A cervical mucus plug is a plug that fills and seals the cervical canal during pregnancy. It is formed by a small amount of cervical mucus.
The mucus plug acts as a protective barrier by deterring the passage of bacteria into the uterus.
Normally during human pregnancy, the mucus is cloudy, clear, thick, and sticky. Toward the end of the pregnancy, when the cervix thins, some blood is released into the cervix which causes the mucus to become bloody. As the woman gets closer to labor, the mucus plug discharges as the cervix begins to dilate. The plug may come out as a plug, a lump, or simply as increased vaginal discharge over several days.