Objective: The aim of this study was to predict spontaneous onset of labor by serial transvaginal ultrasound measurement of cervical length (CL) in a homogeneous population of nulliparous women at term.
Methods: 162 nulliparous women with singleton fetuses in cephalic presentation were examined at weekly intervals from 36 weeks' gestation to delivery. CL was measured by transvaginal ultrasound in the supine and upright positions.
Results: There was a significant decrease in CL in the last 12 days prior to delivery. However, this decrease was small with substantial variation between individuals. Women with spontaneous onset of labor could be divided into three different groups: those with unchanged CL before delivery; those with a fall in CL in the last 2 weeks prior to delivery; and those with a gradual change in CL starting before the last 2 weeks prior to delivery. A single CL measurement below 30 mm between 37 and 38 weeks of gestation predicted spontaneous onset of labor before 41 weeks' gestation with a sensitivity of 46%, specificity of 78%, positive predictive value (PPV) of 82%, negative predictive value (NPV) of 40% in the supine position; and sensitivity of 53%, specificity of 72%, PPV of 81%, NPV of 40% in the upright position.
Conclusion: Between 37 and 38 weeks' gestation, spontaneous onset of labor before 41 weeks can be predicted by a CL measurement, but with low sensitivity and NPV. Inter-individual variations in CL and in CL changes are large, which hampers the value of single and repeated CL measurements for the prediction of spontaneous onset of labor at term.