I would assume there is one Birth record for each birth.
If so, then your expression is only accessing the data in the first related record. To count the values in multiple records would require a different approach.
One option is: PatternCount ( List (Birth 2::Birth outcome ) ; "CS" )
but please note that such an expression will count the "CS" in CS, ERCS and RCS so you may need to rethink how you are counting these.
As usual Phil, you come through. This works perfectly, because I don't need a count of each one, I need a count of the total times any of those three appears.