The Catholic theology of Original Sin begins with the relationship between God the Creator and Adam the creature who is created in the image of God. In simple language, God is a transcendent (supernatural) Divine Pure Spirit without restrictions.
Adam’s nature is an unique unification of the* both* the spiritual world and material world. (Catechism of the Catholic Church, Second Edition, paragraphs 355-368)
Because of his spiritual principle (soul) which is a [partial] image of God’s [total] spiritual nature, Adam begins his life in relationship/friendship with God.
Catholic theology teaches that not only was Adam the first human being, the whole human race is in Adam “as one body of one man.” We know by Revelation that Adam had received original holiness and justice not for himself alone, but for all human nature. His descendants would be born in the state of friendship with God. (CCC 404-405)
Adam is not God.
Because of his creaturely status, the only way that Adam could maintain his original friendship with God was to live in free submission to God. The interesting thing about Original Sin (besides knowing what kind of organic fruit was so tempting ;)) is that Adam wanted to be just as smart and all-knowing as his Creator. What other knowledge is there besides that of good and evil? Instead of following God’s command, Adam preferred his own pride-filled desires and by that very act scorned his Creator. “He chose himself over and against God, against the requirements of his creaturely status and therefore against his own good.” (CCC 396-400) The result shattered humanity’s relationship with divinity
It is Adam’s broken relationship with God which is transmitted by propagation to human descendants. We contracted a state deprived of original holiness and justice otherwise known as Original Sin because the original human did it. Baptism which imparts the life of Christ’s grace erases Original Sin and restores the original relationship with God. Romans 5: 12-21 is fact.
Regarding children in the womb or out. We turn to the fact that God is the Creator and therefore, He is in charge. “Since Christ died for all, and since all men are in fact called to one and the same destiny, which is divine, we must hold that the Holy Spirit offers to all the possibility of being made partakes, in a way known to God, of the Paschal mystery.” (CCC 1260) Who are we to limit the ways God can bring a child to Himself? Is God limited by the wall of the womb?
The last sentence of CCC 1257 explains: “God has bound salvation to the sacrament of Baptism, but He Himself is not bound by His sacraments.”
Link to the Catechism: scborromeo.org/ccc.htm