This is a very complex situation.
Me personally.. i think god has given us technology.. and it's on us to use it wisely.
Living out a life with down syndrome or some other debilitating illness wouldn't be very nice.. not just for that person but for the whole family.
I suppose it all comes down to whether you believe the soul is in the body from conception.
Me personally.. i feel the soul could simply go into another body if the fetus is destroyed. The soul is energy.. and energy can't be destroyed only dissipated.
Perhaps science can one day prove this.. and we could finally know the true and ethical answer to this question..
Surely it's also solved if it turns out souls don't exist. Because then, there are no "pre-born" persons, and we only begin to exist as distinct individuals after we are born and accumulate life experiences that shape who we become.