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Internal os of the cervix

The internal os of the cervix is the proximal opening of the cervical canal into the uterine cavity. It forms the junction between the endocervical canal and the uterine endometrium. The internal os plays a crucial role in reproductive health, pregnancy maintenance, and gynecological imaging. Its patency and competence are critical for fertility, prevention of miscarriage, and safe labor progression.

Synonyms

  • Cervical internal os

  • Internal cervical opening

  • Uterine opening of cervix

Location and Structure

  • Situated at the upper end of the cervical canal

  • Lies between the endocervical canal and the uterine cavity (isthmus region)

  • Bordered superiorly by the endometrium and inferiorly by the cervical stroma

  • Composed of dense fibromuscular tissue with connective tissue support

Relations

  • Superiorly: Uterine cavity (endometrium of lower uterine segment)

  • Inferiorly: Cervical canal leading to the external os

  • Anteriorly: Bladder and vesicouterine pouch

  • Posteriorly: Rectouterine pouch (pouch of Douglas)

Function

  • Serves as the anatomical boundary between uterus and cervix

  • Regulates passage of menstrual blood from uterus to vagina

  • Allows sperm entry into uterine cavity during reproduction

  • Maintains competence in pregnancy, preventing premature passage of amniotic sac

  • Dilates during labor and delivery to allow fetal passage

Clinical Significance

  • Cervical incompetence: Incompetence or weakness of the internal os may lead to recurrent miscarriages or preterm labor

  • Cervical stenosis: Narrowing of the internal os may cause infertility, dysmenorrhea, or hematometra

  • Cancer staging: Location of tumors relative to the internal os is important in cervical and endometrial cancer staging

  • Imaging landmark: Critical reference point in hysterosalpingography, sonohysterography, CT, and MRI

MRI Appearance

T1-weighted images:

  • Internal os appears as a low signal fibromuscular ring at the junction of cervix and uterus

  • Surrounding endometrial cavity may show variable signal depending on cycle phase

T2-weighted images:

  • Internal os shows low signal intensity due to fibrous tissue

  • Endocervical canal and uterine cavity fluid appear bright for contrast

STIR (Short Tau Inversion Recovery):

  • Internal os remains dark

  • Pathological changes (inflammation, edema, tumor invasion) appear bright

T1 Fat-Sat Post-Contrast:

  • Enhances mildly and uniformly under normal conditions

  • Neoplastic or inflammatory infiltration shows heterogeneous enhancement

CT Appearance

Non-Contrast CT:

  • Difficult to delineate internal os clearly; appears as soft tissue junction between uterus and cervix

  • May be indirectly identified by changes in cervical canal caliber

Post-Contrast CT:

  • Normal internal os enhances mildly with uterus and cervix

  • Pathological involvement (tumor, infection) may appear as asymmetric or irregular enhancement

MRI image

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

MRI image

internal os of the cervix MRI AXIAL anatomy image-img-00000-00000

CT image

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