Conscience is a judgement of reason whereby the human person recognizes the moral quality of a concrete act that he is going to perform, is in the process of performing, or has already completed. In all he says and does, man is obliged to follow faithfully what he knows to be just and right. It is by the judgement of his conscience that man perceives and recognizes the prescriptions of the divine law
To be secure in following our conscience, we require a properly informed conscience, i.e., one that is in possession of the truth. And it is the teaching of the Church that guarantees that we possess the truth revealed by God. We form our conscience with the truth of the moral realm, so that we might conform our actions to those of Christ.
In regards to abortion, Jeremiah 1:5, "Before I formed you in the womb I knew you, before you were born I set you apart; I appointed you as a prophet to the nations." and Psalm 139:13, "For you created my inmost being; you knit me together in my mother's womb." says that God knew us while we were in the womb. Therefore, God considers us people even before conception as per Jeremiah.
The lord commands that we not kill. Since God sees us as people before we are even concieved, who are we to decide that God is wrong - that we are just its. Abortion is killing a person. We are responsible for our actions as Leviticus 5:17 says, "If a person sins and does what is forbidden in any of the LORD's commands, even though he does not know it, he is guilty and will be held responsible."
No matter how much someone wants to believe that abortion is not a sin, the bible disagrees and according to Leviticus, they will be held responsible for their actions.
In regards to rights, God gave us the right to choose. We have the right to do whatever we want. We have the right to kill. However, we will not ever get away from God holding us responsible for our choices.
I hope that helps.