Mary is inseparable from Jesus in the divine plan, wherefore the Liturgy applies to her what Holy Scripture says of the eternal Wisdom which is the Word "by whom all was made". Like Christ, the Virgin presides over the whole work of creation, for having been chosen of all eternity to give us the Saviour, it is she, with her Son, whom God had chiefly in view when He created the world.
Salve, sancta parens, enixa puerpera regem: qui caelum terramque regit in saecula saeculorum. * Eructavit cor meum verbum bonum : dico ego opera mea Regi.
Hail, holy Mother! giving birth to thy Child, thou didst bring forth the King, who ruleth the heavens and the earth for ever and ever. Ps. My heart hath uttered a good word : I speak my works to the King.
(Psalm 44:2 from the Introit of mass)
Famulis tuis, quaesumus, Domine, caelestis gratiae munus impertire: ut, quibus beatae Virginis partus exstitit salutis exordium; Nativitatis ejus votiva solemnitas, pacis tribuat incrementum.
We beseech Thee, O Lord, grant to Thy servants the gift of Thy heavenly grace; that as the child-bearing of the Blessed Virgin was the beginning of salvation, so the joyful festival of her Nativity may bring us an increase of peace.
Nativitas tua, Dei Genitrix Virgo, gaudium annuntiavit universo mundo: ex te enim ortus est Sol justitiae, Christus Deus noster: qui solvens maledictionem, dedit benedictionem, et confundens mortem, donavit nobis vitam sempiternam.
Thy Nativity, O Virgin Mother of God, was the herald of joy to the whole world; since from thee arose the Sun of Justice, Christ our God, who, destroying the curse, bestowed the blessing, and confounding death, rewarded us with life everlasting.
(Antiphon at the Magnificat)
From the Catholic Encyclopaedia: http://www.newadvent.org/cathen/10712b.htm