The first counterargument I usually hear (or give) is that patients on respirators or people using dialysis or the like are not somehow less of people–they don’t lose personhood due to their physical dependence.
Additionally, “personhood” should be an intrinsic property of a human, shouldn’t it? It doesn’t make sense–and sounds a bit eerie–to suggest that your personhood depends on factors aside from you. But viability is that type of quality. The Department of Perinatology and Gynecology at the University School of Medical Sciences in Poznan, Poland explained it this way:
Viability exists as a function of biomedical and technological capacities, which are different in different parts of the world. As a consequence, there is, at the present time, no worldwide, uniform gestational age that defines viability. Viability is not an intrinsic property of the fetus because viability should be understood in terms of both biological and technological factors.
In other words, a fetus gestating in the United States, whose mother has access to advanced medical technologies, may be able to survive outside the womb at an earlier gestational age than if the exact same fetus were gestating in Rwanda. If personhood depends on viability, the fetus is a person while in the US, and ceases to be a person if the mother travels to Rwanda. ….This makes no sense. No definition of personhood should boil down to geographic location.