In a way, it's easy to see why the preborn child, especially in the earliest stage of development, is not commonly regarded as a human person (with the rights accorded to persons). When we utter the word "person" most people would immediately conjure up a mental image of a bipedal example of Homo sapiens — probably an adult. But clearly, teenagers, children, toddlers, and babies are also persons. They're just at different stages of development. And so we have to ask: what about the preborn, at any stage of development?
Fr. Tad Pacholczyk writes:
Embryos, of course, are remarkably unfamiliar to us. They lack hands and feet. They don't have faces or eyes for us to look into. Even their brains are lacking. They look nothing like what we are used to seeing when we imagine a human being. But they are as human as you and me. When we look at a scanning electron micrograph of a human embryo, a small cluster of cells, sitting on the point of a sewing pin, we need to ask ourselves a very simple question: "Isn't that exactly what a young human is supposed to look like?" The correct answer to that question doesn't depend on religion or theology, but on embryology. Embryos seem unfamiliar to us on first glance, and we have to make an explicit mental effort to avoid the critical mistake of disconnecting from who we once were as embryos.
Isn't it interesting that everyone who advocates the destruction of embryos, whether through abortion or embryonic stem cell research, was once an embryo? This reminds me of the T-shirt with the two embryos on it, with one saying to the other: "It may be hard to believe right now, but we'll be pro-choice someday."