I really feel you on this, the final stretch is such an anxious time and every little change makes you wonder. At 39 weeks your body is doing so much to prepare, and some discharge changes can be normal, but since you are so close to your due date and the colour is a bit unusual, it would be good to call your gynae or head to the hospital just to get it checked out and have peace of mind. Better to be safe, especially this late in the pregnancy, and they can tell you right away if everything is okay.