When you select "YES" on a leaf node in the Prenatal & Perinatal section of a patient record (ie. a term that has no more sub-terms), a duplicate of that node shows up below the selection tree.
If you change the status from YES or NO to NA on the original node, there is no status change on the duplicate node and the document will still save that term as a YES or NO (rather than a status of NA.)
There is an 'X' that shows up to the left of the duplicate node when you mouse over it. Clicking on it deletes the duplicate node and sets the status for that term as NA.