In this American society we have the privilage of great medical advances, good hospitals, sanitary conditions, etc.
When I hear about viability as an argument for determining when a fetus becomes a person, I have to ask why that fetus can be a person at 22 weeks today becasue of our advances in preemie care, but that fetus would not be a person 100 years ago when we could not support such an undeveloped infant.
The argument for viability says that personhood is only a consideration for the fetus at the earliest stage at which we have seen preemies survive. Prior to that point the fetus is just a blob of cells. Well as a current mature blob of cells I can attest to the fact that I am a person.
The idea of viability tells us nothing about the fetus itself. Viability is not dependant on the fetus. It is dependant upon the conditions in which the mother and child find themselves in. If the mother is pregnant in a third world country where medical treatment is rudimentary, viability is found at a much later stage of development than for the mother who is pregnant in an industrialized country where medical care can better care for a preemie. Viability even changes for the fetus from hospital to hospital. Differences in pediatric staffing, equipment, cleanliness, can all have a drastic effect on viability.
So viability depends on quality medical care, advancement in medical technology, the mother's care of her body during pregnancy, but not one bit of viability has anything to do with the fetus itself.
As an additional note, an infant is not anymore autonomous than the unborn. If you leave an infant alone to fend for itself, it will die.