Mucus plug discharge, or passing the mucus plug, is an early indication of beginning labor, according to WebMD. The mucus plug collects at the opening of the cervix during pregnancy. As the cervix opens and prepares for delivery, the mucus plug releases and enters the vagina.
The mucus plug forms from a small amount of cervical mucus. It fills and seals the cervix during pregnancy and acts like a protective barrier against bacteria entering the uterus. When the mucus plug passes, it appears as a clear, pink or bloody discharge. Once the mucus plug is dispelled, labor may begin soon or as much as two weeks later, according to WebMD.