I think 1 in 20 is a bit too high. 1 in 100 maybe to have a significant impact on your life. There are lots of tests now that so you have the chance to abort.

16% of the world population has a significant disability according to the WHO. Most of them are not born with it: age related (dementia, blindness,..), serious work accidents, lasting effects of diseases,...

Loads of them can get treated or won't alter life as much in the western world.

If your family history has no cases of mental illnesses, chances are very low to get them.

For a serious learning disability (not able to attend normal school), it's probably 1 in 20. Most of these kids will still live a fairly normal life in the western world.

Also, chances of disabilities also increase with the age of the parents. Best to have your kids between 20-35 to keep the odds down. Above 40 is a lot more dangerous than under 30.