There are, essentially, two things that need to be understood in order to come to this conclusion: 1) that killing another person for selfish reasons is wrong, and 2) that yet-to-be-born babies are other persons. For the length of this article we'll presume that the first point is easy for you to recognize (cf. Genesis 9:6, the Sixth Commandment, and Romans 1:18-32; esp. v. 29), and so, our focus shall be on the second idea (that the unborn are human persons).
First, a species only conceives members of its own species. Cats beget cats. Dogs beget dogs. And humans beget humans. You don’t see dogs conceiving antelope. You don’t see fish giving birth to cattle. And you don’t see women conceiving tigers. It just doesn’t work that way. Humans conceive and give birth to other humans (albeit tiny undeveloped humans).
With that, let's look at what the primary differences between us, as adults, and them, as the potentially born.
We are largely independent of others for our survival. We are capable of abstract thought. We have reached a high level of development. We are contributing members of society. And we abide outside a womb.
Fetuses, on the other hand, are dependant upon the host/mother for survival. If they do have abstract thoughts, we cannot know it for fetuses are poor communicators. Fetuses exist at the most rudimentary levels of development. Fetuses contribute nothing to society. And fetuses abide in a womb.
Now the question is this: are these differences enough to keep personhood from unborn infants?
Does our independence make us more valuable than the unborn? Only in a productivity sense. I can help out around the house whereas my unborn child cannot. I can cook, clean, vacuum, and bring home a paycheck. The fetus in my wife’s womb can do none of these things. Of course, neither can my two-year-old. And neither can I when I’m really sick. Does this mean that I am less of a person when I am sick? Or that my two-year-old is not a person either? Of course not. And ending the life of either of us would be wrong.
What about the complexity of our thought lives? Am I more a person because I consider ideas such as the meaning of life and the hope of life beyond the grave? I would hazard to answer this in the negative as again this would make me more worthy of life than anyone who doesn’t think about these things as much or as clearly as I do. Hitler killed thousands of people who did not reach a level of mental capacity to satisfy him; if we base personhood on how well a human thinks, we have vindicated his atrocities.
The difference of our various levels of development operates along similar lines as far as our value as human beings. As far as production value, of course the better developed are more desirable, but this doesn’t affect our intrinsic value as humans. I don’t think any of us are ready to start killing off or enslaving C students and those who run an 8-minute mile simply because they aren’t as well developed as the A students and those who run a 6-minute mile. The C students, the four-year-olds, and the mentally handicapped all retain their intrinsic human value in spite of their inability to perform at the higher levels that those who are better developed are able.
Does personhood depend upon the degree to which we contribute to society? Not remotely. If I get in a car accident this afternoon and am laid up in the hospital for nine months, I am still a person. If someone entered my room in the ward and shot me to death, he would still stand trial for my death for we, as a society, still recognize the inherent value of people (even if they have been injured). We also do not kill hermits in spite of the fact that their contribution to society is negligible.
And does personhood depend on location? Will I be less a person if I am here or if I am there? If location determines personhood, then we should fear constantly to go anywhere, for in being somewhere else, we might lose that which makes us valuable.
In fact, none of these things determine our value as humans. What does determine our value is simply this: we are humans. We are conceived in the image of God. God has breathed into us our human spiritand it is that spirit which gives us our innate value. Therefore, all human beingswhether small or large, new or old, productive or unproductive, intelligent or not, strong or weak, here or thereare innately worthy of respect. To kill any of them as a matter of convenience is morally wrong.