The March for Life is the largest pro-life event in the world. The theme for this year’s march is “Unique from Day One: Pro-Life is Pro-Science.” When I heard about the theme, my immediate thought was it’s about time.

Given the events of the past few years, including other public demonstrations like the March For Science and the Women’s March, there could never have been a better moment to point out the obvious: being pro-life is both pro-science and pro-woman. It is remarkable how much of the science gets lost amid the strong emotions evoked by the abortion debate. The main dividing line between pro-life and pro-choice is not which side cares more about women, families, and their basic freedoms. Rather, the main difference is how each group applies the scientific facts to determine what constitutes women’s rights.

Human Cells, Human Organisms, Human Beings

The scientific question of when the life of a human being begins is often lost or confused with the philosophical and legal questions of what constitutes a human person and which human persons deserve equal protection under the law.

Start your day with Public Discourse

Sign up and get our daily essays sent straight to your inbox.

Based on the biological definition of life at the cellular level, science unequivocally affirms that a newly fertilized human embryo is a living organism. The male sperm cell and the female oocyte (egg) are both human cells, and the fusion of the sperm to the egg unites the two living, human reproductive cells to form a single distinct, live, human embryo. The newly formed human embryo has the complete genetic code needed for the capacities that make it a living organism—capacities such as self-directed activity, growth, maintenance of homeostasis, and reproduction.

In other words: the developing fetus, whose origins trace back embryologically to the single-cell embryo, was never a nonliving entity. Furthermore, because the complete genomic content is present from the human embryo’s conception onward, and the genomic content contains all of the instructions for every function of the developing human across all stages of life, the embryo is equipped with the biological capacities of an organism from the very beginning.

Advancements in our scientific understanding of early human development continue to affirm this concept of the human embryo as a living organism. For instance, scientists have uncovered more about the significance of the crossing over of fetal DNA into maternal circulation, which has furthered our understanding that the developing fetus is not simply receiving from the mother, but also communicating back to her. Other recent studies have provided evidence to suggest that human embryos can direct their own early development in the absence of maternal tissue, which speaks to distinct organismal properties as well. Scientists are now eager to apply this knowledge of the early human organism to save and improve the lives of babies born very prematurely, which is a leading cause of infant mortality. The underlying assumptions for this research are twofold: that the fetus is a distinct organism from the start and that human development occurs along a continuum.

At what point, then, does the living human embryo, which starts as a single-cell organism and rapidly develops along its multicellular and increasingly complex embryonic growth trajectory, become a rights-bearing human person?

Science, Philosophy, and Law

This question is at the heart of the abortion debate, but science is not the power that ultimately determines personhood. Human embryology shows us that certain biological properties, such as the beating of a human heart and the ability to respond to stimuli, evolve at specific points in time. We now know that a baby’s heart begins to beat between the third and fourth week of embryonic life (often before a mother even realizes she’s pregnant). The development of the nervous system also begins incredibly early in life, with the formation of the neural tube beginning around day twenty-two of embryonic life. The neural tube goes on to form the brain and spinal cord, through which, among other functions, the developing human is able to respond to stimuli such as pain, temperature, and so on. Simple errors this early in gestation can lead to devastating outcomes for the developing human, including conditions such as spina bifida or anencephaly.

But neither the presence of a heartbeat nor the development of a brain and spinal cord are in and of themselves the sole property that constitutes a human being. They are properties we recognize in developing human beings, but it is futile to try to designate one of these stages of development as the moment at which the human organism transforms into a human being. The human organism is always a human being, from the moment of conception.

Thus we reach the fork in the road, where pro-life and pro-choice camps must each decide how the scientific evidence informs the philosophical definition of human personhood, which in turn must dictate the legal protections afforded to human persons.

Three Pro-Choice Approaches

On the pro-choice road, there seem to be three main ways to apply the science.

Route One is to concede the science but say it doesn’t matter. An example is Judith Jarvis Thomson’s analogy of the famous violinist, which implies that the developing pre-born human is an uninvited parasite on the mother’s bodily nutrients and ultimately on her health and well-being. Whether or not the pre-born human is a living human being (in her analogy, Thomson assumes it is), the argument is that the bigger issue is whether or not the mother must sacrifice her body for the sake of another. Scientific information about human development doesn’t matter, because abortion is justified by the mother’s will.

Route Two is to waffle about the significance of the science. In essence, Route Two is the “just a clump of cells” position. These abortion proponents concede that the human embryo is initially a living, single cell that develops into a multicellular collection, but they waver on the significance of scientific milestones like a heartbeat and a nervous system. They view the question of when to recognize and protect the human person as a morally grey area. This redirects the argument away from primary consideration of the science of the developing human organism and focuses it instead on what circumstances would justify the removal and termination of the early human fetus or experimentation on human embryonic cells. The premise is that abortion is more clearly wrong once the developing human looks like and clearly functions in a way that is similar to extrauterine human beings.

Possibly as a side-effect of Route Two abortion proponents, fetal pain legislation took off in the 1990s, once it was determined by the Supreme Court that states have a compelling interest in viable humans, which should be balanced with a woman’s “right to choose” abortion. These laws have been heavily criticized by non–Route Two abortion proponents, who argue that the laws confuse science with the philosophy of human personhood. Again, there is no scientifically identifiable developmental stage at which the fetus suddenly becomes a person.

Route Three is to concede that unborn human beings are in fact human beings, but not human persons. Route Three provides support for all abortion, and can be extended to include gender selection and infanticide, because it saves the commitment of personhood until some point after the human baby is born. It is closer in content to Route One, but differs in that its proponents believe that the status of personhood requires the capacities for self-care and self-awareness. That means that those with dementia or other severe physical or intellectual disabilities are not considered human beings. This is arguably the most extreme pro-choice position, but is also the pro-choice position that is most informed by the science of human development.

The Pro-Life Position

By contrast, the pro-life position fully acknowledges the science and embraces this information in its all-encompassing stance that both human life and human personhood begin at conception. Once the sperm and egg cease to be two separate cells and unite to form a zygote, a new human being has come into existence. This human being is also a human person because it contains, from the beginning, the root capacities of personal life (like the ability to make choices and to exhibit self-awareness). While these capacities are not yet exercisable, they exist in radical (root) form. Every human being is, and always is, a human person because every human being shares in human nature.

If human personhood is accorded because of the root capacities that are present at the formation of the distinct human embryo, then the rest of that human’s development also falls under the protections afforded to human persons. That means that disabled or injured humans are also persons worthy of dignity and respect. Humans with an extra chromosome 21 (Down Syndrome) are equal in status to humans whose brains or neural tubes do not fully develop (anencephaly, spina bifida), and equal in status to humans who develop normally before and after birth. These human persons are equal in status to humans nearing the end of life, who may have lost some of the capacities they once had—people who suffer from dementia, or can no longer feed themselves, for instance.

The Marriage of Science and Philosophy

The beauty of this marriage of science and philosophy is that it values the dignity of all human life from its natural beginning to its natural end. We show no partiality to “healthy” or “wanted” humans. We do not give preference to those who charm us with their eloquence or dazzle us with their physical beauty, nor do we discard those who cannot or will not provide material benefits to us.

Today, as we march, we march for the disabled, the elderly, the broken, and the vulnerable among us. We march for all women, and with special compassion for the women who have been violated, manipulated, raped, and abused in heart-wrenching ways. We march for the fathers in our lives and for the fathers who didn’t get a right to choose. We march for the men and women we have lost, and for the change we want to see in our world. We march with hope for the future, with love in our hearts, and with the knowledge that science helps us to understand ourselves as unique human beings from our conception on.

We march for life.