For any given couple, the risk of having a disabled child is 2%.

So even if you got Serena Williams and Michael Phelps together 15 years ago, you're still looking at only about 98% certainty of a healthy child.

If a couple has already had a disabled child together then would you consider it child endangerment for them to deliberately have another natural child? Should this couple adopt their next child?

Personally I do consider it to be child endangerment. I don't even think that they should get a surrogate mother unless they are are certain from DNA testing that the the condition came from the mother's genes. Even in these cases I would be a bit iffy about re-using sperm that contributed to a disabled child in the past.

They should adopt.