Reference range for cervical length in twin pregnancies

Background. Twin pregnancies are at increased risk of preterm delivery, and cervical length measurement by transvaginal ultrasound examination has been shown to be an effective method of screening for this complication. The aim of the present study is to establish a reference range for cervical length measurement throughout gestation in twin pregnancies. Methods. Prospective study involving 144 nonselected twin pregnancies that delivered after 34 weeks and underwent ultrasound examination every 4 weeks for transvaginal measurement of cervical length. Mean cervical length measurements, 95% confidence intervals for the estimate and 95% prediction intervals were established throughout gestation by linear regression analysis. Results. There was a significant correlation between cervical length measurements and gestational age [cervical length (mm) = - 0.758 * gestation age (weeks) + 56.69, r =- 0.43, p < 0.0001], and mean cervical length shortened by approximately 0.8 mm per week, from 47 mm at 13 weeks to 32 mm at 32 weeks. The corresponding lower limits for the 95% prediction intervals were 29 and 15 mm, respectively. Conclusion. Cervical length shortens gradually throughout gestation in twin pregnancies.