Normal Cervical Canal Diameter
The normal cervical canal length in pregnancy is approximately 30-40 mm (3.0-4.0 cm) when measured via transvaginal ultrasound with an empty bladder, with a mean length of approximately 32.5 mm. 1
Measurement Standards in Pregnancy
Transvaginal ultrasound is the gold standard for cervical length measurement, and all clinical measurements should be performed using this approach with standardized protocols as described by the Perinatal Quality Foundation or Fetal Medicine Foundation. 1
Key Measurement Parameters:
- Mean cervical length: 32.5 mm (range varies by gestational age and parity) 2
- Short cervix threshold: ≤25 mm is used to diagnose short cervix in singleton pregnancies without prior preterm birth 1
- Critical short cervix: ≤20 mm requires intervention with vaginal progesterone before 24 weeks 1
- Severely shortened cervix: <15 mm carries approximately 42.5% risk of cervical dilation or pregnancy loss before 24 weeks 3
Important Technical Considerations:
Bladder filling significantly distorts cervical measurements - with partial bladder filling, the apparent cervical length increases to a mean of 46 mm compared to 32.5 mm with an empty bladder, representing artificial elongation rather than true anatomy. 2 This is why all clinical measurements must be performed with an empty bladder to avoid false reassurance. 1, 2
Twin Pregnancy Variations
In twin pregnancies, cervical length measurements differ from singleton pregnancies:
- Mean cervical length in dichorionic twins: 34.9 ± 8.6 mm 1
- Mean cervical length in monochorionic twins: 32.8 ± 10.1 mm (significantly shorter than dichorionic) 1
- High-risk threshold in twins: <15 mm is predictive of preterm labor regardless of management strategies 1
Non-Pregnant Cervical Measurements
In non-pregnant women, the cervical canal length averages 33.95 mm (range ± 9.23 mm) by anatomical measurement, with a negative correlation to age (cervix shortens with advancing age). 4
Clinical Context: Cervical Spinal Canal (Different Structure)
Important caveat: The cervical spinal canal (in the neck vertebrae) has completely different normal values and should not be confused with the uterine cervical canal:
- Cervical spinal canal sagittal diameter: ranges from 15.33-20.46 mm at C1-C7 levels 5, 6
- Cervical spinal stenosis threshold: ≤13.7 mm in Chinese populations, ≤14 mm in Caucasians 5
This distinction is critical - the question asks about cervical canal diameter, which in obstetric/gynecologic context refers to the uterine cervix, not the spinal canal. 1, 4