Transvaginal cervical length measured to a strict technique — empty bladder, sagittal view of the whole canal, no pressure, three measurements, shortest valid — below ~25 mm before 24 weeks identifies women who benefit from progesterone or cerclage.
Orient first
- Transabdominal measurement is unreliable (bladder filling lengthens the cervix).
- Thresholds and the recommended action depend on history (singleton, twins, prior preterm birth — verify local guidance).
- Funnelling and sludge are additional signs; dynamic change with fundal pressure may be recorded.
Acquire the study
- Transvaginal ultrasound with an empty bladder; sagittal plane showing the internal os, external os and the whole canal; image filling ~75% of the screen.
The manoeuvre
- Sagittal plane: both os and the endocervical mucosa visible; withdraw the probe until the image blurs, then advance just enough.
- Measure the closed canal from internal to external os in mm (trace if curved).
- Take three measurements over ~3 min; report the shortest technically adequate.
- Funnelling: width and length in mm; amniotic fluid sludge.
What confirms it
- A short cervix below the threshold on a technically adequate transvaginal measurement.
What licenses you to exclude it
- A normal measurement at 18–24 weeks reduces but does not remove the risk.
The classic misread
- Excessive probe pressure lengthening the cervix, or a lower segment contraction mimicking a long cervix.