This article at Catechism.cc, excerpted from my book The Catechism of Catholic Ethics, presents a theological argument, based on the conceptions of Jesus and of Mary, that life for all human persons begins at conception. The origin of each human life, in body and soul, at conception, is important to the moral definition of abortion. The article concludes with the following:
There should be no debate among the faithful about the time of ‘ensoulment’, or about when a prenatal becomes a human person. The dogmas of the Incarnation and of the Immaculate Conception necessarily imply that, in the very same instant, the body is created, and the soul is created, and body and soul are one. A human being, at any stage of life, in any condition whatsoever, has a body and a soul; every human being with a body and a soul is a human person. The soul in particular is made directly by God, in the image of God. Therefore, human life must be protected from the moment of conception. All prenatal human beings are innocent human persons created by God and in the image of God.
Read the article at Catechism.cc