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Isthmus of uterus

The isthmus of the uterus is the narrow region that connects the body of the uterus to the cervix. It is about 1 cm long in the non-pregnant uterus and forms the transition between the endometrial cavity of the uterine body and the cervical canal. During pregnancy, it undergoes significant changes and contributes to the formation of the lower uterine segment, which is critical for fetal growth and obstetric management.

Synonyms

  • Uterine isthmus

  • Lower uterine segment (in pregnancy)

  • Uterine transition zone

Location and Extent

  • Situated between the uterine body above and the cervix below

  • Lies at the level of the internal os of the cervix

  • In the non-pregnant state, it is about 1 cm in length and poorly defined

  • In pregnancy, it elongates and expands, forming the lower uterine segment

Relations

  • Anteriorly: Vesicouterine pouch and urinary bladder

  • Posteriorly: Rectouterine pouch (pouch of Douglas) and rectum

  • Superiorly: Body of uterus

  • Inferiorly: Cervix

Function

  • Acts as a junctional zone between uterine body and cervix

  • Plays a role in expansion during pregnancy, forming the lower uterine segment

  • Facilitates cervical dilation and effacement during labor

  • Important site for surgical incisions in cesarean section (lower segment approach)

Clinical Significance

  • During pregnancy, it becomes thin and distensible, forming the lower uterine segment

  • Landmark for cesarean section incision (preferred due to reduced risk of bleeding and rupture)

  • In gynecology, lesions such as fibroids, adenomyosis, or endometrial carcinoma may extend into the isthmus

  • Isthmico-cervical incompetence can lead to recurrent pregnancy loss or preterm birth

  • Pathological enlargement may mimic uterine body or cervical pathology on imaging

MRI Appearance

T1-weighted images:

  • Appears as a band of low-to-intermediate signal intensity between uterine body and cervix

  • Endometrial cavity at this level shows high signal if  blood is present

T2-weighted images:

  • Demonstrates low-signal intensity myometrium surrounding the high-signal endometrial stripe

  • In pregnancy, lower uterine segment appears stretched and thinned with intermediate signal intensity

STIR (Short Tau Inversion Recovery):

  • Normal isthmus shows low-to-intermediate signal

  • Pathology (inflammation, adenomyosis, or tumor) appears as bright hyperintensity

T1 Fat-Sat Post-Contrast:

  • Enhances mildly and uniformly in normal conditions

  • Neoplastic or inflammatory involvement shows heterogeneous or irregular enhancement

CT Appearance

Non-Contrast CT:

  • Appears as a soft tissue structure continuous with uterus and cervix

  • Difficult to differentiate from uterine body and cervix unless pathology is present

  • Hemorrhage or fluid may be visible in endometrial cavity

Post-Contrast CT:

  • Enhances homogenously with rest of the uterus

  • Pathology (fibroid, carcinoma, adenomyosis) shows focal or heterogeneous enhancement

  • In pregnancy, lower uterine segment can be seen as a thinned, enhancing band below the uterine body

MRI image

isthmus of uterus MRI sagittal image anatomy image-img-00000-00000

CT image

isthmus of uterus CT sagittal image anatomy image-img-00000-00000