What is the dogma of the Immaculate Conception? It pertains to Mary's being conceived without Original Sin, not to the virginal and miraculous conception of Jesus Christ in Mary's womb.
We declare, pronounce, and define that the doctrine which holds that the most Blessed Virgin Mary, in the first instance of her conception, by a singular grace and privilege granted by Almighty God, in view of the merits of Jesus Christ, the Savior of the human race, was preserved free from all stain of original sin, is a doctrine revealed by God and therefore to be believed firmly and constantly by all the faithful.The dogma was explicitly and infallibly defined in 1854 by Pope Pius IX in the Constitution Ineffabilis Deus. That it was defined in 1854 does not mean that it is a new belief of the Church. If you read the Constitution, you will see that Pope Pius IX presents historical, liturgical, and theological evidence for the belief throughout the history of the Church. (The dogmas defined at the Council of Nicea didn't come into existence then, either, but the proper understanding and definition of the belief was then given.)
Declaramus, pronuntiamus et definimus doctrinam quae tenet beatissimam Virginem Mariam in primo instanti suae conceptionis fuisse singulari Omnipotentis Dei gratia et privilegio, intuitu meritorum Christi Jesu Salvatoris humani generis, ab omni originalis culpae labe praeservatam immunem, esse a Deo revelatam, atque idcirco ab omnibus fidelibus firmiter constanterque credendam.
I would also suggest reading the document he issued prior to Ineffabilis Deus. In 1849, he wrote Ubi Primum, preparing the Church for the definition of the dogma. Here are some excerpts from the first few paragraphs:
1. ... there was in the entire Catholic world a most ardent and wondrous revival of the desire that the most holy Mother of God -- the beloved Mother of us all, the immaculate Virgin Mary -- be finally declared by a solemn definition of the Church to have been conceived without the stain of original sin.Here are a few helpful links for understanding, celebrating, and defending this important dogma of the faith:
2. Both to Our Predecessor and to Us this most devout desire was clearly and unmistakably made manifest by the petitions of illustrious bishops, esteemed canonical chapters, and religious congregations, among whom was the renowned Order of Preachers. These appeals vied with one another in the insistent request that official permission be granted for the word Immaculate to be publicly used and be added to the sacred liturgy, particularly in the Preface of the Mass of the Conception of the Blessed Virgin. ...
3. Moreover, Venerable Brethren, many of you have sent letters to Our Predecessor and to Us begging, with repeated insistence and redoubled enthusiasm, that We define as a dogma of the Catholic Church that the most blessed Virgin Mary was conceived immaculate and free in every way of all taint of original sin. Nor do we lack today eminent theologians -- men of intellectual brilliance, of virtue, of holiness and sound doctrine -- who have so effectively explained this doctrine and so impressively expounded this proposition that many persons are now wondering why this honor has not already been accorded to the Blessed Virgin by the Church and the Apostolic See -- an honor which the widespread piety of the Christian people so fervently desires to have accorded to the Most Holy Virgin by a solemn decree and by the authority of the Church and the Holy See.