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Internal os

The part of the cervical canal, forming an internal narrowing of the uterine cavity
The cervix uteri in female reproductive system is located at the lower part of the uterus and resembles a neck as it forms a narrow canal in between the uterine cavity and the lumen of the vagina. Its function is to allow the passage of blood during menstrual period, as well as semen during copulation. It also serves as the birth canal from where fetus would pass through at the time of parturition. There are two openings in the cervix uteri and are termed internal os and external os. The opening into the uterus is termed internal os or internal orifice of the cervix uteri. It corresponds to the slight constriction referred to as the isthmus, which can be found on the surface of the uterus, midway from the apex and the base. The other end that opens into the vagina is referred to as the external os.
During pregnancy, the internal os should not dilate before the fetus has reached full term. The dilation of the internal os early on during pregnancy is called funneling and may lead to pregnancy loss. Therefore, bed rest is recommended when funneling is observed via an internal ultrasound.

  • internal orifice of the uterus
  • internal orifice of the cervix uteri


  • external os
  • See also:

    • cervix uteri
    • uterine cavity
    • uterus

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