How is ultrasound used to assess cervical length, and what is the significance?

Ultrasound (transvaginal) measures cervical length and visualises the internal os - an advantage over digital examination, which cannot - detecting early changes of cervical incompetence.

In selected high-risk women it is done from 14-16 weeks and 2-weekly thereafter; a cervical length below 25 mm predicts preterm labour risk and may indicate the need for a cervical cerclage.

Clinical relevance. Sonographic cervical length predicts preterm birth and guides cerclage decisions in high-risk women.

More Detail

Cervical length assessment

Sonographic cervical length is used to predict preterm birth in at-risk women.

Advantage over digital examination

• Ultrasound can visualise the internal os and the whole canal, detecting early funnelling/shortening (cervical incompetence) that a digital examination misses.

How it is used

• In a selected (high-risk) population - e.g. previous preterm birth or cervical surgery.
• Measured from 14-16 weeks and repeated every 2 weeks through the relevant period.

Significance

• A cervical length below 25 mm is the generally used cut-off to predict the risk of preterm labour.
• It may indicate the need for a cervical cerclage in high-risk women (or progesterone).