Cervical length and the risk of preterm delivery

The cervix is made up of muscle and connective tissue, and sits directly between the uterus and the vagina. For the majority of a woman’s pregnancy her cervix stays thick and closed, but towards the end of pregnancy hormones signal to the cervix to soften and shorten in preparation for the delivery of the fetus.

Women with shorter cervixes can be at a greater risk of premature birth and related associated health risks for their babies. As a result, healthcare professionals carefully measure and monitor cervical length specifically in women with a history of a prior cervical procedure or a prior preterm birth in an effort to identify those at risk to intervene and minimize the likelihood of preterm delivery.

When can the length of the cervix be monitored?

The cervical length can be monitored using transvaginal ultrasound most accurately after 16 weeks gestation to determine whether or not a woman will be at an increased risk for preterm delivery.

What is the average length of the cervix?

Before the cervix begins to shorten it is usually between 3-5 cm. A shorter-than-average cervix after 16 weeks gestation is 2.5 cm or less.

Why would a woman have a greater risk of having a cervix that is shorter than average?

There are a number of things that increase one’s risk for a short cervix. If a woman has given birth preterm before or if she has had surgery on her cervix, she may be at a greater risk of having a short cervix. A woman’s risk factors will determine how closely and frequently her healthcare provider will want to monitor her cervical length.

What should pregnant women do if they have a short cervix?

It is very important for women with short cervixes to be aware of the signs of early labor. This will help them know when to contact their healthcare providers. Based on how short the cervix is, their healthcare provider may recommend additional monitoring and/or interventions. Depending on the intervention recommended to them by their providers, it is also important that women follow the plan as strictly as possible, taking all necessary measures to avoid preterm labor.

If you have any questions about cervix length and a pregnant woman’s risk for preterm delivery, speak to your healthcare provider, as they will be able to discuss your potential risk.

  • Joseph R Wax. “How to manage a short cervix to lower the risk of preterm delivery.” OBG Manag. 22(5):36-50. Web. May 2010.
  • A Naim. “Changes in cervical length and the risk of preterm labor.” Am J Obstet Gynecol. 186(5):887-9. Web. May 2002.
  • “Preterm (Premature) Labor and Birth.” ACOG. FAQ087 from the American College of Obstetricians and Gynecologists, Nov 2016. Web.
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