The always brilliant Professor Robert P. George nails it:
[A]t 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.
And all the people said: Amen.