The mucus plug is the small amount of mucus that has sealed the cervix for nine months during pregnancy. It has been a barrier during the pregnancy to make sure that no bacteria is able to enter the uterus.
As labor approaches, the cervix thins and dilates, and the plug isn't large enough to fill the space anymore. The plug may come out all at once, or it may come out slowly in the vaginal discharge over a couple of days.
Losing the mucus plug is a sign that your cervix is dilating and your body is starting to prepare for labor. You may lose it as early as one to two weeks before the signs of true labor appear or just as labor itself is beginning.
A mucus plug may be clear, slightly pink, or may be tinged with blood. The plug may consist of stringy mucus, or it may be sticky discharge. Many women don't even notice when they lose it because there is already so much vaginal discharge during pregnancy and especially during the time leading up to labor and delivery.