The cervical mucus plug inhibits, but does not block, the passage of ascending bacteria from the vagina during pregnancy.

OBJECTIVE: To evaluate the microbial load and the inflammatory response in the distal and proximal parts of the cervical mucus plug.

DESIGN: Experimental research.

POPULATION: Twenty women with a normal, singleton pregnancy.

SAMPLE: Vaginal swabs and specimens from the distal and proximal parts of the cervical mucus plug.

METHODS: Immunohistochemistry, enzyme-linked immunosorbent assay, quantitative polymerase chain reaction and histology.

RESULTS: The total bacterial load (16S rDNA) was significantly lower in the cervical mucus plug compared with the vagina (p = 0.001). Among women harboring Ureaplasma parvum, the median genome equivalents/g were 1574 (interquartile range 2526) in the proximal part, 657 (interquartile range 1620) in the distal part and 60,240 (interquartile range 96,386) in the vagina. Histological examinations and quantitative polymerase chain reaction revealed considerable amounts of lactobacilli and inflammatory cells in both parts of the cervical mucus plug. The matrix metalloproteinase-8 concentration was decreased in the proximal part of the plug compared with the distal part (p = 0.08).

CONCLUSION: The cervical mucus plug inhibits, but does not block, the passage of Ureaplasma parvum during its ascending route from the vagina through the cervical canal.