Correct Answer » B Explanation

Face Presentation

Face presentation occurs due to complete extension of the fetal head, making the face the presenting part.

Denominator

• Mentum (chin)

Presenting/Engaging Diameter

Submentobregmatic diameter

• Extends from:
  • Junction of chin and neck (submentum)
  • To the center of bregma
• Measures:
  • 9.5 cm

This is the diameter that engages in a true face presentation.

Explanation of Other Options

A. Mentovertical — Incorrect

• Presenting diameter in brow presentation
• Extends from:
  • Chin (mentum)
  • To highest point of vertex
• Measures:
  • 13.5 cm
• Largest AP diameter of fetal skull
• Usually incompatible with vaginal delivery

C. Occipitofrontal — Incorrect

• Seen in partially deflexed vertex presentation
• Diameter:
  • ~11.5 cm

D. Suboccipitobregmatic — Incorrect

• Diameter in well-flexed vertex presentation
• Measures:
  • 9.5 cm
• Smallest AP diameter for normal vaginal delivery