Mucus Plug
The cervical mucus plug forms early in pregnancy to create an impermeable and immunological barrier from potentially harmful bacteria located in the lower genital tract. It is expected that when formed properly, this plug can protect the amniotic cavity from infection.
It is known, however, that some bacteria can get through the plug, and an estimated 1 in 3 preterm deliveries are thought to occur due to infection that originated in the lower genital tract. Research is ongoing to determine the properties of the mucus plug in pregnant women to help identify a potential screening tool for preterm delivery risk.
The mucus plug falls out in some women sometime between 37 and 42 weeks of pregnancy, while others lose their plug after labor has already begun. While losing the plug prior to contractions is a good sign the cervix is ripening, labor could still be days or weeks away.
Background
Many women know the mucus plug as the “bloody show” that appears at the end of pregnancy as the body begins to prepare for labor. The visualization of this plug on tissue or a pantyliner gets most women excited that labor is near. While labor itself could still be days (or weeks) away, the plug does serve as proof the cervix is prepping for delivery by softening and ripening enough for the plug to fall out.
The mucus plug forms a physical and immunological barrier against bacteria present in the vagina, as any bacteria present must go through the cervix before infecting the uterus and/or amniotic cavity.
The increase in progesterone in early pregnancy changes the structure and composition of cervical mucus. It becomes thicker and more gelatinous, creating a plug of white blood cells – necessary for fighting infection.
The mucus from this plug continues to flow into and out of the vagina (increase in discharge) throughout pregnancy so the plug always contains "fresh" mucus to keep pathogens out of the uterus. An abnormal plug – through either length or consistency – increases the risk for infection and preterm birth.
Timing and Expulsion
A pregnant woman usually loses her mucus plug between 37 and 42 weeks, or during labor. The plug can be lost all at once or gradually; some women may never notice their plug fall out.
Near the end of pregnancy, estrogen increases while progesterone decreases (reversal from above); this causes the cervical collagen to degrade, resulting in softening and ripening of the cervix. The cervix then degrades slightly, and shortens in length and diameter (dilation and effacement). This causes the plug so fall/slip out.
Therefore, although the observation of the plug does not necessarily mean that labor is imminent (but it could be), it is a sign that cervical change has occurred enough for the plug to fall out, which does indicate progress.
Appearance
During ovulation, cervical mucus has a stretchy and stringy consistency (known as spinnbarkeit) to help sperm travel through the cervix and into the uterus. This mucus changes to a thicker, stickier, but somewhat still elastic consistency during pregnancy that fills the entire cervical canal.
The term “bloody show” fits the appearance of the plug because it is a large “glob” of jelly-like mucus that is much larger in amount than at any other point in menstruation or pregnancy and may be tinged with blood. This tinge comes from thin blood vessels in the cervix rupturing as it begins to ripen and soften. It can also have tints of yellow, green, pink, or brown. It has also been described as a “large, well-defined structure, of approximately 10 grams” (less than ½ an ounce).
Although a little blood is normal, if women notice the blood is more than just “streaks”, it could indicate a problem with the placenta and women should call their HCP immediately.
Preterm Delivery
All women have bacteria in the lower genital tract, but these microorganisms usually do not gain access to the amniotic cavity during normal, full-term pregnancies.
When bacteria in the lower genital tract ascends through the mucus plug and infects the placenta, amniotic fluid, and/or the fetus, it can lead to contractions, preterm premature rupture of membranes, preterm delivery, and even stillbirth. One of every three preterm infants is born to a woman who has an intra-amniotic infection.
Therefore, the cervical mucus plug inhibits but does not block all harmful pathogens from entering the cervix or fetal membranes in all pregnancies. It is not completely understood why some women develop an ascending infection while others do not.
This gap may be answered by studying the composition of the mucus plug in women who contract an amniotic infection; it is possible that alterations of the composition or formation of the cervical plug may affect the mucus’ ability to protect against ascending bacteria.
It is theorized that in women considered high risk for preterm birth, cervical mucus fails to develop into the thickened and impermeable state described above, which makes it easier for bacteria to travel to the amniotic cavity.
In a study of the examination of cervical mucus from women considered low-risk, the mucus was opaque and paste-like, while the mucus of women considered high-risk resembled egg whites, similar to cervical mucus during ovulation.
Harmful organisms that appear to be get through the plug include:
Treptococcus agalactiae (Group B Strep)
Escherichia coli (E. coli)
Mycoplasma hominis (kidney infection)
Ureaplasma urealyticum (urinary tract infection)
Just as cervical mucus is used to provide a visual clue for ovulation, it is assessed that cervical mucus can be used to potentially assess a woman’s risk of preterm delivery via infection. However, the composition of cervical mucus for the screening of preterm delivery is not studied often, and therefore more research is needed.
A short cervix may also be a risk factor for preterm delivery, not necessarily due to strength (see Insufficient Cervix) but because bacteria has a shorter distance to travel before reaching the amniotic cavity.
Action
Women should call their HCP immediately if:
They notice bleeding that is more than just “streaks”, as this could be a problem with the placenta.
They believe they are in preterm labor, due to the presence of bleeding or contractions.
Gaps
Can the cervical mucus of pregnant women potentially be examined in the late second trimester/early in the third trimester to determine those who may be at risk for preterm birth?
Resources
Click here to see an MRI of the mucus plug as well as a graphic (NCBI).