary more than any other created person in Christ with every spiritual blessing in the heavenly places and chose her in Christ before the foundation of the world, to be holy and blameless before him in love. (Catechism of the Catholic Church, 492)

God qualified Mary for her magnificent role in salvation history by the power of his amazing grace.

So we can pray with the confidence of an Angel . . . Hail Mary, full of grace, the Lord is with thee. Blessed art thou among women . . .