The First Nature of Any Being Contains Actualized Potencies
Some argue that the unborn are not yet human because they lack rational intelligence, the distinctive difference of the human species. Rationality is certainly not missing in the arsenal of an unborn child's potentialities. If the child lives, he or she will certainly exercise that potentiality in a recognizable way.
The "first nature" of any living being contains a bundle of potencies which are actualized according to the growth cycle of individual being. Just because the actualization of the capacity for human reasoning cannot be detected in a child's prenatal existence is no reason to assume the child lacks that capacity. There are many things a child won't do right away, such as forming words, sentences, and grasping concepts. But as any mother will tell you, children learn these things far sooner than you would expect.
Both of the arguments above suffer from another defect -- they focus on certain aspects or qualities of a human life and not the life itself. When you talk to parents about the birth of a child they will often tell you about the sudden recognition that a new life has been thrust into their hands. A human being who did not exist now exists and has come forth from you. The contrast between existence and non-existence that is experienced at childbirth points back to the existence that came into being at the moment of conception. Sperm and egg joined together, and a new being, a third being, was created out of the union. That third being, the child, has never been before, except, perhaps, in the mind of God.
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In your sense the potential for life is in everything, it is everywhere. I actually agree - when a man and a woman think, "child", right then is when the child comes into being, even in the absolute physical sense. This is because the physical as well as the metaphysical potentialities are present, all that remains is the act of consummation, which can either take place organically, or in a test tube. When a man and a woman together think "child", but cannot bear one, that is true death of the unborn.
5 minutes before conception those same potencies are already in existence. We carry some of the "potencies" of all our descendants with us right now. In the unbroken chain of descent there are a series of miraculous transformations, one of which is conception. Why do you glorify conception over the others? After conception it is still the genetics of the mother that rule the cell. A fetus's own genetic combination doesn't take over until a short while later, but that is just another of the miracles that occur.
How about the eggs first cell division? That is a big one, where the fetus will die if the chromosomes can't dance together. How about gastrulation? An incredible, complex, cellular convolution where the fetus has to prove that it has what it takes. Many fetuses don't make it there. How about implantation - the point where the fetus and mother meet and negotiate terms of gestation. Whatever we decide about the morality of abortion has no effect on this fundamental, biological "choice" by the womb. How about when the nascent immune system first defines 'self'? How about a heart beat or a meiotic division or a nerve impulse or a breath? Life is a string of miracles, especially during fetal development, and it is not clear to me which one deserves to be held up as the most miraculous. Each milestone is the realization of one of your bundled potencies, and an absolute requirement for all the subsequent ones.
I believe that a fetus gains humanity incrementally as it passes each subsequent test. Before the milestones are past a fetus is only potentially a human being - potential is not the same as actual.