Don't inconvenience me! Kill all retards!
/sarc.
The fact is if you're baby is developed enough for the doctor to detect mental retardation, the idea that "it's just a clump of cells" flies in the face of reality. Nevermind the number of diagnoses they get wrong, aborting perfectly healthy babies.
Calling it a fetus doesn't mean you haven't a) potentially damaged your girlfriend/spouse/streetwalker's womb permanently, b) almost certainly damaged her psychologically, c) snuffed out a human heartbeat.
Now if you're all fine with that, then on a selfish, personal note I'm glad you're disposing of your would-be progeny. If you aren't going to man up and realize that blowing your top inside a woman has actual, permanent consequences, it's probably better that you didn't have children.
That being said, abortion is a moral atrocity whose backers insist that it should be rare, and yet we all know that for some abortion is the birth control of first resort. Irresponsible men love abortion; what better way to cop out of child support than pushing the girl towards abortion. It's like soft-resetting irl!
Except it isn't. There is nothing about retardation which makes anyone less human, and if you want to know who the modern day heroic mothers and fathers are, they are the mothers and fathers who are told by a doctor their baby won't be 6'4, have blond hair, blue eyes, and speak perfect German, and they still keep him or her and raise them to be proper adults.
A girl getting pregnant because you screwed her isn't an "accidental" pregnancy. It was perfectly predictable and arguing otherwise is the height of selfish denial. You play, you pay. So says the procreative history of homo sapiens.