Role of mid-trimester transvaginal cervical ultrasound in prediction of preterm delivery

Ultrasonographic cervical length assessment is increasingly being utilized clinically to identify women at risk for spontaneous preterm delivery. In a randomised prospective longitudinal study involving 200 women, we measured cervical length and internal os diameter by transvaginal scan at 20 - 24 weeks and analysed their ability to predict preterm birth. The risk of spontaneous preterm delivery increased steeply as cervical length decreased. At cut off value of < or = 2.5cm, the cervical length measurements had sensitivity, specificity, positive predictive value and negative predictive value of 77%, 95%, 56% and 98% respectively. However, internal os diameter lacked sensitivity and specificity. Our data suggests that the duration of pregnancy is directly related to length of the cervix: the shorter the cervix, the greater the chance of preterm delivery.