Read on another thread here about the fact that it seems only to be in UK USA that children in 'one child families' are labelled an 'only child'.
I sometimes find myself, when people say, 'is he your only child' trying to justify myself only having one child, by saying ' Yes, I started late.. too old to have another etc etc.' Apparently, in other countries the 'only child' label doesnt exist. Similar to the fact that, in France, there is no word for 'bullying' as it is not something that they experience over there (heard that on Radio 4 today - explained by a French woman).
The more I think about it, the more I feel that the expression 'only child' has negative connotations about it - and I feel it should be banned!! JMO.