Observations on the sonographic measurement of cervical length and the risk of premature birth.
OBJECTIVE: There is increasing evidence that sonographic measurement of cervical length may be a useful predictor of the risk of spontaneous premature birth. The purpose of this study is to determine whether the measurement of cervical length in a high-risk population at 24 weeks gestation, or the relative change over 24-28 weeks gestation, is more accurate in predicting the risk of spontaneous preterm birth before 35 weeks gestation. METHODS: Over a 4-year period from 1993-1996, 443 patients with a singleton pregnancy who were at increased risk of preterm birth were studied by serial endovaginal sonography performed at 24 and 28 weeks gestation. RESULTS: There was a positive association between a short cervix and increased risk of preterm birth (F = 13.3, P < .0001). The variable with the highest predictive value for preterm birth was the cervical length at 24 weeks gestation. Changes over time did not substantially improve the predictive accuracy for spontaneous preterm birth. CONCLUSIONS: We conclude that a short cervix as determined by endovaginal sonography has a significant association with preterm birth in a high-risk obstetric population. Measurements taken at 24 weeks gestation are most accurate in assessing this risk, and serial observations of the cervix over time have less accuracy for predicting preterm birth.