With the “Declaration on Procured Abortions” you are on the right track.
The Catechism of the Catholic Church states in #2323, “Because it should be treated as a person from conception, the embryo must be defended in its integrity, cared for, and healed like every other human being.”
The Church actually teaches as De Fide that the soul is the “form of the body.” In so doing, it is saying that the soul is the principle of the life of the person. (The Council of Venice, held from 1311to 1312: Denzinger 491, cf. 738, 1658)
So only when there is life is there a human soul. Death is the departure of the soul from the body. This means that at no time can one have a living body without a soul.
So the Church DOES INDEED teach that human life begins at conception. What it does not explain is HOW this happens. In the case of twins, it could be that the first embryo has a soul and is a person. Then another embryo develops from it. Thus we have another person with another soul. Or it could be that the first embryo splits into two other embryos with the first embryo dying at the conception of two new persons. At this point, we don’t have enough information to explain what exactly has happened.
Fr. Vincent Serpa, O.P.