I was already in labor when I lost it. However I know some people who continued to lose it for weeks before they began labor...it regenerates. Not a very good indicator of labor onset.
I lost mine about a week or two before my planned c-section... Although I went into labor an two hours before my surgery and my baby about an hour early
Re: ? from 3rd trimester
I lost my MP after my water broke, lol.
It doesn't always happen first.