I think one should be very careful about any term which implies that a child is an accident or unwanted in any way. Even if this might in fact be true.
Just imagine the situation from the child's ( and then the adult he quickly becomes) point of view.
I was born when my 20 year old parents had been married 18 months- so nothing obviously untoward about my arrival. But one small comment from them about my birth coming unexpectedly and too soon from a financial point of view (ie they were extemely poor at the time), has coloured my entire relationship with them.
No child should have to think of themselves as being unwelcome or unwanted.