Secular option is "when the brain is made you can say it is a life". That is the standard used for most pro-abortion laws and 99% of the abortions happens before that
First, this example you are saying is RIDICULOUSLY rare. Like "Every time the Halley comet appears in the sky at the same time of a 2d Metroid release" rare.
(21 weeks being a little bit more than 4 months... you are saying more than the double of that)
The BIG POINT of an abortion is not suffer the months of problems related to the pregnancy for a kid you don't want.
THAT BEING SAD, working on your crazy terms :
I am 100% in favor of "mother's body, mother's choice".
If was in the USA, I would say that "as soon as the baby don't need the womb" it is probably easier to the mother agree to deliver and send to adoption .... but in my country I am perfectly fine saying that killing the baby is perfectly fine alternative to what is in this kid's future