Cervical Dilatation Recording with Open External Os and Closed Internal Os
When the external os is open but the internal os is closed, cervical dilatation should be recorded as 0 cm (closed cervix), as the internal os is the anatomically and clinically relevant landmark that determines true cervical dilatation.
Rationale Based on Cervical Anatomy and Clinical Assessment
The internal cervical os is the critical anatomical landmark for assessing cervical dilatation and predicting labor progression or preterm birth risk:
The internal os is where physiologically significant cervical changes begin during labor and preterm cervical insufficiency, making it the reference standard for cervical assessment 1.
Transvaginal ultrasound evaluation specifically measures from the external os to the internal os to assess cervical status, emphasizing the internal os as the definitive endpoint 1.
In clinical practice, a cervix with a closed internal os functions as a closed cervix regardless of external os status, as the internal os provides the structural barrier preventing membrane prolapse and maintaining pregnancy 1.
Clinical Context and Implications
Normal Physiologic Variation
Opening of the internal cervical os can occur normally in parous women as early as 24-25 weeks gestation in up to 52% of women who deliver at term, often appearing as V-shaped funneling 2.
The external os may be patulous in multiparous women without clinical significance if the internal os remains closed 2.
Risk Assessment Considerations
When funneling (internal os dilatation) is present with a closed external os, this represents a different clinical scenario with significant implications for preterm birth risk 3.
Cervical length measurement from the external os to the internal os provides the most clinically relevant prognostic information, with lengths <15 mm associated with 77% sensitivity and specificity for delivery within 7 days in symptomatic patients 1.
Documentation Standards
Record the cervical dilatation as 0 cm and document the specific findings:
- Note that the external os is open (specify diameter if measurable)
- Confirm the internal os is closed
- Document cervical length if ultrasound is available
- Note any funneling characteristics if present 1
This approach aligns with the principle that the internal os determines functional cervical competence and is the anatomically relevant landmark for clinical decision-making regarding labor management, preterm birth risk, and pregnancy maintenance 1.