Do you really believe that anyone who aborts a child under any circumstance would simply not get rid of the evidence? I can’t imagine that there would be follow up on a dead baby to make sure they were correct in their diagnosis.
There certainly would be if the mother asked for it. Many would, some just to reassure themselves that it was indeed not a chromosomally normal baby, and in some cases because if an anatomical abnormality had been detected, but no genetic testing had been done before the abortion, the anatomical abnormality might have been due to some other kind of genetic disorder that is (unlike Downs) hereditary. Most women would want to confirm before attempting another pregnancy that the abnormality was indeed just due to randomly-occurring Downs.
And of course, there’s no shortage of unethical people who are eager to find some way to sue a doctor and hit the jackpot. If a doctor had actually suggested and proceeded with an abortion based on suspected Downs, without chromosomal confirmation, that’s practically winning the lottery in the malpractice-suit-for-profit business, because there IS a huge chance that the fetus was not Downs affected, and an absolutely guaranteed payout if it wasn’t.