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The metabolic view: There is no one point when life begins. The sperm cell and egg cell are as alive as any other organism.
The genetic view: A new individual is created at fertilization. This is when the genes from the two parents combine to form an individual with unique properties.
The embryological view: In humans, identical twinning can occur as late as day 12 PC. Such twinning produces two individuals with different lives. Even conjoined ("Siamese") twins can have different personalities. Thus, a single individuality is not fixed earlier than day 12. (In religious terms, the two individuals have different souls). Some medical texts consider the stages before this time as "pre-embryonic." This view is expressed by scientists such as Renfree (1982) and Grobstein (1988) and has been endorsed theologically by Ford (1988), Shannon and Wolter (1990), and McCormick (1991), among others. (Such a view would allow contraception, "morning-after" pills, and contragestational agents, but not abortion after two weeks.)
The neurological view: Our society has defined death as the loss of the cerebral EEG (electroencephalogram) pattern. Conversely, some scientists have thought that the acquisition of the human EEG (at about 27 weeks) should be defined as when a human life begins. This view has been put forth most concretely by Morowitz and Trefil (1992). (This view and the ones following would allow mid-trimester abortions).
The ecological/technological view: This view sees human life as beginning when it can exist separately from its maternal biological environment. The natural limit of viability occurs when the lungs mature, but technological advances can now enable a premature infant to survive at about 25 weeks gestation. (This is the view currently operating in many states. Once a fetus can be potentially independent, it cannot be aborted.)
The immunological view: This view sees human life as beginning when the organism recognizes the distinction between self and non-self. In humans, this occurs around the time of birth.
The integrated physiological view: This view sees human life as beginning when an individual has become independent of the mother and has its own functioning circulatory system, alimentary system, and respiratory system. This is the traditional birthday when the baby is born into the world and the umbilical cord is cut.