namessurnames
My partner has changed her name in the past, for reasons not related to marriage, so I was wondering whether her maiden name would be considered her name at birth, or simply her pre-marital name?
For example:
Which is considered the maiden name? Lady A or Lady B?
It is proper to refer her as Ms Smith (for example) regardless of her martial status. Since the woman in question kept her maiden name, I would opt for this title or use the appropriate occupational title (e.g. Doctor Smith, Professor Smith, Major Smith, etc.)
Miss is a title for an unmarried woman, and is not acceptable.
If the person writes under a "false name" then pseudonym is the answer.
If the person has chosen to call him or herself with one name only, such as Adele, Prince, Pelé or Sting, then the term is mononym. For example, “Her mononym is Shakira.”
A single name by which a person, thing etc is known. For example, Madonna (the pop musical artist)
Talking of Prince, when he changed his name to the so-called ‘love’ symbol 
, for a number of years he was known as "The Artist Formerly Known as Prince", "TAFKAP", or just "The Artist". Likewise anyone could rename him or herself and say:
I'm [new first name], formerly known as [old first name].
OR
I'm [new first name], but I used to be called [old first name].
If a person dislikes their first name (for whatever reason), they can just call themselves with a different name. Sometimes a family nickname sticks, and that person grows up with that name. Changing a name by deed poll might include a lengthy process depending on one's nationality.
The most common use is a name change through a deed of change of name (often referred to simply as a deed poll). Deeds poll are used for this purpose in countries including the United Kingdom, Ireland, New Zealand, Hong Kong and Singapore. In the UK, a deed poll can also be used to change a child's name, as long as everyone with parental responsibility for the child consents to it and the child does not object to it. The child's parents execute the deed poll on the child's behalf. In some other jurisdictions, a person may simply start using a new name without any formal legal process.
Wikipedia
Oxford Dictionaries:
pseudonym
(noun)
a fictitious name, especially one used by an author.
"I wrote under the pseudonym of Evelyn Hervey"synonyms: pen name, assumed name, alias, professional name, sobriquet, stage name, nickname
Best Answer
If you want to be strict then Lady C's maiden name is 'A' going by the definition of maiden.
However, in this specific case the lady has changed her surname from A to B through choice, not marriage, possibly using a system similar to the UK Deed Poll. In the UK a clause given by the UK Deed Poll Service would apply which explicitly disallows the use of a previous registered name.
From UK Deed Poll Service
In that case Lady A would be forced to use Lady B as her maiden name, however it is rare that birth certificates are changed after a Deed Poll name registration so her birth surname would remain as A. (This restriction may vary from country to country depending on local laws and regulations.)
In a more general case...
Of course, any lady can choose what she wishes to write as her maiden name but the meanings and uses of the word maiden, suggest otherwise.
Unless otherwise noted all quotes are from OED1.
Unmarried here is the issue does it mean was married and now no longer is? ... the definition isn't entirely clear.
Maiden defn2. A virgin has spawned a couple of maiden ..... words
Fairly clear that maiden here means a virgin, but that doesn't stop Miss A marrying Mr B and still remaining a virgin (much to Mr B's dismay one would imagine and possibly the reason why Mr B might file for divorce making Mrs B unmarried again and free to tie the knot with the unwitting Mr C) but Miss A is now called Miss B so when she marries Mr C is her maiden name A or B?
More maidenish things could sway me towards her maiden name being A. These are some of the other definitions for Maiden that I skipped earlier on.
Excluding the recurring fruit mentions, defn. 5 seems pretty much the same as virgin to me - not been conquered, tried, worked, disturbed, ploughed all could easily sound a little like euphemistic terms.
But it is defn.6. that seals it. The first of it's kind and there are a number of Maiden .... firsts.
Quotes from Wikipedia
I think a combination of unmarried, virgin, and first pretty much marks maiden name down as the first surname a lady had.
Interestingly, defn.4 : That has yielded no results gives us
Wikipedia attribution:
Wikipedia contributors, "Maiden speech," Wikipedia, The Free Encyclopedia, http://en.wikipedia.org/w/index.php?title=Maiden_speech&oldid=626049739 (accessed September 27, 2014)
Wikipedia contributors, "Maiden voyage," Wikipedia, The Free Encyclopedia, http://en.wikipedia.org/w/index.php?title=Maiden_voyage&oldid=620824032 (accessed September 27, 2014).
Wikipedia contributors, "Maiden flight," Wikipedia, The Free Encyclopedia, http://en.wikipedia.org/w/index.php?title=Maiden_flight&oldid=619347591 (accessed September 27, 2014).