What is aborted?

In the modern age, we have very detailed observations, confirming beyond any reasonable doubt, that the cell produced by sperm-egg fusion (the zygote) is a human organism; i.e. a human being. We know this because immediately upon the binding and fusion of the gametes (a rapid event taking less than a quarter second to complete), the newly formed zygote enters into a sequence of molecular events that determine and direct its subsequent maturation and growth. The fact that the zygote autonomously initiates the process of embryonic development distinguishes it from a mere human cell [a patch of skin cells, or sperm or egg] and clearly indicates that it is a full and complete, albeit immature, member of the human species.

—Maureen Condic, Ph.D., Associate Professor of Neurobiology and Anatomy at the University of Utah, "Q&A with the Scholars: Science and the Beginning of Life"

The science of embryology establishes that from the earliest stages of development, the unborn are distinct, living, and whole human beings. Breaking that down, the embryo is:

Unique—not a part of his or her mother, but genetically distinct; possibly with a different blood type, for example
Living—growing, developing, undergoing metabolism at a rapid pace; also responds to stimuli
Complete—everything needed for development is already present; simply needs time to grow
Human—living things always produce after their own kind, so humans beget only complete, unique humans

While the early embryo does not look like an adult, it does look exactly as a developing human should at that stage of development. "Embryo" and "fetus" are simply stages of life in a span that includes others—such as "infant," "toddler," "adolescent," and "adult."

Scripture teaches that all humans have value because they bear the image of their Maker (Genesis 1:26-28, James 3:9). Because humans bear the image of God, the shedding of innocent blood is strictly forbidden (Exodus 23:7, Proverbs 6:16-19, Matthew 5:21). We are never to kill innocent human beings.

Abortion is the intentional killing of an innocent human being. The same passages in Scripture that forbid the shedding of innocent blood apply to the unborn as they do to every other human being.

The SLED test

There is no morally significant difference between the embryo you once were and the adult you are today that would justify killing you at that earlier stage of development. Here's a helpful acronym that reminds us of four non-essential differences between people:

S	You were once very small as an embryo, but body SIZE does not make a person more or less valuable. A 7-foot basketball player is not "more human" than a toddler.
L	As a fetus you were more developed than you were at the embryo stage, but less developed than you are today. Why should LEVEL OF DEVELOPMENT matter? Two-year-old girls are less developed than 18-year-old women—should we kill them because of it? Of course not, for the same reason that parents don't have a greater right to live than their teenage children, even though they're more developed physically and intellectually.
E	Your ENVIRONMENT also has no bearing on what you are. The journey of eight inches down the birth canal does not change the essential nature of the unborn—from someone we can kill to someone we cannot.
D	DEGREE OF DEPENDENCY—Each of us are dependent on sources outside ourselves for survival, to one degree or another. Being more dependent than someone else has no bearing on a person's value.

From The Moral Question of Abortion, by Stephen D. Schwarz (Loyola Press, 1990); see also "How to Defend Your Pro-Life Views in 5 Minutes or Less," by Scott Klusendorf, Life Training Institute (http://prolifetraining.com/resources/five-minute-1/).