I want to know why this joke is supposed to be hilarious [closed]

Solution 1:

The doctor is saying that the test shows a disability.

In many countries, there are regulations about parking spaces that are reserved for individuals with disabilities; you will usually be able to spot them, because there will be a stick figure in a wheelchair painted on the ground on them. Since they're reserved for individuals with disabilities, they will frequently be unused even when the rest of the parking lot is full.

So, when the doctor is saying "The good news is that your child will always find a parking space," he's using black humor, by calling attention to the silver lining of your child being able to find parking spaces easily, instead of the fact that your child will be born disabled.