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External cervical os: anatomy, location and function

The external os is the lower opening of the cervical canal into the vagina. It lets menstrual blood out and sperm in, and it opens fully (about 10 cm) in labour.

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Highlighted in the model: External cervical os. 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.

External cervical os at a glance

Also called
External os
What it is
The external os is the lower opening of the cervical canal into the vagina.
Location
It sits at the tip of the part of the cervix that projects into the vagina.
Function
It lets menstrual blood out and sperm in, and it opens fully (about 10 cm) in labour.
Body system
Reproductive · single, midline structure · female body
FMA ID
FMA76836 (Foundational Model of Anatomy)

Key facts

  • It is small and round before vaginal birth and becomes a transverse slit afterwards.
  • It is the opening seen in a speculum examination.

Clinical relevance

Cells for cervical screening are taken from around the external os, where the transformation zone usually lies.

Structures next to the external cervical os

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

References

  1. Foundational Model of Anatomy: External cervical os (FMA76836). 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.

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