Before we can know how to treat unborn children (an ethical question), we must know what they are biologically. This is a question of science.
Here's what science tells us about the unborn.
Why the unborn is a human being
When a sperm successfully fertilizes an oocyte (egg), a new cell, called a zygote, is generated by their union. The zygote represents the first stage in the life of a human being. This individual, if all goes well, develops through the embryonic (first eight weeks) and fetal (eight weeks until birth) periods and then through infancy, childhood, and adolescence before reaching adulthood.
Four characteristics of the unborn human (the zygote, embryo, or fetus) are important:
Distinct. The unborn has a DNA and body distinct from her mother and father. She develops her own arms, legs, brain, nervous system, heart, and so forth.
Living. The unborn meets the biological criteria for life. She grows by reproducing cells. She turns nutrients into energy through metabolism. And she can respond to stimuli.
Human. The unborn has a human genetic signature. She is also the offspring of human parents, and humans can only beget other humans.
Organism. The unborn is an organism (rather than a mere organ or tissue)—an individual whose parts work together for the good of the whole. Guided by a complete genetic code (46 chromosomes), she needs only the proper environment and nutrition to develop herself through the different stages of life as a member of the species.
These facts about the unborn are established by the science of embryology and developmental biology. They are confirmed by embryology texts, scientific journals, and other relevant authorities.
"Human development begins at fertilization when a sperm fuses with an oocyte to form a single cell, a zygote," explains the textbook The Developing Human: Clinically Oriented Embryology. "This highly specialized, totipotent cell marks the beginning of each of us as a unique individual."
"The development of a human being begins with fertilization," notes Langman's Medical Embryology, "a process by which the spermatozoon from the male and the oocyte from the female unite to give rise to a new organism, the zygote."
The scientific evidence, then, shows that the unborn is a living individual of the species Homo sapiens, the same kind of being as us, only at an earlier stage of development. Each of us was once a zygote, embryo, and fetus, just as we were once infants, toddlers, and adolescents.
Objections to the humanity of the unborn
Many people, however, still dispute the biological humanity of the unborn. Here are some of the most common science-related objections.
Life is continuous
Some people point out that the sperm and egg are alive. Indeed, life, in a broad sense, is continuous (stretching back to the beginning of life on Earth). So it's not accurate, they claim, to say that life "begins" at conception.
It's true that life in general is continuous, but the life of an individual human being is not continuous. It has a beginning and an end. The beginning is called conception. "Although life is a continuous process," explains the textbook Human Embryology & Teratology, "fertilization … is a critical landmark because, under ordinary circumstances, a new, genetically distinct human organism is formed."
Comparison to body parts
Many people note that human organs, tissues, and cells (including the sperm and egg) are living and genetically human. But merely being alive and human doesn't make them human beings. Neither, the argument goes, does it make the unborn a human being.
The difference, however, is that the unborn is a whole organism—an individual member of the species—and other cells and tissues are mere parts. So the unborn isn't just living and human (in the adjective sense of those words); she's a life and a human (in the noun sense). None of us was ever a kidney or a skin cell or a sperm cell. But each of us was once an embryo.
Lack of unity
Some people think that the cells of a very early embryo are too unspecialized or insufficiently unified for the embryo to count as an individual human being. The embryo, they say, is more akin to a mass or ball of cells.
From the zygote stage forward, however, the unborn human clearly exhibits the molecular composition and behavior characteristic of a self-integrated and self-directed organism rather than a mere collection of cells. That's why she can go on to develop the specialized tissues and organs that she does.
"From the moment of sperm-egg fusion," concludes embryologist Maureen L. Condic, a professor at the University of Utah School of Medicine, in a detailed scientific analysis, "a human zygote acts as a complete whole, with all the parts of the zygote interacting in an orchestrated fashion to generate the structures and relationships required for the zygote to continue developing towards its mature state."
Twinning
Before about 14 days post-conception, some embryos split into two embryos (identical twins). Therefore, some think, embryos before this point aren't yet individual, unitary human beings.
But the fact than one organism can give rise to two doesn't mean it isn't an individual organism. A flatworm, as Patrick Lee observes, can be cut to produce two separate flatworms, and that doesn't mean a flatworm isn't a flatworm. The evidence of embryology shows that human embryos, likewise, are unitary and individual organisms even if twinning later occurs.
Parallel with brain death
The irreversible cessation of brain function indicates the death of a human being. Some people argue, then, that the life of a human being cannot begin until brain activity begins.
But the reason (total) brain death matters is that it means the body can no longer function as an integrated whole (even if some cells and tissues are still alive). The brain, in older humans, is essential for that purpose. Before the development of the brain in the first place, however, the very young embryo does not require it in order to function as an organism and direct her own growth (including the development of her brain).
Thus, while a brain-dead patient is a corpse in the process of decay, an embryo is a living and growing individual.
Science and morality
If the basic scientific facts pertaining to the nature of the unborn are straightforward, why do so many people claim that "no one knows when life begins" or that a human embryo isn't human? The biggest reason is that science is conflated with morality, philosophy, or religion.
When someone says that the unborn is not yet "human" or "alive," he is often using those terms in a non-scientific way. He doesn't mean that the unborn isn't biologically human or alive. He means that the unborn isn't valuable or doesn't have human rights. He means that the unborn doesn't yet have the characteristics (e.g., "viability," self-awareness, an infant-like appearance) he thinks would make her "human" or "alive" in this philosophical sense.
So there are two distinct issues here. First, the scientific issue: Is the unborn a human being in the biological sense—a living human organism? The answer, unequivocally, is yes.
Second, the moral or philosophical issue: How should we treat these human beings who have not yet been born? Do they have a right to life? Do all members of our species matter, or only some? This is where the controversy actually lies.
Human embryos and fetuses are human beings. That's what science tells us. Is human equality true? That's what the abortion debate is really about.
A version of this article first appeared in the November-December 2017 issue of MCCL News.