Based on universally accepted scientific criteria, a new cell, the human zygote, comes into existence at the moment of sperm-egg fusion, an event that occurs in less than a second. Upon formation, the zygote immediately initiates a complex sequence of events that establish the molecular conditions required for continued embryonic development. The behavior of the zygote is radically unlike that of either sperm or egg separately and is characteristic of a human organism. Thus, the scientific evidence supports the conclusion that a zygote is a human organism and that the life of a new human being commences at a scientifically well defined “moment of conception.”
Listen to this interview with Robert P. George, who says "at all stages of our lives — from the embryonic through the fetal, infant, child, and adolescent stages and into adulthood — we are human beings with dignity and the right to life. Our dignity does not come from having achieved a certain level of intellectual proficiency or even conscious awareness. … We have our dignity in virtue of the kind of entity we are: that is human being, a creature with a rational nature. And we became that when we came to be."
Contraception, abortion, and the eugenics movement by Jonah Goldberg, an excerpt from Liberal Fascism.
Raymond Dennehy, "THE ONTOLOGICAL BASIS OF HUMAN RIGHTS", The Thomist 42 (July 1978): 434-463.
Instruction Dignitas Personae on Certain Bioethical Questions