Transvaginal ultrasound measurement of cervical length is useful for supporting or excluding the diagnosis of cervical insufficiency or preterm labor when the diagnosis is unclear. The maternal cervix is measured by ultrasound as a screening tool, since a short cervix is often detected on ultrasound examination before it can be appreciated on physical examination.
All patients have a cervical length measurement at their anatomical survey, most of the time by transabdominal imaging. If there are other risk factors for preterm birth, if the cervix cannot be adequately imaged, or the cervix appears short on transabdominal scanning, then a transvaginal ultrasound will be done for a more detailed assessment.
“Cervical shortening” is one of the earliest signs of preterm labor and can precede labor by several weeks. The cause is often unclear. A short cervical length before 24 weeks can be predictive of preterm birth and is most predictive in women with early and significant cervical shortening.
A long cervix (≥30 mm) before 34 weeks of gestation has a high negative predictive value for preterm birth, meaning preterm birth is unlikely to occur.