Sonographic parameters for diagnosing fetal head engagement during labour

The purpose of this study was to investigate the diagnostic performance of the head-perineum distance, angle of progression, and the head-symphysis distance as intrapartum ultrasound parameters in the determination of an engaged fetal head. Two hundred and one women in labour underwent both ultrasound and digital vaginal examination in the estimation of fetal head station. The transperineal ultrasound measured head-perineum distance, angle of progression, and head-symphysis distance for values correlating with digital vaginal examination head station. Using station 0 as the minimum level of head engagement, correlating cut-off values for head-perineum distance, angle of progression, and head-symphysis distance were obtained. Receiver operating characteristics were used in determining the diagnostic performance of these cut-off values for the detection of fetal head engagement. With head-perineum distance of 3.6 cm the sensitivity and specificity of sonographic determination of engaged fetal head were 78.7 and 72.3%, respectively. A head-symphysis distance of 2.8 cm also had sensitivity and specificity of 74.5 and 70.8%, respectively, in determining engagement, whilst an angle of progression of 101 degrees was consistent with engagement by digital vaginal examination with 68.1% sensitivity and 68.2% specificity. Ultrasound shows high diagnostic performance in determining engaged fetal head at a head-perineum distance of <= 3.6 cm, head-symphysis distance of <= 2.8 cm, and angle of progression of >= 101 degrees.