Human Embryology and Teratology, 3rd ed.," from publisher Wiley-Liss, asserts that fertilization is the "critical landmark" when a new, genetically distinct human organism is formed. Yet, the text explains, "life is a continuous process" throughout the pregnancy.
As Harvard University Medical School professor Micheline Matthews-Ross testified before a 1981 U.S. Senate Judiciary Committee, "It is scientifically correct to say that an individual human life begins at conception … and that this developing human always is a member of our species in all stages of life" (New York Times, April 26, 1981).
In other words, Matthews-Ross was saying, a baby is a baby — from fertilization, to heartbeat, to birth. Yes, the baby of five weeks in the womb differs from the newborn, but so does the toddler differ from the teen. Scientifically, we pass through different stages as we grow, but we don't pass from person to non-person, or vice versa.
At that same 1981 government hearing, Dr. Watson A. Bowes of the University of Colorado Medical School asserted: "The beginning of a single human life is from a biological point of view a simple and straightforward matter — the beginning is conception. This straightforward biological fact should not be distorted to serve sociological, political or economic goals."