Those who defend the gradualist thesis, such as Daniel Callahan and Robert Wennberg, argue that the unborn entity increases in value as it develops physically. Unlike the theories critiqued above, in this view there is no one decisive moment at which the unborn entity moves from nonperson to person. For example, the one-celled zygote has less value than the three-month fetus while the three-month fetus has a lesser right-to-life than the eight-month fetus.
There have been a number of critiques of this position which space does not permit me to articulate here. 
However, our critique of the major decisive-moment theories is sufficient to refute gradualism. That is to say, since none of the decisive moments we have already gone over can be shown to eradicate the full humanness of the unborn entity at any stage of her development, it follows that there are no philosophical, scientific, or moral grounds by which to say that the unborn gradually becomes fully human. For she would still need to achieve full humanness at some decisive moment. That is, someone who is fully human cannot gradually become more fully human.
Certainly it is true that the unborn human physically develops gradually, as is true of humans at later stages (e.g., infancy, childhood, adolescence). But it does not follow from this fact that the unborn human is any less human than the infant, the child, or the adolescent. They are nonetheless fully human although they are gradually developing.
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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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