How is cervical length measured?

The cervix plays an important role during pregnancy: it’s the barrier between your uterus and the outside world until you’re ready to deliver your little one. It’s a small, cylindrical piece of tissue that connects the vagina to the uterus, and measuring its length can help your healthcare provider determine whether you’re at risk for preterm delivery.

The normal range of cervical length for pregnant women is 3 to 5 cm, though the cervix will get shorter as your baby grows and your uterus expands. Having a cervix shorter than that during pregnancy puts you at increased risk of preterm delivery because it will get shorter as your pregnancy progresses. If it gets too short, it can open before your due date, causing preterm birth and many potential problems for your baby. The length of your cervix at 18 to 24 weeks is likely the best predictor of preterm birth, besides having already delivered preterm in the past.

A short cervix is defined as one shorter than 2.5 cm before 24 weeks. It can be difficult or impossible to diagnose a short cervix using just an abdominal ultrasound, and a physical exam isn’t the perfect diagnostic tool either. A transvaginal ultrasound is the “gold standard” for the measurement of cervical length, according to a study published in Obstetrics & Gynecology. Using a sterile technique, the transvaginal ultrasound is “objective, reproducible, and acceptable to patients.” Acceptable means it won’t exactly be a day at the spa, but most patients don’t have trouble with it.

In a transvaginal ultrasound, a healthcare provider will, using lubricating gel, insert a probe two to three inches into a woman’s vagina. The probe will release sound waves that create a picture of the internal organs, making it easy for doctors to see the size and shape of the cervix. The procedure can take 30 minutes to an hour, and the sensation is similar to that of a pap smear. There’s no significant risk in a transvaginal ultrasound, and your healthcare provider will get a clear picture of whether or not you have a short cervix.

To find out whether you have a short cervix, you’ll likely need to request a measurement from your healthcare provider. They should screen you for a short cervix if you have a history of preterm delivery or cervical surgery, but it can benefit all pregnant women to be screened for short cervixes because of how helpful it is in predicting preterm birth.

  • “Cervical insufficiency and short cervix.” March of Dimes. March of Dimes Foundation. August 2015. Web.
  • “Preterm (Premature) Labor and Birth.” ACOG. American College of Obstetricians and Gynecologists. November 2016. Web.
  • “Management of Pregnancies With Cervical Shortening: A Very Short Cervix Is a Very Big Problem.” Obstetrics & Gynecology. U.S. National Library of Medicine. National Institutes of Health. 2009. Web.
  • “Transvaginal ultrasound.” MedlinePlus. U.S. National Library of Medicine. May 16, 2016. Web.
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