What is internal OS
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Internal Os
The internal os is the upper opening of the cervical canal — the anatomical boundary between the cervix and the uterine body (corpus).
Anatomy
The cervix has two openings (os):
| Opening | Location |
|---|---|
| Internal os | Where the cervical canal opens into the uterine cavity (superior end) |
| External os | Where the cervical canal opens into the vagina (inferior end) |
The internal os corresponds to the isthmus — a slight narrowing at the junction of the cervix and uterine corpus. The endocervical canal runs between these two openings and:
- Transports sperm into the uterine cavity
- Allows escape of menstrual blood
- Secretes cervical mucus into the female reproductive tract
- Dilates significantly during childbirth (parturition)
Clinical Significance
-
Cervical incompetence (insufficiency): A weakened internal os may dilate prematurely during pregnancy, leading to second-trimester pregnancy loss. Treatment includes cervical cerclage (a suture to reinforce the internal os).
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Cervical ectopic pregnancy: Implantation within the cervical canal below a closed internal os is a diagnostic criterion — confirmed on ultrasound showing the gestational sac below the internal os with no intrauterine pregnancy.
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Labor/Induction: Cervical assessment on ultrasound measures the internal os width and posterior cervical angle to predict readiness for labor induction.
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Stenosis: Narrowing of the internal os can cause dysmenorrhea (painful periods) and hematometra (blood accumulation in the uterus).
Here is an ultrasound image showing the internal os and cervical canal in the sagittal plane:
Transvaginal sagittal ultrasound showing the internal os (IO) at the junction of the cervical canal and lower uterine segment, external os at the distal end.
- Berek & Novak's Gynecology (cervical ectopic — closed internal os criterion)
- SEER Training — Anatomy of the Female Pelvis | TeachMeAnatomy — The Cervix
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