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.