Head entering pelvis. Biparietal diameter is indicated with shading (9.25 cm). A, Suboccipitobregmatic diameter: complete flexion of head on chest so that smallest diameter enters. B, Occipitofrontal diameter: moderate extension (military attitude) so that large diameter enters. C, Occipitomental diameter: marked extension (deflection) so that largest diameter, which is too large to permit head to enter pelvis, is presenting.