Engagement of the fetal head is a determinant element when deciding on operative vaginal delivery. In routine practice, engagement is a clinical diagnosis based on transvaginal digital examination. Transperineal ultrasound might provide complementary information useful for measuring the fetal head-perineum distance (HPD). The purpose of this work was to determine the cutoff HPD distinguishing engagement from non-engagement.

This single-center prospective study approved by the institutional review board was conducted between December 25, 2012 and August 31, 2015 in 411 nulliparous women; 20 did not provide informed consent and were excluded; analysis concerned 391 patients. Clinical diagnosis – engagement or non-engagement depending on results of the transvaginal digital examination (Farabeuf's and Demelin's signs) – was compared with the ultrasound HPD measurement.

The clinical diagnosis was non-engagement at complete dilatation in 96 patients (24.6%). The cutoff HPD distinguishing between engagement and non-engagement was 57mm (AUC 83.5% [95%CI 79.3–87.8]), with 75.0% [65.5–82.6] sensitivity, 75.9% [70.7–80.5] specificity, 50.3% [42.2–58.4] positive predictive value, and 90.3% [86.0-93.4] negative predictive value.

In this series, the HPD cutoff distinguishing between engagement and non-engagement was 57mm. Below this cutoff level, the head should be considered engaged, beyond non-engaged. Nevertheless, the pertinence of this cutoff level is hampered by the imprecision of the gold standard used for the clinical diagnosis (transvaginal digital examination). In case of doubt, we recommend, in addition to considering the obstetrical setting, to combine transperineal ultrasound with transvaginal digital examination to avoid deleterious failure of operative vaginal delivery.