I agree with sevenfold. If a disabled baby can be terminated up to birth then the same rule should apply for a non disabled baby.
Yes, some conditions are not detected until the 20 week scan.
There is another twenty weeks between that scan until the baby is born. It does not take another twenty weeks to determine the extent of a condition - usually conditions that are incompatible with life are abundantly clear, but the point here is that it is not just conditions that are incompatible with life that are eligible for termination up to birth - any disability is. Let's look at it in another way:
Let's say that a baby is diagnosed with downs following the amnio, but the twenty week scan shows there are no heart defects or other associated health problems. The mother is offered a termination but decides not to go through with one.
At 30 weeks she is again offered a termination but decides not to go through with it. Bearing in mind here that because the baby is said to have a severe disability, albeit it is not incompatible with life.
Then at 39 weeks the mother has a change of heart and decides she can't face the possibility of bringing a baby with downs into the world, So she goes to see her consultant and says that she has changed her mind and that she would in fact like a termination. And because the baby is severely disabled the consultant agrees and she is booked into hospital the next day.
Except that night she goes into labour at home, and it's a quick labour, and the baby is delivered at home before she can get to hospital. A healthy baby, albeit it has Downs.
Now, that baby was due to be killed the next day under the agreement that a baby with a disability can be terminated up to term.
so - who thinks the baby should be killed anyway due to the fact it has a severe disability? And if not, why not, given it was already viable at the point it was scheduled to be killed in utero?