There will always be those whose beliefs, religious or otherwise, say to them that life begins at conception. They therefore see that there are two competing rights - that of the mum, that of the baby. But that's a slightly different debate to this one.
Let me see if I can simplify it.
Women can choose to have abortions, and any colour of baby could be aborted, yes?
Suppose it is decided that babies who are black are going to have a worse quality of life due to bullying from society/are seen as less valuable to society/generally earn less/some other weird and unfathomable reason.
Is it therefore ok to abort all the babies who are black because after all, no baby has an automatic quality to life?
If your answer is no, of course not, then how can that same argument apply to those with a disability?
Scenario Two:
You are a baby in the womb who is 30 weeks old. Your parents have discovered you have six fingers rather than five. They have you aborted, even though you had a chance of life at that stage. Nothing wrong with your quality of life, your economic chances. And yes, we can do this, right up to full term if the baby has a disability. And we sometimes do.
OK, say you are that exact same baby who is 24 weeks old but has the 'right' number of fingers. Say you are born very early that week. Society spends £100,000 keeping you alive and you grow up to be a loved and valued member of society.
One finger. That's all it took for you to go from a bit of refuse to be disposed of/left to die because it's faulty, to a person worthy of huge resources and worthy of love.
Explain that to the baby in question.
If we're debating ethics, we might want to take time to consider where our ethics are right now.