There is a little subjectivity to what makes a fetus viable. Not only does not everyone agree about what makes a person alive, but doctors do not always agree about weather or not a fetus will be able to survive once it is born. This is a complicated issue, and it is an emotional issue.
As a person who is pro-choice, I think that it is fine to set a standard gestation period for which a legal abortion must fall within. However, I think that doctor's and their patients should be protected in extenuating circumstances, allowing abortion outside of that timeframe in the event that the progression of the pregnancy will be harmful to the mother's life.
No, it is my belief that a universal decision cannot be made on when a fetus is considered viable for purposes of abortion laws. This is because everyone has their own opinions on whether or not abortion should even be legal, let alone making them decide on a time during pregnancy when abortion is still a viable option.
To determine fetal viability is the job of doctors expert in the field. It has to be decided on the spot, on a case-by-case basis, and many medical factors are involved. A special complicating factor is the fact that babies born early can sometimes be sustained only by the application of modern, expensive technology. These decisions should therefore not be subject to a universal decision.