In a face presentation, the chin (mentum) is the denominator.
A left or right mentoanterior position is most common, occuring in 80% of face presentations.
Persistent mentoposterior makes vaginal delivery almost impossible.
As the mentoanterior face descends, the submentobregmatic diameter (9.5 cm) passes through the cervix. A short rotation of the foetal head brings the chin to rest behind the pubic arch. The head is born by a flexion movement.
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