A mucus plug is a thick, sticky substance that forms in the cervix during pregnancy. It's made up of secretions from the cervical glands and is designed to protect the developing fetus by creating a barrier against bacteria and other potential pathogens. As the body approaches labor, the mucus plug will typically thin out and eventually be expelled during contractions or even earlier if it's dislodged before labor begins. Losing the mucus plug is often considered one of the first signs that labor is imminent.