Consider the following from Catholic Answers:
IMMACULATE CONCEPTION
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."
The Catechism of the Catholic Church states that she was "redeemed in a more exalted fashion, by reason of the merits of her Son" (CCC 492). She has more reason to call God her Savior than we do, because he saved her in an even more glorious manner!
But what about Romans 3:23, "all have sinned"? Have all people committed actual sins? Consider a child below the age of reason. By definition he can’t sin, since sinning requires the ability to reason and the ability to intend to sin. This is indicated by Paul later in the letter to the Romans when he speaks of the time when Jacob and Esau were unborn babies as a time when they "had done nothing either good or bad" (Rom. 9:11).
We also know of another very prominent exception to the rule: Jesus (Heb. 4:15). So if Paul’s statement in Romans 3 includes an exception for the New Adam (Jesus), one may argue that an exception for the New Eve (Mary) can also be made.
Paul’s comment seems to have one of two meanings. It might be that it refers not to absolutely everyone, but just to the mass of mankind (which means young children and other special cases, like Jesus and Mary, would be excluded without having to be singled out). If not that, then it would mean that everyone, without exception, is subject to original sin, which is true for a young child, for the unborn, even for Mary—but she, though due to be subject to it, was preserved by God from it and its stain.
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Does it not stand to reason that God would choose to be born of a woman 'conceived without sin'.
No. He didn't spend His life in a sin free bubble. He spent it with sinners. That's why He came.