Before deciding how we ought to treat the unborn—a moral question—we must first be clear about what the unborn is. This is a scientific question, and it is answered with clarity by the science of human embryology.
When sperm fertilizes egg
The facts of reproduction are straightforward. Upon completion of the fertilization process, sperm and egg have ceased to exist (this is why "fertilized egg" is an inaccurate term); what exists is a single cell with 46 chromosomes (23 from each parent) that is called a zygote. The coming into existence of the zygote is the point of conception—the beginning of the life of a new human organism. The terms zygote, embryo and fetus all refer to developmental stages in the life of a human being.
Four features of the unborn
Four features of the unborn (i.e., the human zygote, embryo or fetus) are relevant to his or her status as a human being. First, the unborn is living. She meets all the biological criteria for life: metabolism, cellular reproduction and reaction to stimuli. Moreover, she is clearly growing, and dead things (of course) don't grow.
Second, the unborn is human. She possesses a human genetic signature that proves this beyond any doubt. She is also the offspring of human parents, and we know that humans can only beget humans (they cannot beget dogs or cats, for instance). The unborn may not seem to "look" human (at least in her earlier stages), but in fact she looks exactly like a human at that level of human development. Living things do not become something different as they grow and mature; rather, they develop the way that they do precisely because of the kind of being they already are.
Third, the unborn is genetically and functionally distinct from (though dependent on and resting inside of) the pregnant woman. Her growth and maturation is internally directed, and her DNA is unique and different from that of any other cell in the woman’s body. She develops her own arms, legs, brain, central nervous system, etc. To say that a fetus is a part of the pregnant woman’s body is to say that the woman has four arms and four legs, and that about half of pregnant women have penises.
A whole organism
Fourth, the unborn is a whole or complete (though immature) organism. That is, she is not a mere part of another living thing, but is her own organism—an entity whose parts work together in a self-integrated fashion to bring the whole to maturity. Her genetic information is fully present at conception, determining to a large extent her physical characteristics (including sex, eye color, skin color, bone structure, etc.); she needs only a suitable environment and nutrition to develop herself through the different stages of human life.