Society for Maternal-Fetal Medicine (SMFM); Jennifer McIntosh, DO; Helen Feltovich, MD; Vincenzo Berghella, MD; and Tracy Manuck, MD
Worldwide, 15 million babies are born too soon every year, causing 1.1 million deaths, as well as short- and long-term disability in countless survivors. In high-income countries, preterm birth (PTB) is the leading cause of death in children <5 years, and globally it is second only to pneumonia. Few prognostic tests are available to predict which pregnancies will deliver preterm. The majority (2/3) of PTBs are spontaneous, and recurrence risks are high; a history of a prior spontaneous PTB is historically the strongest risk factor for spontaneous PTB.
The purpose of this document is to review the indications and rationale for cervical length (CL) screening to prevent PTB in various clinical scenarios.
Q: What is the clinical significance of a sonographically short cervix?
Women with a history of a prior spontaneous PTB account for only 10% of all births <34 weeks’ gestation. Currently, mid-trimester CL assessment by transvaginal ultrasound is the best clinical predictor of spontaneous PTB. A CL below the 10th percentile for gestational age is considered “short.” At 18 to 24 weeks’ gestation, the 10th percentile corresponds to a CL of less than 25 mm.
The risk of spontaneous PTB is inversely proportional to the length of the cervix; those with the shortest CL have the highest risk of prematurity.
Women with a history of a prior spontaneous PTB and a short CL are at the highest risk. Nonetheless, the finding of a short CL, irrespective of prior pregnancy history, has been consistently and reproducibly associated with an elevated risk of spontaneous PTB across all gestational age cutoffs and multiple patient populations.
Q: Should transabdominal or transvaginal ultrasound be used?
Transvaginal ultrasound is considered the “gold standard” measurement when assessing CL. In contrast to transabdominal ultrasound, transvaginal ultrasound measurements are highly reproducible, and measurements are unaffected by maternal obesity, cervical position, and shadowing from fetal parts. Transvaginal ultrasound is also much more sensitive than transabdominal ultrasound using CL cutoffs that are typical for screening for a short cervix.
Transvaginal ultrasound is safe, and when it is performed by trained operators, results are reproducible with a low interobserver variation rate of 5%–10%.