Mean Cervical Length Research Articles

Identifying twin gestations at low risk for preterm birth with a transvaginal ultrasonographic cervical measurement at 24 to 26 weeks' gestation

Objective: Because twins are a high-risk group for preterm birth, many clinicians routinely use prophylactic interventions such as home bed rest, hospital bed rest, oral tocolytics, or home uterine activity monitoring to prevent preterm delivery. We sought to identify twin gestations at low risk for spontaneous preterm birth with transvaginal ultrasonography of the cervix to avoid the unnecessary use of prophylactic interventions in these pregnancies. Study Design: We measured cervical length at 24 to 26 weeks' gestation by transvaginal ultrasonography in women with twin gestations referred to our prematurity prevention clinic. Each delivery was classified as (1) spontaneous preterm birth <34 weeks' gestation, (2) delivery at ≥34 weeks' gestation with intervention, or (3) delivery at ≥34 weeks' gestation without intervention. Intervention included strict bed rest at home or in the hospital, either parenteral or oral tocolysis, or both, or home uterine activity monitoring. Indicated preterm deliveries and patients with cerclage were excluded from this analysis. The ability of transvaginal cervical length to predict women who would deliver at ≥34 weeks without intervention was evaluated. A cervical length of 35 mm was chosen by scatter diagram as the best cutoff to discriminate between the group delivered at term without intervention and the other two groups. Results: Of 85 women with twin gestations who underwent ultrasonographic cervical length measurements at 24 to 26 weeks' gestation, 17 had spontaneous preterm birth at <34 weeks, 23 were delivered at ≥34 weeks but required intervention, and 45 were delivered at ≥34 weeks without intervention. The mean cervical length for those delivered at ≥34 weeks' gestation without intervention (36.4 ± 5.8 mm) was significantly greater ( p < 0.0001) than the mean for those delivered preterm (27.4 ± 8.5) and those delivered at ≥34 weeks' gestation who required intervention (27.7 ± 10.5 mm). The sensitivity, specificity, and positive and negative predictive values of a cervical length >35 mm for predicting delivery at ≥34 weeks' gestation are 49%, 94%, 97%, and 31%, respectively. Conclusion: A transvaginal ultrasonographic measurement of the cervix of >35 mm at 24 to 26 weeks in twin gestations can identify patients who are at low risk for delivery before 34 weeks' gestation.

Cervical ultrasonography compared with manual examination as a predictor of preterm delivery

OBJECTIVE: Our purpose was to compare the accuracy of ultrasonographic and manual cervical examinations for the prediction of preterm delivery. STUDY DESIGN: One hundred two singleton pregnancies at high risk for preterm delivery were followed up prospectively from 14 to 30 weeks with both serial cervical ultrasonography measurements and manual examinations of the length of the cervix. The primary outcome studied was preterm (<35 weeks) delivery. RESULTS: Excluding six induced preterm deliveries, 96 pregnancies were analyzed. The mean cervical length measured by ultrasonography was 20.6 mm in pregnancies delivered preterm ( n = 17) and 31.3 mm in pregnancies delivered at term ( n = 79) ( p = 0.003); the mean cervical lengths measured by manual examination were 16.1 mm and 18.6 mm in the same preterm and term pregnancies, respectively (not significant). The sixteenth- and twentieth-week ultrasonographic cervical lengths predicted preterm delivery most accurately ( p < 0.0005). The 25th percentiles of ultrasonographic (25 mm) and manual (16 mm) cervical lengths showed relative risks for preterm delivery of 4.8 (95% confidence interval 2.1 to 11.1, p = 0.0004) and 2.0 (95% confidence interval 0.5 to 4.7, p = 0.1), respectively; sensitivity, specificity, and positive and negative predictive values were 59%, 85%, 45%, 91%, and 41%, 77%, 28%, and 86%, respectively. CONCLUSION: Cervical length measured by ultrasonography is a better predictor of preterm delivery than is cervical length measured by manual examination. Cervical ultrasonography in patients at high risk for preterm birth seems to be most predictive of preterm delivery when it is performed between 14 and 22 weeks' gestation. (Am J Obstet Gynecol 1997;177:723-30.)

Measurement of the pregnant cervix by transvaginal sonography: an interobserver study and new standards to improve the interobserver variability.

Transvaginal sonography has become an important tool for assessing the gravid cervix uteri, especially in patients at risk for cervical incompetence and preterm delivery. The purpose of our prospective study was to evaluate interobserver variability in measurements of the cervical length and, as a second step, improve the reproducibility and interobserver discrepancy by the introduction of quality control standards when producing and measuring the image of the cervix. Before the introduction of these standards we obtained 46 measurements and observed a mean cervical length (internal to external os) of 33.7 mm (range 26-52 mm) and an interobserver average discrepancy of 3.04 mm (range 0-6 mm). Later, measurements were obtained only when the following conditions were assured and visualized on the screen: (1) the internal os is either flat or is an isosceles triangle; (2) the whole length of the cervical canal can be observed; (3) a symmetric image of the external os can be obtained; and (4) the distance from the surface of the posterior lip to the cervical canal is equal to the distance from the anterior lip to the cervical canal. After the implementation of these quality control standards, 70 measurements were performed obtaining a mean length of 35.3 mm (range 27-51) and an average interobserver discrepancy of 1.24 mm (range 0-4). Thus the measurement error was significantly lower following introduction of quality control standards. Furthermore, the coefficient of variation dropped from 7.1% to 3.3% after the introduction of the guidelines. We believe that these guidelines could be helpful by making the measurements of the cervical length more accurate, reducing the interobserver variability, improving the reproducibility and promoting the role of transvaginal sonography in monitoring the incompetent cervix.

Transvaginal ultrasonographic cervical assessment for the prediction of preterm delivery.

To determine the usefulness of transvaginal ultrasonographic cervical assessment for the prediction of preterm delivery in an apparently normal population, 729 pregnant women (between 15 and 34 weeks' gestation) were randomly enrolled in the study in ten tertiary perinatal centers in Japan. Cervical parameters, including cervical length, internal os dilatation, and funneling depth, were measured by transvaginal ultrasound. The predictive values of these measurements for preterm delivery were investigated in a prospective fashion. Among various cervical parameters, cervical length showed the best correlation with pregnancy outcome. Cervical length (mm) was gradually decreased as the gestational age progressed, the regression line being y = 41.21-0.22x. When the mean cervical length minus 1 standard deviation at each gestational age was chosen as a cut-off value, the group with a shortened cervix showed a significantly high preterm delivery rate exclusively in the primigravidae (odds ratio: 4.86, 95% CI: 1.85-12.72). Internal os dilatation, in contrast, was a useful predictor in multiparous women (odds ratio: 6.00, 95% CI: 1.65-21.71). It was concluded that tranvaginal ultrasonographic cervical assessment, especially the measurement of cervical length, was effective for the prediction of preterm delivery in the primigravidae.

Ultrasound evaluation of cervical shortening after loop excision of the transformation zone (LETZ)

To assess cervical shortening after loop excision of the transformation zone (LETZ), and confirm the validity of ultrasound measurement of cervical length. Subjects (n = 29) were patients at the colposcopy clinic of Beth Israel Hospital who underwent vaginal ultrasound measurement of cervical length before and after LETZ. The pathologic specimen was measured by ruler. Mean cervical length (+/- S.D.), mean percentage (+/- S.D.) of cervical length removed, and correlation (r) between ultrasound and pathology specimen measurement were determined. Mean (+/- S.D.) cervical length measurements prior to LETZ were 3.2 +/- 0.9 cm and after LETZ were 2.6 +/- 0.9 cm, with a difference of 0.7 +/- 0.4 cm. The pathologic specimen (mean +/- S.D.) was 0.6 +/- 0.3 cm. The correlation between ultrasound and pathology measurement was r = 0.9 (p = 0.0001). Mean (+/- S.D.) cervical length was shortened by 22 +/- 12%. There is excellent correlation between ultrasound and ruler measurement of the cervix. There is significant cervical length shortening after LETZ, but further study is needed to see whether this persists or leads to pregnancy complications.

Single transvaginal sonographic measurement of cervical length early in the third trimester as a predictor of preterm delivery

To evaluate the accuracy of a single sonographic measurement of cervical length early in the third trimester as a predictor of preterm delivery. A total of 771 women attending the antenatal clinic at the Maharaj Nakorn Chiang Mai Hospital between January 1, 1990, and November 30, 1993, with singleton gestations, cervical competence, accurate dates of last menstrual period, and gestational ages between 28-30 weeks, were recruited into the study. Forty-one were excluded, ten because of induced preterm delivery, 24 because of measurement problems, and seven because they were lost to follow-up. We analyzed the remaining 730 women. Ninety-one (12.5%) women ended with preterm births and the remaining 639 (87.5%) delivered at term. The mean (+/- standard deviation) cervical lengths of the term and preterm groups were statistically significantly different at 37 +/- 5 and 34 +/- 6 mm, respectively (P < .001). The likelihood ratio of cervical length at various cutoff points was calculated. The appropriate cutoff point based on the receiver operating characteristic curve (35 mm) was associated with a significantly increased likelihood of preterm delivery (20 versus 7%) and was detected in two-thirds of preterm births. This cutoff point gave a sensitivity and specificity of 65.9 +/- 5.1% (95% confidence interval [CI]) and 62.4 +/- 5.2% (95% CI), respectively. A single transvaginal sonographic measurement of cervical length at 28-30 gestational weeks can be used to predict the risk of preterm delivery, using a cutoff point of 35 mm, but its cost-effectiveness should be assessed further.

Evaluation of Threatened Preterm Delivery by Transvaginal Ultrasonographic Measurement of Cervical Length

To establish a nomogram for the length of the uterocervical canal and to determine whether this can be used to predict preterm delivery.Cervical length was measured by transvaginal ultrasonography in 32 women (21 primigravid, 11 multigravid) with threatened preterm delivery, and in 177 normal singleton pregnancies between 18-37 weeks' gestation. Regression analysis was used to create the nomogram. Student t test was used to compare the groups.A linear relationship was found between cervical length and gestational age (r = -0.4, P < .001). Comparison of cervical length on admission in the patients with threatened preterm delivery showed that 11 preterm deliveries occurred in women who had a mean cervical length of 23.2 mm (range 17-29), whereas 21 term deliveries occurred in women with a mean cervical length of 31.7 mm (range 21-42). The difference was significant (P < .001). A cervical length of less than 20 mm on admission had a positive predictive value of 100%. These patients had preterm delivery despite tocolytic therapy during hospitalization.The risk of preterm delivery is high in women whose cervical length on admission is less than 30 mm, and strict management is required for those with a cervical length of less than 20 mm.