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Isthmus of uterus: anatomy, location and function

The isthmus is the short, narrow part of the uterus between the body and the cervix. In late pregnancy it stretches and thins to become the lower uterine segment, which takes up the fetal head.

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Highlighted in the model: Isthmus of uterus. Click any part to read about it, or switch off “Highlight topic” to see the whole body. Use Male / Female to switch bodies. 3D data: BodyParts3D, © The Database Center for Life Science; knee ligaments, menisci, tonsils, palate and parotid glands from the Human Reference Atlas Visible Human Male model and female parts from its Visible Human Female model (HuBMAP / NIH); lung lobes built from the BodyParts3D chest; all CC BY 4.0; meshes simplified and recoloured. The female body shares the male skeleton, muscles and organs except its pelvis, reproductive organs, breasts, bladder, urethra and skin.

Isthmus of uterus at a glance

Also called
Lower uterine segment (in pregnancy)
What it is
The isthmus is the short, narrow part of the uterus between the body and the cervix.
Location
It is about 1 cm long and lies at the level of the internal cervical os.
Function
In late pregnancy it stretches and thins to become the lower uterine segment, which takes up the fetal head.
Body system
Reproductive · single, midline structure · female body
FMA ID
FMA17752 (Foundational Model of Anatomy)

Key facts

  • It is the narrowest part of the uterine cavity outside pregnancy.
  • The bladder lies directly in front of it, separated by the vesico-uterine pouch.
  • The Human Reference Atlas model labels this part the lower uterine segment.

Clinical relevance

Caesarean sections are usually done through a transverse incision in the lower uterine segment, which bleeds less and heals more strongly than the upper segment.

Structures next to the isthmus of uterus

The nearest named structures in the 3D model, measured from the centre of each part:

References

  1. Foundational Model of Anatomy: Isthmus of uterus (FMA17752). EMBL-EBI Ontology Lookup Service.
  2. OpenStax. Anatomy and Physiology 2e, §27.2 Anatomy and Physiology of the Ovarian Reproductive System. Rice University, 2022.
  3. Mitsuhashi N. et al. (2009). BodyParts3D: 3D structure database for anatomical concepts. Nucleic Acids Research 37, D782–D785. Data © The Database Center for Life Science, CC BY 4.0.
  4. Browne K., Schlehlein H. 3D Reference Organ Set for United, Female (Visible Human Female). Human Reference Atlas 3D Reference Object Library, HuBMAP / NIH. Data from the NLM Visible Human Project. CC BY 4.0.

Explore the whole body in the interactive 3D human anatomy model, or browse the A–Z list of human structures.