In a normal pregnancy, the mucus plug acts as a barrier at the entrance of the uterus to protect the fetus. If no complication occurs, it remains there until labor. When labor begins, the pregnant woman starts losing the mucus plug slowly.
If you start losing your mucus plug before the 37th week of pregnancy, you should visit your doctor as soon as possible, as it is a common symptoms of preterm labor.
If your water breaks and you lose the mucus plug at the same time, it's likely that labor is commencing. The difference between them is easy: while the mucus plug is often thick and contains blood, amniotic fluid is clear and odorless.