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.
Source: Ad Caeli Reginam (Vatican.va)