According to the Bible and biology, a human being comes into existence at conception. From this point onward, there is no question as to what this life will become should it continue to grow and develop. It will not be a cat or a bird or a fish. It is clearly human.
Several scriptures confirm that God considered babies in the womb to be people with whom He was already working (see Jeremiah 1:5 and Job 31:15) and beings capable of reacting to things happening outside their mother's wombs (Luke 1:41, 44).
The belief that a human being doesn't become a human being until he or she breathes on his or her own outside the womb is sometimes derived from Genesis 2:7, which says: "And the LORD God formed man of the dust of the ground, and breathed into his nostrils the breath of life; and man became a living being."
Yet, while this is the way Adam (and apparently Eve) came into existence, it is not the way the rest of us have come to have life. We came into existence through conception and being carried in our mother's wombs—not being shaped first as full-grown adults and then having life breathed into us.
The Bible explains that life is in the blood (Leviticus 17:14). Within four or five weeks of conception babies already have their own blood system. In the womb, via the mother's blood, a baby receives nutrients and oxygen—thus receiving "the breath of life" prior to being born. A baby in the womb has blood circulating through its body and is "breathing" by loose analogy like a fish breathes in the water. As a fish gathers the oxygen from the water, so, too, the baby draws oxygen from its mother's blood.
The book Principles of Anatomy and Physiology (Tortora and Anagnostakos, fifth ed., pp. 750-751) states about this process: "They [chorionic villi, microscopic fingerlike projections that make up the placenta] continue growing until they are bathed in maternal blood sinuses called intervillous spaces. Thus, maternal and fetal blood vessels are brought into proximity.
"It should be noted however, that maternal and fetal blood do not normally mix. Oxygen and nutrients from the mother's blood diffuse into the capillaries of the villi. From the capillaries the nutrients circulate into the umbilical vein. Wastes leave the fetus through the umbilical arteries, pass into the capillaries of the villi, and diffuse into the maternal blood" (emphasis added).
The blood of the mother and the baby don't mix. The baby has its own blood and is equipped to receive the oxygen and nutrients and dispose of waste materials via its mother. Its heart is beating, circulating its own oxygenated and deoxygenated blood through its body. It is even conscious at times—looking around, kicking its feet and sucking its thumb (quite unlike the initial creation process of Adam or Eve).
These are a few of the reasons we believe babies in the womb are "living souls," and thus, why it is murder to abort them.