Cervical length assessment by ultrasound as a predictor of preterm labour—is there a role for routine screening?

Abstract

Transvaginal ultrasonography has recently been shown to be an objective, reproducible and reliable method to assess the cervix and predict the risk of preterm delivery in high‐risk pregnancies. Assessment of the cervix includes cervical length measurement (CLM) and measurement of dilatation of the internal os in a dynamic functional examination. There is an inverse correlation between cervical length and the frequency of preterm delivery. The high negative predictive value avoids unnecessary interventions such as tocolysis or cerclage in high‐risk pregnancies. In contrast, a length of 25 mm or less at 28–30 weeks of gestation is associated with a significantly increased incidence of preterm delivery. Studies in women with high risk for preterm delivery, i.e. contractions, premature rupture of the membranes and history of preterm delivery, have shown a high sensitivity and a high positive predictive value, however in low‐risk groups they have failed to show a high sensitivity. From large observational studies in low‐risk populations we know that the 50th percentile of the cervical length is 35 mm at 24 weeks of gestation3. Advantages of CLM as a screening test include the fact that sonographical assessment of the cervix is a widely accepted and well‐standardised method, which requires only a relatively short period of training. Disadvantages of screening are two factors, the first being the low sensitivity of the test and the low prevalence of preterm deliveries in a low‐risk population, resulting in cut off values being set at a very low level (i.e. 5th percentile) in order to get acceptable specificity. Secondly, screening is only worthwhile if an effective preventive therapy is available. The debate about tocolysis and cerclage is not yet concluded. Therefore we would not currently recommend cervical length measurement as a screening tool—but as a routine method in high risk gravidas with or without symptoms. Further interest should be focused on scoring systems combining ultrasound with biochemical, endocrinological and maybe molecular cell methods such as the measurement of fetal DNA in maternal blood to prevent preterm deliveries in the general population.