"All these things developed from the Pontiffs and the theologians, and a terminology was created in which Mary is soon called the 'spiritual Mother of men, Queen of heaven and earth'; in other ways, 'New Eve, Mediatrix, Dispensatrix of all graces,' and indeed, 'Co-Redemptrix'...To that which pertains to the title, 'Co-Redemptrix,' and 'Associate of Christ the Redeemer,' some things must be added.
Already in the tenth century, the title of 'Redemptrix' was used: 'Holy Redemptrix of the world, pray for us.' When in the fifteenth and sixteenth century, this familiar title was used, already an immediate cooperation of the Blessed Virgin in the work of our Redemption was recognized, and to the name, 'Redemptrix' is added 'co,' and therefore the Mother of God was called, 'Co-Redemptrix,' while Christ continued to be called, 'Redeemer.' From that time to the seventeenth century, the title Co-redemptrix was brought into use not only in devotional works of piety and holiness, but also in a great number of theological tracts. This also pertains to the Roman pontiffs, as it has occurred in certain texts of St. Pius X and Pius XI..."
-From the first schema on the Blessed Virgin Mary presented to the Council Fathers of Vatican II on November 23, 1962, as quoted in the book "With Jesus": The Story of Mary Co-Redemptrix by Dr. Mark Miravalle.