what does the term (-2) mean?

First time mom here, yesterday at my appointment I was checked to see my progress. After being checked my midwife (who is a very vauge person) she mumbled out 50% and said I'm at -2. When I went to ask her what she meant she had already left the room and moved onto her next patient. I'm assuming she meant 50% effaced but I have no idea what -2 means. Can anyone tell me what it means?