EWTN Catholic Q&A
Sacramentals - Miraculous Medal
Question from M. Pierce on 11-30-2011:

What is the history of the Miraculous Medal?

Why do spanish speaking people say the following phrase/prayer when praying the rosary - "Mary, conceived without original sin pray for us."

Why don't we say that prayer in addition to the Fatima prayer?

Answer by Fr. Jay Toborowsky on 12-02-2011:

I'm sure there are plenty of websites and booklets with the history of the Miraculous Medal, but the essential part of it is that the Blessed Mother asked St. Catherine Laboure to have the medal created (and described to her what it should look like). The phrase you mention is on the medal itself, and part of the Miraculous Medal Novena prayers. I don't know if it is a "Spanish" thing specifically, but has more to do with personal devotions that anyone may have.