Cervical Length Measurement

Cervical length measurement by transvaginal ultrasound estimates the risk of spontaneous preterm birth. The cervix shortens as part of normal labour, and a premature shortening or funnelling of the internal os predicts early delivery.

A short cervix in the mid-trimester is the strongest single predictor of spontaneous preterm birth in singleton pregnancies. The measurement identifies women who need closer surveillance or intervention.

Usage. It is used in women with a history of spontaneous preterm birth or late miscarriage, in selected high risk groups, and in symptomatic women with threatened preterm labour.

Indications. Previous spontaneous preterm birth, previous late miscarriage, previous cervical surgery, uterine anomaly, and symptoms of threatened preterm labour between 24 and 34 weeks. Universal mid-trimester screening is applied in some services.

Preparation and technique. The bladder is emptied. A transvaginal probe is inserted just far enough to visualise the cervix, avoiding pressure that artificially lengthens it. A sagittal view shows the internal os, canal and external os. The length is the straight line between the internal and external os. At least 3 measurements are taken over 3 to 5 minutes, and the shortest is recorded. Dynamic change under fundal pressure or coughing is noted.

Contraindications and precautions. Placenta praevia with bleeding requires caution with the transvaginal probe. Transperineal scanning is used when the transvaginal route is not possible, with lower accuracy.

Interpretation.

• A length of 25 mm or less between 16 and 24 weeks is short and associated with increased risk of preterm birth.
• A length under 15 mm is very short, with a high risk of delivery within a few weeks.
• Funnelling is a V- or U-shaped widening of the internal os with membranes in the canal. It adds risk when the length is also short.
• Amniotic fluid sludge, meaning dense echogenic material near the internal os, is associated with intra-amniotic infection and preterm birth.

Causes of a short cervix. Cervical insufficiency, previous cervical surgery (cone biopsy), uterine anomaly, infection, and hormonal factors.

Critical values and findings needing immediate action.

• A cervix under 15 mm, or a dilated cervix with bulging membranes, requires same day specialist assessment.
• A short cervix with symptoms of labour, bleeding or fever requires immediate evaluation for infection and preterm labour.

Limitations and artefacts. Full bladder, probe pressure and uterine contractions alter measured length. Examiner variability is significant. Thresholds are validated mostly in singleton pregnancies.

Follow up tests. Repeat cervical length at 1 to 2 weeks, foetal fibronectin, high vaginal swab, urine culture, and assessment for intra-amniotic infection when sludge or symptoms are present.

Special groups. In twin pregnancy, a threshold of about 25 mm at 18 to 24 weeks is used, with local variation. Women with cone biopsy or uterine anomaly have shorter baseline lengths.