Shortened cervix and preterm labor risk

During pregnancy, the cervix is a closed and sealed tunnel between the uterus and the vagina. Before or during labor and delivery, the cervix stretches and flattens (effacement). At 24 weeks of pregnancy, the average cervix is about 35 mm (1.4 in.) long.1

Using ultrasound imaging, researchers have found that at the end of the second trimester (about 24 weeks), the shorter a woman's cervical length, the more likely she is to deliver preterm. (This type of cervical measurement requires special training and experience to perform accurately.) Cervical length of less than 25 mm (1 in.) before 35 weeks is considered a reliable sign of increased preterm labor risk. However, most women with a shortened cervix do go on to deliver at term.2

How useful is this information?

If you have preexisting risk factors for preterm labor, the absence of a shortened cervix is a reassuring sign that preterm labor is not imminent.

There is a lack of proven treatment options for preventing preterm labor over a period of weeks. So, knowing that you have a shortened cervix and increased preterm labor risk may not change the way your doctor treats your pregnancy.2 If you learn that you are at high risk, make sure that you know the symptoms of preterm labor and that you know what to do if you have them.


  1. Cunningham FG, et al. (2005). Preterm birth. In Williams Obstetrics, 22nd ed., pp. 855–880. New York: McGraw-Hill.
  2. American College of Obstetricians and Gynecologists (2001, reaffirmed 2008). Assessment of risk factors for preterm birth. ACOG Practice Bulletin No. 31. Obstetrics and Gynecology, 98(4): 709–716.

Last Updated: January 14, 2009

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