Many scientists and experts, along with some members of the general population, utilize the gradualist thesis to determine when human life begins. Two major proponents of this theory are Daniel Callahan and Robert Wennberg. Francis Beckwith (1994) states that these two men, along with other defenders of the gradualist thesis, argue that "the unborn entity increases in value as it develops physically." This theory does not give a specific moment when the fetus makes the shift from nonperson to person. Instead, the one-celled zygote has less value than the three-month fetus, and the three-month fetus has less value and a lesser right-to-life than the eight-month fetus. The newborn baby has the greatest right to life.

    Josefina Figueira-McDonough (1990) observes that a consensus exists that the fetus is an emergent human being. Yet she raises the question, "When should the fetus be recognized as a person, a being who has the kinds of characteristics that are taken to be relevant to compel a recognition of human personhood?" Francis Beckwith (1994) agrees that there needs to be a moment when the preborn baby is seen to achieve humanness. He argues that once humanness is achieved, we do not continue to gradually become more human. He concludes by agreeing that from conception until death, humans develop gradually. However, he states that we do not gradually become more human.
 
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