Interesting article about a couple suing their doctor for wrongful birth:
Rick and Danika VanVooren argued that their doctors were negligent in not informing them about the birth defects that would have prompted them to terminate the pregnancy. They sued in 2002.
"They assert in each cause of action that, but for the negligence of the defendants, they would have terminated the pregnancy," the Idaho Supreme Court wrote.
The court then cited a section of Idaho law that bars lawsuits that say but for act or omission of another person a pregnancy would have been aborted.
Unfortunately, the article doesn't elaborate on the legal concepts involved. I would have found a more detailed discussion useful.