Its no one's fault.
Its just that medical science is till working on avoiding human defects and its not very far.
The complex human birth system, involving millions of matching processes may fault some times. Genetics, nourishment, food, environment or hundreds of other forces can effect an unborn child or his early growth. Medical sciences are working on it and they have learned to interfere with genetics and DNAs. There is enough evidence that many of the genetic mismatches will be removed in time. Knowledge is helping people to diminish the existence of genetic abnormalities in newborns and such. For example, in many US states (24), inter-family marriages are not allowed by law because their children could face difficulties due to their abnormalities.
At the same time, being specials is not an excuse, especially not in west. We have examples of many accomplished people in our society who have helped their communities immensely. Top of the line is one of the greatest scientist of our time, "Sir Stephen Hawkins," the British scientist, who gave the world the Big bang theory, has been confined to his wheelchair for most of his life yet has managed to amaze the world with his intelligence. (See Wikipedia for his achievements.)
On a personal note, Back home in Pakistan, I know two very bright young brother and sister, both blind by birth who achieved their Master's degree from Univ. of Karachi. They are now teaching in good institutions. This just shows that there are millions of successful people in the world who overcome their hurdles and in spite of problems, are great achievers.