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Second-trimester evaluation of cervical length for prediction of spontaneous preterm birth

Vincenzo Berghella, MD
Section Editors
Charles J Lockwood, MD, MHCM
Deborah Levine, MD
Deputy Editor
Vanessa A Barss, MD, FACOG


This topic will describe the normal distribution of cervical length, provide a clinical definition for a short cervix, and discuss our approach and technique for cervical length screening.


Cervical shortening (effacement) is one of the first steps in the parturition process, preceding labor by several weeks. As cervical length decreases in the second trimester, the risk of spontaneous preterm birth increases [1-5], especially when effacement occurs early in the second trimester [6]. Because effacement begins at the internal cervical os and progresses caudally [1,5], it is often detected on ultrasound examination before it can be appreciated on physical examination. The cause of preterm cervical shortening is often unclear. It has been attributed to several sources, including occult uterine activity, uterine overdistention, congenital or acquired cervical insufficiency, decidual hemorrhage, infection, inflammation, and biological variation.


Normally, cervical length is stable between 14 and 28 weeks of gestation and is described by a normal, bell-shaped curve [1,7]:

15 mm – 2nd centile

20 mm – 5th centile


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Literature review current through: Jan 2016. | This topic last updated: Dec 14, 2015.
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