The Immaculate Deception

And indeed, illustrious documents of venerable antiquity, of both the Eastern and the Western Church, very forcibly testify that this doctrine of the Immaculate Conception of the most Blessed Virgin, which was daily more and more splendidly explained, stated and confirmed by the highest authority, teaching, zeal, knowledge, and wisdom of the Church, and which was disseminated among all peoples and nations of the Catholic world in a marvelous manner — this doctrine always existed in the Church as a doctrine that has been received from our ancestors, and that has been stamped with the character of revealed doctrine. For the Church of Christ, watchful guardian that she is, and defender of the dogmas deposited with her, never changes anything, never diminishes anything, never adds anything to them; but with all diligence she treats the ancient documents faithfully and wisely; if they really are of ancient origin and if the faith of the Fathers has transmitted them, she strives to investigate and explain them in such a way that the ancient dogmas of heavenly doctrine will be made evident and clear, but will retain their full, integral, and proper nature, and will grown only within their own genus — that is, within the same dogma, in the same sense and the same meaning.

https://www.papalencyclicals.net/pius09/p9ineff.htm

In Catholic thought, Mary was never subject to sin nor original sin. These are some articles discussing the exegetical and historical issues. Links on the subject:

Triablogue:

Some Early Sources On The Sinlessness Of Mary

More Early Sources On Whether Mary Was Sinless

Some Later Sources On The Sinlessness Of Mary

TheCouncil:

Was Mary Sinless?

JustforCatholics:

The Dogma of the Immaculate Conception

Immaculate Conception and the Church Fathers

Turretinfan:

Augustine and the Immaculate Conception

Pre-Responding to the “Immaculate” => “Immaculate Conception” Argument