I am not sure I really get the argument about viability. Is the reason the limit is set at viability because theoretically the baby could survive outside of the womb?
Obviously, the limit has to be set somewhere, but it seems a bit arbitrary, because (I realise I may get flamed for this, but as a feminist, it increasingly bothers me) if you terminate a pregnancy, you really do ensure that there is no hope of survival (so why argue about survival chances?).
I guess, and I am just trying to think it through, the argument is really whether a woman is seen to have the right to terminate a pregnancy, and (this is where viability comes in) how society decides what is morally acceptable should she choose to do so.
The argument would be that while a foetus is in the womb, it has the chance to develop into a fully formed baby. Whereas abortion to me seems an act of violence against that foetus. I don't see how the viability of the foetus makes any difference. As I said, I expect to be flamed for this, but as a feminist it really bothers me. One of the founding tenets of feminism was surely pacifism.
Surely the arguments should be about the reasons why women seek abortion, going beyond (and not necessarily negating) the fact that they should have a right to do so. How we, as a society, make child-raising and childcare easier to afford/balance with education, employment and other opportunities, reduce the stigma of being a single parent, don't penalise mothers in the workplace, ensure better access to contraception and that men use it (don't pressure women into sex), address the social acceptability of rape, see childrearing as a societal/male responsibility - and so on. It's really interesting to note that, in Soviet Russia, where women had more rights in the workplace and more equality, abortion rates were lower than countries where such things didn't exist.
To reduce the argument to the fact that the foetus would not survive outside the womb just seems to me really reductive and wrong.