Babies are viable when they can survive with help from any adult. Before some fetal age, they are completely dependent on the body of the mother - if they are removed from it, no matter how gently, they will die (perhaps there is some way to make a fetal transplant, but that wouldn't necessarily change the equation so much).
Once past some age (and especially after birth), they just need someone to take care of them, it doesn't have to be their mother.
Wouldn't it then make more sense to require the mother to take care for the fetus/baby before the age of viability (as she's the only one who can), not after that (and/or after birth) when anyone can? Right now, it's the reverse.
Now, the viability standard is important because it is very hard to consider a non-viable entity "a person" in any serious sense. If it can't even perform the basic functions of its organism, how can it be considered to have rights, and has never been able to do so, how can it be considered a person?
What I am sayin here is that if there was a process by which we could take an unwanted fetus and implant it into someone else / allow that child to exist outside of the physical body of a person that doesn't want it, then sure, that is possibly a better alternative to abortion. Short of that, people should not be forcing other people to give births against their will.
I never understood this argument. There is no baby that is "viable" (able to survive on their own) (well, to be honest, most adults aren't either), and parents are required to care for them (and criminally liable if they fail to do so).