What you're asking about is eugenics. People with downs syndrome can lead rewarding lives. Many people who work with those of us with downs say that they have the highest satisfaction with life of anyone out there. They do require a lot of costly and time consuming care in early life, but there is no reason a child born with downs can't have a life they find fulfilling.

Given that, I would say if a mother's only reason for aborting is the downs syndrome, that probably leans towards unethical. However if there are any other reasons a mother might choose to abort that fetus, it's ethical. But terminating a pregnancy because the child has an "undesirable" trait is both eugenics and ableism which rarely result in ethical decision making.