Cervical length (CL) measurement by transvaginal ultrasound (TVS) is the most reliable predictor of spontaneous preterm birth. It's a technically demanding measurement with specific criteria that must be met for the number to be valid and clinically actionable.

Why Cervical Length Matters

Spontaneous preterm birth (< 37 weeks) complicates ~10% of pregnancies and is the leading cause of neonatal morbidity and mortality. Cervical shortening precedes preterm birth by weeks — a short cervix on midtrimester screening identifies women who may benefit from progesterone supplementation, cervical cerclage, or closer monitoring.

When is CL measured?

Universal screening: 18–24 weeks (typically combined with the anatomy scan)
Symptomatic patients: Any GA with contractions, pelvic pressure, or preterm labor
High-risk patients (prior preterm birth, uterine anomalies, multiple gestation): Serial measurement from 16 weeks

Why TVS, Not Transabdominal?

Transabdominal (TAS) CL measurement is inaccurate — the full cervix is often obscured by the fetal presenting part, maternal body habitus, or bowel. A full bladder can compress and artificially lengthen the cervix. TVS is the standard for CL measurement — it provides direct visualization of the entire endocervical canal without bladder interference.

TVS Cervical Length Protocol

Equipment and Setup

Probe: Endovaginal (high-frequency, 5–9 MHz).

Patient preparation:

Empty bladder — critical. A full bladder artificially lengthens the cervix by compressing the anterior and posterior walls together.
Confirm the patient has voided before the exam.

Position: Lithotomy position, standard TVS setup.

Step-by-Step Technique

1. Insert the probe and identify the cervix in sagittal view.

Locate the endocervical canal — it appears as a thin echogenic line connecting the internal os to the external os.

2. Identify the internal os.

The internal os is where the lower uterine segment transitions to the cervix — identified by the characteristic V-shape where the uterine walls converge. This is the upper measurement point.

3. Identify the external os.

The external os is the most caudal visible point of the endocervical canal, often seen as a dimple or the point where the canal meets the vagina.

4. Measure the closed portion of the endocervical canal.

Place the caliper from the internal os to the external os along the endocervical canal. If the canal is curved (common), you may need to add two straight-line segments (sum of two or three straight lines along the canal axis).

5. Apply gentle probe pressure — then withdraw.

Excessive pressure from the probe will artificially lengthen the cervix by compressing it. After obtaining your image, withdraw the probe slightly and re-image to confirm the measurement is consistent without compression artifact.

6. Observe for dynamic changes (30 seconds to 3 minutes).

Watch for:

Funneling: The internal os opens, taking on a V, U, or T shape (widening from above). The funnel length is the open portion of the internal os from the top of the funnel to the point where it re-approximates.
Sludge: Echogenic debris or particulate matter within the amniotic fluid near the internal os — associated with intrauterine infection.

7. Record at least 3 measurements.

Take the shortest measurement (not the average) — transient shortening is more clinically significant than an artificially long measurement due to uterine contraction or probe compression.

What Is a Valid Measurement?

A valid CL image must show:

Both os visible — internal and external os clearly identified
Symmetric anterior and posterior lips of the cervix
Endocervical canal visible as an echogenic or hypoechoic line
No probe pressure — if the cervix bows anteriorly or looks compressed, withdraw the probe
Clear measurement landmarks

Threshold Values and Clinical Interpretation

Cervical Length	Interpretation
≥ 25 mm (16–24 wks)	Low risk for preterm birth
20–24 mm	Intermediate — increased risk; consider repeat measurement and risk stratification
< 20 mm	Short cervix — significantly elevated risk; treatment is indicated
< 10 mm	Critical — extremely short; emergent evaluation for cerclage or hospitalization

The most widely used clinical threshold: < 20 mm at 18–24 weeks in a singleton pregnancy.

Evidence-based interventions for short cervix:

Vaginal progesterone: For singleton pregnancies, CL < 20 mm, no prior preterm birth — reduces preterm birth risk by ~45%
Cerclage: For CL < 25 mm with prior spontaneous preterm birth (singleton), or progressive shortening in high-risk patient
Pessary: Some evidence in selected populations

Funneling: What It Means

Funneling occurs when the internal os opens, shortening the functional cervical length.

How to describe funneling:

Shape: V, U, or T-shaped opening
Funnel width: Widest transverse dimension of the open funnel
Funnel length: Distance from top of funnel to the point of canal re-approximation
Functional (closed) CL: The length of the remaining closed canal — this is the clinically relevant measurement, not the total cervical length

Example: Total cervical length 28 mm, funnel length 10 mm → functional CL 18 mm (< 20 mm threshold is met).

Multiple Gestation

CL thresholds and management differ for twins:

Short CL (< 20 mm) in twins: vaginal progesterone does NOT reduce preterm birth (no benefit in twin gestations — unlike singleton)
Management is more complex; cerclage not routinely indicated in uncomplicated twins
CL < 25 mm in twins: increased surveillance, patient education on preterm labor symptoms

Pitfalls

Full bladder: Artificially elongates the cervix — patient must void first.

Probe pressure: Compresses and lengthens the canal — withdraw probe and confirm.

Internal os identification: Confused with the lower uterine segment or a contraction — watch for dynamic changes (contractions resolve; a short cervix persists).

Contraction artifact: If the uterus is contracting during the exam, the lower uterine segment may appear elongated and mimic a long cervix. Allow time to pass and re-measure when relaxed.

Reporting Language

Normal: "Transvaginal cervical length measured at 38 mm with a closed internal os. No funneling or dynamic change observed over 3 minutes of observation. Endocervical canal is well-visualized."

Short cervix with funneling: "Transvaginal cervical length measures 14 mm (shortest of three measurements). Funneling is present at the internal os with a V-shaped funnel measuring 8 mm in depth and 11 mm in width. Functional closed cervical length is 14 mm. Findings indicate significantly shortened cervix at risk for preterm birth. Clinical correlation and obstetric management recommended."

SonoBuddy's OB protocols include first trimester, anatomy survey, growth scan, and cervical length guides.

Cervical Length Measurement: Preterm Risk and TVS Technique