Objective: The subjectivity and inter- and intra-observer variability of transvaginal (TV) digital examination in the diagnosis of fetal head engagement set a real problem in the strategic choice of the mode of delivery. We conducted a preliminary study on the feasibility of using ultrasound in the diagnosis of fetal head engagement during labour.
Material and method: From 3rd January to 20th February 2007, a prospective monocentered study was set up by comparing the fetal head position in the pelvic cavity obtained by TV digital examination with the ultrasound measurement of the perineum-fetal head distance. Sixty-five measurements were obtained from 45 patients (single pregnancies, cephalic presentations), during labour and/or at complete cervical dilatation, by a single operator.
Results: Whenever the perineum-fetal head distance was greater than 60 mm, the fetal head was not engaged in the pelvic cavity, with a specificity of 89% and a negative predictive value of 94.1%. However, if the distance is less or equal to 60 mm, the fetal head was engaged with a sensitivity of 97.8% and a positive predictive value of 95.6%. It was even possible to define the fetal head station in the cavity as the distances are now known: high cavity : 50 mm, mid cavity : 38 mm, low cavity : 20 mm.
Conclusion: This technique based on a single distance measurement is not difficult and the reference point is easily localized. An abdominal probe is sufficient, making the diffusion of this method quite easy. With its excellent negative predictive value, transperineal ultrasound would allow obstetricians avoid difficult vaginal extractions, localize correctly the fetal position in the cavity and obtain the exact fetal head orientation, even in case of scalp blood humps. The feasibility and relevance of this technique must be confirmed by larger studies.