how can they possibly diagnose mental illness in the womb ?
People are lonely. The general population is by and large self-centered. There is no sense of community. Daily life involves slaving for someone else doing unfulfilling work that nobody would perform if they had a choice other than "earning a paycheque".
The worst part is that the majority of people who discuss mental illness publicly are not themselves affected by this kind of debilitating sense of there being no meaning to life. And no, having a "family member" or "close friend" who suffers does not make one qualified to talk on their behalf. It's disgusting how self-centered people are, discussing the "mental illness" of their friends or family - the claim is always "because we care" or "we can help them", but the true motivations are entirely selfish.
If so, what makes you think this is the case?
The world has changed and sped up and kids who don't cope with life these days get all kinds of labels. For all we know, they would be fine if they were growing up on a quiet farm with a more organic pace of life and disconnected from our many gadgets and what not -- as was the norm for everyone on the planet not too many decades ago.
If you look at history, each era has some widespread "mental health" issue. Modern peoples consider many of these labels bogus. For example, at one time women were believed to suffer a condition known as "Hysteria" which was treatable with medically induced hysterical paroxysms -- aka orgasms: https://www.psychologytoday.com/blog/all-about-sex/201303/hy...
http://www.huffingtonpost.com/2013/11/21/female-hysteria_n_4...
http://www.motherjones.com/media/2012/05/hysteria-sex-toy-hi...
You would be well advised to be skeptical of extremely widespread cases of "mental disease" found to be prevalent in a particular generation during a particular time span. In another generation, they are likely to have a different explanation and ADHD may well be a term that only occurs in history books and articles mocking the concept.
I do know of these cases, of course, but the field of mental illness is very much different today than what it was in the days of "Hyesteria" - both with regards to diagnostics, and to research. For instance, there are well known structural and functional abnormalities associated with certain mental illnesses - which would seem to imply that the current categorizations aren't completely off.
While you may be right in the technical sense that future research can make our current understanding obsolete, I don't think there's any doubt that some people suffer as a consequence of what is currently conceptualised as a behavioural disorder called ADHD.