SINGLE TRANSVAGINAL SONOGRAPHIC MEASUREMENT OF CERVICAL LENGTH EARLY IN THE 3RD TRIMESTER AS A PREDICTOR OF PRETERM DELIVERY

Objective: To evaluate the accuracy of a single sonographic measuremen t of cervical length early in the third trimester as a predictor of pr eterm delivery. Methods: A total of 771 women attending the antenatal clinic at the Maharaj Nakorn Chiang Mai Hospital between January 1, 19 90, and November 30, 1993, with singleton gestations, cervical compete nce, accurate dates of last menstrual period, and gestational ages bet ween 28-30 weeks, were recruited into the study. Forty-one were exclud ed, ten because of induced preterm delivery, 24 because of measurement problems, and seven because they were lost to followup. We analyzed t he remaining 730 women. Results: Ninety-one (12.5%) women ended with p reterm births and the remaining 639 (87.5%) delivered at term. The mea n (+/- standard deviation) cervical lengths of the term and preterm gr oups were statistically significantly different at 37 +/- 5 and 34 +/- 6 mm, respectively (P < .001). The likelihood ratio of cervical lengt h at various cutoff points was calculated. The appropriate cutoff poin t based on the receiver operating characteristic curve (35 mm) was ass ociated 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 a 5.2% (95% CI), respectively. Conc lusion: A single transvaginal sonographic measurement of cervical leng th at 28-30 gestational weeks can be used to predict the risk of prete rm delivery, using a cutoff point of 35 mm, but its cost-effectiveness should be assessed further.