Question
| A. |
Occipitofrontal diameter |
| B. |
Suboccipitofrontal diameter |
| C. |
Mentovertical diameter |
| D. |
Suboccipitobregmatic diameter |
|
Correct Answer » C Explanation |
 |
Clinical Reasoning for the Correct Answer:
– In marked deflexion of the fetal head (“brow presentation”), the chin is not tucked and the brow (forehead) presents first.
– The engaging diameter in this position is the mentovertical diameter (measured from the chin to the highest point on the vertex: 13.5 cm).
– This is the largest diameter of the fetal skull and makes vaginal delivery very difficult or impossible.
Why Option A is Incorrect (Occipitofrontal diameter):
– Occipitofrontal diameter (11.5 cm) presents in a deflexed (not markedly deflexed) vertex presentation.
– Classic for moderate deflexion, but not as severe as brow presentation.
Why Option B is Incorrect (Suboccipitofrontal diameter):
– Suboccipitofrontal diameter (10 cm) presents in a partially flexed head, as seen in some vertex positions.
– Not associated with marked deflexion or brow presentation.
Why Option D is Incorrect (Suboccipitobregmatic diameter):
– Suboccipitobregmatic diameter (9.5 cm) is the engaging diameter when the head is fully flexed (vertex presentation).
– This is the optimal position for vaginal delivery, not marked deflexion.
Types of Head Presentations and Corresponding Diameters
| Presentation | Head Position | Presenting Diameter | Measurement |
|---|---|---|---|
| Well-flexed (Vertex) | Chin on chest | Suboccipitobregmatic | ~9.5 cm |
| Incomplete flexion | Partial extension | Suboccipitofrontal | ~10 cm |
| Marked deflexion (deflexed vertex) | Nearly extended head | Mentovertical | ~13.5–14 cm |
| Complete extension (face) | Head fully extended | Submentobregmatic | ~9.5 cm |
| Brow presentation (partial extension) | Forehead presents | Mentovertical (largest) | ~13.5 cm |
