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Some pro-choice biologists counter that human life does not begin until the fetus becomes viable, by which they mean sufficiently developed to survive outside the uterus. In 1973, when the Supreme Court gave women the legal right to have abortions up to the moment of viability, that age was placed at between 24 to 28 weeks. Since then the age at which a fetus is considered viable by medical experts has slowly dropped; doctors are now able to keep alive fetuses as young as 20 weeks and weighing 500 gm (1.1 Ibs.). Indeed, Dr. Norman Post of the Medical School of the University of Wisconsin-Madison believes that the day will eventually arrive when all fetuses can be kept alive—in the laboratory if not in a nursery.
But is any of this relevant? Some experts argue that it is futile to rely on biological data at all in trying to determine when life begins. "Most biological data can never be decisive," says Lisa Cahill, a Catholic and assistant professor of theology at Boston College. "Any particular biological line that might be drawn, such as implantation or viability, is relative to the individual fetus, and each fetus reaches each stage at a slightly different time." Yet even if every fetus developed at precisely the same rate, a consensus would never be reached on when human life begins. "The question is unresolvable," says Fost. "It's not a question that doctors or religious authorities can be helpful on because it's not certifiable. It is just a matter of individual opinion."