Sonographic longitudinal cervical length measurements in nulliparous women at term; prediction of spontaneous onset of labour

Introduction The aim of this study was to predict spontaneous onset of labour by serial transvaginal ultrasound in a homogenous population of nulliparous women at term.

Methods One hundred sixty two nulliparous women with a singleton fetus in cephalic position were examined at weekly intervals from 36 weeks of gestation to delivery. Cervical length (CL) was measured by transvaginal ultrasound in supine and upright position.

Results There was a significant decrease in CL in the last twelve days prior to delivery, however this decrease was small with a substantial variation between individuals. Women with a spontaneous onset of labour could be divided into three different groups: unchanged CL until shortly before delivery, a drop in CL in the last two weeks prior to delivery, and a gradual change in CL starting before the last two weeks prior to delivery. A single CL measurement smaller than 30mm in supine and upright position between 37 and 38 weeks of gestation was predictive for spontaneous onset of labour before 41 weeks of gestation (sensitivity 46%; specificity 78%; PPV 82%; NPV 40% and sensitivity 53%; specificity 72%; PPV 81%; NPV 40% respectively).

Conclusion Between 37 and 38 weeks of gestation, spontaneous onset of labour before 41 weeks of gestation can be predicted by a CL measurement, but with low sensitivity and NPV. Inter-individual variation in CL and in CL changes is large. This hampers the predictive value of single and repeated CL measurements for the spontaneous onset of labour.