The Catholic catechism has a section on abortion (2270-2275). It starts with this:
Human life must be respected and protected absolutely from the moment of conception. From the first moment of his existence, a human being must be recognized as having the rights of a person - among which is the inviolable right of every innocent being to life.
Before I formed you in the womb I knew you, and before
you were born I consecrated you.
My frame was not hidden from you, when I was being made in secret,
intricately wrought in the depths of the earth.7
Essentially, the position is that at the moment of conception, the embryo is fully human, that it should be "treated from conception as a person" (CCC 2274). Because of that, destroying the embryo (or worse, the fetus) is murder. Therefore, the ban against abortion is based on the fifth commandment (do not murder).