The pro-life position rests on the following premises:
1. The fetus is a human being.
2. There is no consistent, objective distinction between "person" and "human being."
3. Human beings possess human rights.
4. Bodily integrity is not sufficient to justify most abortions.
A "human being" is a member of the species homo sapiens. While there is considerable debate over when a human organism becomes a "person," there is not much debate over when a human organism begins biologically: "Biologically speaking, fertilization (or conception) is the beginning of human development. Fertilization normally occurs within several hours of ovulation (some authors report up to 24 hours) when a man’s sperm, or spermatozoon, combines with a woman’s egg, or secondary oocyte, inside a woman’s uterine tube (usually in the outer third of the uterine tube called the ampulla)."
Many pro-choicers concede that unborn children are human beings, but deny that the fetus is a "person" deserving of full human rights. Their views of what else is necessary to achieve personhood vary widely. Some of the more common positions are that to be a "person," a human being must also:
Have a heartbeat (which begins at 3-4 weeks gestation)
Produce brain waves (which begins at 6-7 weeks gestation)
Pro-lifers find these personhood restrictions arbitrary and inconsistent. Many of the proposed criteria would, if applied consistently, deny the personhood of newborns, people with disabilities, and other vulnerable groups. For more on the practical problems of separating "person" from "human being," see the "Related Articles".
Suppose it was universally acknowledged that the fetus is a human being and agreed that there is no consistent, meaningful distinction between a "human being" and a "person." These two premises are still not sufficient to hold a pro-life perspective. Pro-lifers insist that abortion kills a human being and that killing human beings is (generally) wrong.
The general wrongness of killing people is, thankfully, a near-universal moral judgment (sociopaths aside). The right to life is widely acknowledged by secular governments, human rights documents, and countless religious and philosophical traditions.
You do not have to be religious to value human life. You do not have to be religious to see the humanity of the fetus. And you do not have to be religious to be pro-life.
Some pro-choicers sidestep the above premises entirely by basing their position solely on a strict adherence to the value of bodily integrity or autonomy. We agree that bodily autonomy is an important value. However, it is not absolute.
The overwhelming majority of pregnancies occur as a result of consensual sex. It is unjust to deny a pre-born child's right to life in favor of a bodily autonomy right that could have been vindicated earlier, and without violence, through the practice of abstinence or contraception.
Some of this information came from our partner organization, Secular Pro-life.