Is it True that “no one knows when life begins”?
It is often claimed by abortion-rights advocates that “no one knows when life begins.” In reality, no one who knows anything about prenatal development seriously doubts that individual biological human life is present from conception.
What the abortion-rights advocates probably mean when they say that “no one knows when life begins” is that no one knows when full humanness is attained in the process of human development by the individual in the womb. Thus, from a legal perspective they are arguing: since no one knows when full humanness is attained, abortion should remain legal. I believe, however, that there are at least four problems with this argument.
It is a two-edged sword. If no one knows when full humanness is attained, then we cannot prevent a Satan-worshipping neighbor, who believes that full humanness begins at the age of two, from sacrificing his one-and-a-half-year-old son to the unholy one. After all, who knows when life begins?
If it is true that we don't know when full humanness begins, this is an excellent reason not to kill the unborn, since we may be killing a human entity who has a full right to life. If game hunters shot at rustling bushes with this same philosophical mind-set, the National Rifle Association's membership would become severely depleted. Ignorance of a being's status is certainly not justification for killing it.
As the biological facts of prenatal development indicate, we have excellent reason to believe that full humanness is present from the moment of conception, and that the nature of prenatal and postuterine existence is merely the unfolding of human growth and development which does not cease until death. In other words, the unborn—like the rest of us—are not potential human beings, but human beings with much potential.
By permitting abortion for virtually any reason during the entire nine months of pregnancy, abortion-rights advocates have decided, for all practical purposes, when full humanness is attained. They have decided that this moment occurs at birth, although some of them—such as Peter Singer and Michael Tooley—also advocate infanticide. 
The very abortion-rights advocates who claim that “no one knows when life begins” often act as if protectable human life begins at birth. Since actions speak louder than words, these “pro-choicers” are not telling the truth when they claim they "don't know when life begins."
See Michael Tooley, Abortion and Infanticide (Oxford: Clarendon Press, 1983); and Peter Singer and Helga Kuhse, “On Letting Handicapped Infants Die,” in The Right Thing to Do, James Rachels, editor (New York: Random House, 1989). [up]
Author: Francis J. Beckwith, adapted from a series in Christian Research Journal, Spring 1991. Provided with permission by Summit Ministries and the author.
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For further reading on abortion issues
Francis J. Beckwith, Politically Correct Death: Answering the Arguments for Abortion Rights (Grand Rapids, Michigan: Baker Book House, 1993).
Francis J. Beckwith, Abortion and the Sanctity of Human Life (Joplin, Missouri: College Press, 2000).
Stephen Schwarz, The Moral Question of Abortion (Loyola University Press, 1990).
Randy Alcorn, Prolife Answers to Prochoice Arguments (Sisters, Oregon: Multnomah Press, 2000).