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External os of the cervix

The external os of the cervix is the opening of the cervical canal into the vagina. It represents the distal end of the cervix and provides the communication between the endocervical canal and the vaginal cavity. Its shape and appearance vary depending on parity, hormonal status, and pathology. The external os is an important landmark in gynecological examination, obstetrics, and cervical cancer screening.

Synonyms

  • Cervical external os

  • Vaginal opening of the cervix

  • Cervical mouth (clinical usage)

Location and Structure

  • Located at the inferior end of the cervix, projecting into the vaginal lumen

  • Marks the junction of the endocervical canal with the vaginal cavity

  • In nulliparous women: appears as a small, circular opening

  • In multiparous women: appears as a transverse slit due to childbirth changes

  • Surrounded by columnar epithelium of the endocervix internally and squamous epithelium of the ectocervix externally, forming the squamocolumnar junction (important in pathology and cervical screening)

Relations

  • Anteriorly: Anterior fornix of the vagina

  • Posteriorly: Posterior fornix of the vagina

  • Laterally: Vaginal walls

  • Superiorly: Cervical canal and internal os

Function

  • Serves as the exit of cervical secretions into the vaginal cavity

  • Functions as part of the barrier system of the cervix against ascending infection

  • Changes shape and patency during menstrual cycle, pregnancy, and labor

  • Provides an anatomical site for cytology sampling (Pap smear) and colposcopy

Clinical Significance

  • Changes in shape help determine parity and cervical status

  • Dilatation of the external os is key in labor progression

  • Site of common pathologies: cervical ectropion, cervical polyps, carcinoma of the cervix

  • Inspected during routine gynecologic exam, colposcopy, and hysteroscopy

  • Imaging is crucial for assessing cervical cancer extension and os involvement

MRI Appearance

T1-weighted images:

  • Cervical stroma around the external os appears as low-to-intermediate signal intensity

  • Mucus in the canal may appear intermediate-to-high signal

T2-weighted images:

  • External os and cervical canal contents show high signal intensity (mucus and fluid)

  • Cervical stroma remains low signal, providing contrast

  • Pathology (inflammation, tumor) shows bright or heterogeneous signal

STIR (Short Tau Inversion Recovery):

  • External os region appears dark

  • Inflammatory or neoplastic lesions show bright hyperintensity

T1 Fat-Sat Post-Contrast:

  • Normal cervical os enhances mildly and homogeneously

  • Cervical carcinoma at the os demonstrates irregular, heterogeneous enhancement

  • Infected or inflamed lesions may show diffuse or rim enhancement

CT Appearance

Non-Contrast CT:

  • External os not well delineated separately; cervix seen as soft tissue density within pelvis

  • Tumor or bulky pathology may enlarge or distort the external os region

Post-Contrast CT:

  • Cervical tissue enhances homogeneously under normal conditions

  • Malignancy appears as irregular or heterogeneous enhancement at or around the external os

  • Abscess or necrosis shows rim enhancement with central low density

MRI image

External os of the cervix MRI AXIAL anatomy image-img-00000-00000_00001

MRI image

External os of the cervix MRI sagittal anatomy image-img-00000-00000

CT image

External os of the cervix ct sagittal anatomy image-img-00000-00000