From Ad Caeli Reginam by Pope Pius XII on Proclaiming the Queenship of Mary:
8. From early times Christians have believed, and not without reason, that she of whom was born the Son of the Most High received privileges of grace above all other beings created by God. He “will reign in the house of Jacob forever,” “the Prince of Peace,” the “King of Kings and Lord of Lords.” And when Christians reflected upon the intimate connection that obtains between a mother and a son, they readily acknowledged the supreme royal dignity of the Mother of God.
9. Hence it is not surprising that the early writers of the Church called Mary “the Mother of the King” and “the Mother of the Lord,” basing their stand on the words of St. Gabriel the archangel, who foretold that the Son of Mary would reign forever, and on the words of Elizabeth who greeted her with reverence and called her “the Mother of my Lord.” Thereby they clearly signified that she derived a certain eminence and exalted station from the royal dignity of her Son.
10. So it is that St. Ephrem, burning with poetic inspiration, represents her as speaking in this way: “Let Heaven sustain me in its embrace, because I am honored above it. For heaven was not Thy mother, but Thou hast made it Thy throne. How much more honorable and venerable than the throne of a king is her mother.” And in another place he thus prays to her: “. . . Majestic and Heavenly Maid, Lady, Queen, protect and keep me under your wing lest Satan the sower of destruction glory over me, lest my wicked foe be victorious against me.”