How measuring cervical length helps predict preterm birth

A good way for providers to predict a woman’s risk of preterm delivery is to measure her cervical length in the second trimester of pregnancy. Normally in pregnancy, the cervix stays thick and closed until weeks 32-34, when it starts to get softer and shorter in preparation for labor. But women who deliver preterm tend to experience shortening of their cervix earlier in pregnancy, with a cervix shorter than 2.5 cm before 24 weeks. In these cases, the cervix is shortening too early in the pregnancy and the chance of preterm labor is higher. Healthcare providers can use these measurements to determine if a woman is at risk of going into labor too early, and if so, to determine a plan to manage her pregnancy and keep her and her baby healthy.

Risk factors and screening

Certain risk factors, like a multiple pregnancy or a previous preterm birth, increase a woman’s chances of delivering preterm. Women who have risk factors like these are considered high-risk for preterm delivery, and they are likely to be screened for a short cervix at a routine check-up in the second trimester.

But while risk factors are important, they’re not a perfect predictor of whether a woman will go into preterm labor. In fact, according to the Mayo Clinic as well as studies in the American Journal of Obstetrics & Gynecology, a large percentage of women who deliver preterm don’t have a history of preterm birth. Even in low risk pregnancies, when women don’t have risk factors for preterm labor, a transvaginal ultrasound to screen for cervical length can help to predict a woman’s risk of preterm delivery. However, women who are low risk for a preterm birth aren’t routinely screened for a short cervix at their checkups unless they request it from their provider.

The bottom line

Measuring cervical length between weeks 16-24 weeks of pregnancy is a reliable way for healthcare providers to determine a woman’s risk of preterm labor. If your provider hasn’t taken a cervical measurement yet, it might be worth asking for one, because even if you aren’t high risk for a preterm birth, it might give you more peace of mind in your pregnancy. For women who learn they have a short cervix, their provider helps them manage their pregnancy so that their baby is delivered happy, healthy, and on time.

  • Sarah J. Kilpatrick, MD, PhD. “Prediction and Prevention of Preterm Birth: What have we learned since 2001?” ModernMedicine. UBM Media, LLC., Feb 2013. Web.
  • ES Miller, et al. “Second-Trimester Cervical Length Screening Among Asymptomatic Women: An Evaluation of Risk-Based Strategies.” Obstet Gynecol. 126(1):61. Web. 2015.
  • Gustaaf Albert Dekker, et al. “Risk Factors for Preterm Birth in an International Prospective Cohort of Nulliparous Women.” PLOS ONE. Web. Jul 2012.
  • “The preterm prediction study: a clinical risk assessment system.” Am J Obstet Gynecol. 174(6):1885-93. Web. Jun 1996.
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  • EF Werner. “Universal cervical-length screening to prevent preterm birth: a cost-effectiveness analysis.” Ultrasound Obstet Gynecol. 38(1):32. Web. 2011.
  • J Owen, et al. “Can shortened midtrimester cervical length predict very early spontaneous preterm birth?” Am J Obstet Gynecol. 191(1):298. Web. 2004.
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