The mucous plug begins to form at approximately seven weeks gestation, according to Pregnancy Corner. The plug is an accumulation of mucous that WebMD describes as resembling a big glob of slime. It forms a tight seal, like a cork in a wine bottle, at the mouth of the cervix to protect the fetus from bacteria. It is routinely expelled before the baby is delivered.
The mucous plug is expelled when a woman's body begins to prepare for the birth of her baby, according to WebMD. The plug may be a brownish blob that appears in the toilet or on the woman's undergarment. The appearance of the mucous plug doesn't necessarily mean that labor is imminent; it's just one sign that the body is preparing itself for the birth. The plug is flushed out when the opening of the cervical canal begins to widen in anticipation of the birthing process.
Actual labor is signalled when a woman's water breaks, which is a rupture of the fluid-filled amniotic sac that has protected and cushioned the baby. Unlike the expulsion of the mucous plug, the breaking of the amniotic sac is considered a clear signal that birth is imminent and the woman should go to the hospital or birthing center, according to the Mayo Clinic.