Uy, ang mucus plug is actually a good sign na something's happening down there, your body's preparing na talaga! A few of us moms went through the same, one friend even had hers for a few days before actual labor kicked in, so walang exact timeline talaga. Since you're almost 39 weeks na, best talaga to keep in touch with your OB para ma-monitor kung when you should come in, especially if may changes like regular contractions, bleeding, or water breaking, don't wait lang if something feels off kasi they'll know better kung ready na ang baby mo!