Saleem's parents have blood types A and O. Genetically Saleem should be one of those two blood types. He is neither, which proves that is not their biological son. Mary, much later, confesses to switching Saleem and Shiva at birth in order to give the poor baby a life of privilege.
The first change in Saleem's life as a result of the blood type proof is that his aunt and uncle pick him up from the hospital, and Saleem lives with them for a while. Saleem does eventually move back in with his parents but notices that his father, Ahmed, no longer treats him as the favorite child. Instead his attention is more focused on his younger sister. It is at about this time that Mary finally confesses her actions. Ahmed is already upset at the blood type evidence, but Mary's confession pushes him over the edge. He gets drunk and berates his wife, Amina. Due to that, Amina flees to Pakistan with her two kids.
Pakistan becomes a second home for Saleem, and it is there that he loses most of his family due to air raids, loses his memory, gets conscripted into the military, gets his memory back, has a child, and finally has his powers sterilized.