Earlier this year, Northwest Perinatal Center provided new recommendations on the use of progesterone to prevent recurrent spontaneous preterm birth. In so doing, we recognized that a renewed review and recommendation for the use of transvaginal cervical length measurement as a tool to predict recurrent spontaneous preterm birth was due.


Transvaginal ultrasound cervical length measurements are used to stratify the risk of preterm birth within the current pregnancy. A cervical length measuring less than 25 mm between 16
and 24 weeks gestation increases the risk of preterm birth. This risk is inversely correlated with the length of the cervix. Vaginal progesterone has been shown to reduce the risk of preterm birth by 40% among people with a short cervix < 25 mm between 16 and 24 weeks gestation. Universal screening has been adopted nationally, and at WHA a transabdominal cervical length is obtained for all pregnancies between 16 and 24 weeks gestation. For people with at the highest risk of preterm labor, such as those with a history of spontaneous preterm birth, it became common practice nationally to perform serial transvaginal cervical length assessments to both risk stratify and/or to start vaginal progesterone when a short cervix is identified.


Review of the current literature reveals that there is no appreciable increase in the predictive value of transvaginal cervical length when performed serially. In addition, other findings such as funneling of the internal os and change with fundal pressure do not add to the predictive value of the exam. The presence of intraamniotic “sludge” or “debris” does appear to be associated with a higher risk of spontaneous preterm birth.


Offer a single transvaginal cervical length measurement to people with a history of spontaneous preterm birth prior to 37 weeks gestation. This study can most often occur at the time of the anatomic survey and is useful from 16 to 24 weeks gestation. Please place an order for both anatomic survey and transvaginal cervical length when appropriate. For people with a history of spontaneous preterm birth < 32 weeks gestation or a history of cerclage, consider NWP consultation to make additional recommendations for cervical length screening.


  • Iams J.D., Goldenberg R.L., Meis P. J. et al. The length of the cervix and the risk of spontaneous premature delivery. NEJM. 1996; 334:567-573.
  • Hassan S.S. Romero R. Vidyadhari D. et al. Vaginal progesterone reduces the rate of preterm birth in women with a sonographic short cervix: a multicenter, randomized, double-blind, placebo-controlled trial. Ultrasound Obstet Gynecol. 2011; 38: 18-31.
  • Conde-Agudelo A. and Romero R. Predictive accuracy of changes in transvaginal sonographic cervical length over time for preterm birth: a systematic review and metaanalysis. AJOG. Dec2015:789-801.
  • ACOG Practice Bulletin Number 234. Prediction and Prevention of Spontaneous Preterm Birth. 2021.