It’s important to understand what the doctrine of the Immaculate Conception is and what it is not. Some people think the term refers to Christ’s conception in Mary’s womb without the intervention of a human father; but that is the Virgin Birth. Others think the Immaculate Conception means Mary was conceived "by the power of the Holy Spirit," in the way Jesus was, but that, too, is incorrect. The Immaculate Conception means that Mary, whose conception was brought about the normal way, was conceived without original sin or its stain—that’s what "immaculate" means: without stain. The essence of original sin consists in the deprivation of sanctifying grace, and its stain is a corrupt nature. Mary was preserved from these defects by God’s grace; from the first instant of her existence she was in the state of sanctifying grace and was free from the corrupt nature original sin brings.

When discussing the Immaculate Conception, an implicit reference can be found in the angel’s greeting to Mary. The angel Gabriel said, "Hail, full of grace, the Lord is with you" (Luke 1:28). The phrase "full of grace" is a translation of the Greek word kecharitomene. This word represents the proper name of the person being addressed by the angel, and it therefore expresses a characteristic quality of Mary.

The traditional translation, "full of grace," is more accurate than the one found in many recent versions of the New Testament, which give something along the lines of "highly favored daughter." Mary was indeed a highly favored daughter of God, but the Greek implies more than that (and it never mentions the word for "daughter"). The grace given to Mary is at once permanent and of a unique kind. Kecharitomene is a perfect passive participle of charitoo, meaning "to fill or endow with grace." Since this term is in the perfect tense, it indicates a perfection of grace that is both intensive and extensive. So, the grace Mary enjoyed was not a result of the angel’s visit, and was only as "full" or strong or complete as possible at any given time, but it extended over the whole of her life, from conception onward. She was in a state of sanctifying grace from the first moment of her existence to have been called "full of grace."

www.catholic.com/library/Immaculate_Conception_and_Assum.asp

Immaculate Conception as Patroness of the United States
In 1846, the American bishops--one Archbishop and twenty-two bishops, at the Sixth Provincial Council of Baltimore, 1846--placed the United States "under the special patronage of the holy Mother of God, whose immaculate conception is venerated by the piety of the faithful throughout the Catholic Church." Already in 1791, Bishop John Carroll's Baltimore Synod had named the Virgin Mary as patroness of the Diocese of Baltimore, which then embraced the whole United States.
Nine years after the American bishops chose Mary in her Immaculate Conception as patroness of the United States, Pius IX defined the Immaculate Conception. For more than four centuries before the definition of the doctrine in 1854, Catholic people pledged themselves to honor and defend the conception of Mary as sinless.

www.catholic.com/library/Mary_Mother_of_God.asp