During pregnancy, the cervix is covered by a glob of mucus that is referred to as the mucus plug. As a result, it helps protect the baby by literally plugging the uterus. This helps prevent bacteria and other forms of infection from breaching the cervical barrier.
As the third trimester is nearing its end, the cervix begins to soften and dilate open. This opening causes the mucus plug to slowly release. It is a sign that the cervix is preparing itself but is not really an indicator that labor is soon to occur.
Further, this process may also result in the “bloody show.” For more information, see below.