A cervical mucus plug (operculum) is a plug that fills and seals the

cervical canal The cervical canal is the spindle-shaped, flattened canal of the cervix, the neck of the uterus. Anatomy The cervical canal communicates with the uterine cavity via the internal orifice of the uterus (or internal os) and with the vagina via the ...

during pregnancy. It is formed by a small amount of cervical

mucus Mucus ( ) is a slippery aqueous secretion produced by, and covering, mucous membranes. It is typically produced from cells found in mucous glands, although it may also originate from mixed glands, which contain both serous and mucous cells. It is ...

. The mucus plug acts as a protective barrier by deterring the passage of

bacteria Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria were among ...

into the

uterus The uterus (from Latin ''uterus'', plural ''uteri'') or womb () is the organ in the reproductive system of most female mammals, including humans that accommodates the embryonic and fetal development of one or more embryos until birth. The uter ...

, and contains a variety of antimicrobial agents, including

immunoglobulin An antibody (Ab), also known as an immunoglobulin (Ig), is a large, Y-shaped protein used by the immune system to identify and neutralize foreign objects such as pathogenic bacteria and viruses. The antibody recognizes a unique molecule of the ...

s, and similar

antimicrobial peptides Antimicrobial peptides (AMPs), also called host defence peptides (HDPs) are part of the innate immune response found among all classes of life. Fundamental differences exist between prokaryotic and eukaryotic cells that may represent targets for a ...

to those found in

nasal mucus The nasal mucosa lines the nasal cavity. It is part of the respiratory mucosa, the mucous membrane lining the respiratory tract. The nasal mucosa is intimately adherent to the periosteum or perichondrium of the nasal conchae. It is continuous wit ...

. Normally during

pregnancy Pregnancy is the time during which one or more offspring develops ( gestates) inside a woman's uterus (womb). A multiple pregnancy involves more than one offspring, such as with twins. Pregnancy usually occurs by sexual intercourse, but ca ...

, the mucus can be described as cloudy, clear, thick, salty and sticky, although

viscoelasticity In materials science and continuum mechanics, viscoelasticity is the property of materials that exhibit both viscous and elastic characteristics when undergoing deformation. Viscous materials, like water, resist shear flow and strain linearly wi ...

may be the most appropriate term to describe it. 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 pregnant 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 Vaginal discharge is a mixture of liquid, cells, and bacteria that lubricate and protect the vagina. This mixture is constantly produced by the cells of the vagina and cervix, and it exits the body through the vaginal opening. The composition, amou ...

over several days. The mucus may be tinged with brown, pink, or red blood, which is why the event is sometimes referred to as "bloody show". Loss of the mucus plug does not necessarily mean that delivery or labor is imminent. Having intercourse or a

vaginal examination The Papanicolaou test (abbreviated as Pap test, also known as Pap smear (AE), cervical smear (BE), cervical screening (BE), or smear test (BE)) is a method of cervical screening used to detect potentially precancerous and cancerous processes in ...

can also disturb the mucus plug and cause a pregnant woman to see some blood-tinged discharge, even when labor does not begin over the next few days.Signs of labor - BabyCenter
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References

Obstetrics Midwifery {{anatomy-stub