Engaging diameter of vertex presentation is:
A. Suboccipito frontal
B. Suboccipito bregmatic
C. Submento bregmatic
D. Mento vertical
Correct Answer: B. Suboccipito bregmatic
Explanation:
Engaging diameter of vertex presentation: Suboccipito-bregmatic diameter (9.5 cm)
Explanation
In vertex presentation, the fetal head is well flexed. Because of this good flexion, the smallest anteroposterior diameter of the fetal head presents at the pelvic inlet.
- The diameter that lies between the undersurface of the occiput (suboccipital region) and the center of the bregma is called the suboccipito-bregmatic diameter.
- Its length is 9.5 cm, which is the smallest diameter of the fetal skull.
- Since engagement occurs when the widest transverse diameter (BPD) and the corresponding AP diameter pass through the pelvic inlet, the suboccipito-bregmatic diameter is the engaging diameter in a normal vertex presentation.
Key point to remember
- Good flexion → smallest diameter → easier engagement
- Vertex presentation → Suboccipito-bregmatic diameter (9.5 cm
Other Options Details
These are important diameters of the fetal skull, commonly asked in obstetrics.
I’ll describe each option, its measurement, presentation, and clinical significance.
A. Suboccipito-frontal diameter
- Extent: From the suboccipital region (below occiput) to the centre of the frontal bone
- Length: ≈ 10 cm
- Seen in: Incomplete flexion of head (deflexed vertex)
- Clinical importance:
- Larger than the normal engaging diameter
- Labour may be slightly difficult compared to well-flexed head
B. Suboccipito-bregmatic diameter ✅ (Most important)
- Extent: From suboccipital region to centre of bregma
- Length: 9.5 cm
- Seen in: Well-flexed vertex presentation
- Clinical importance:
- Smallest AP diameter of fetal skull
- Ideal for normal vaginal delivery
- This is the engaging diameter in vertex presentation
C. Submento-bregmatic diameter
- Extent: From submentum (under chin) to bregma
- Length: 9.5 cm
- Seen in: Face presentation with complete extension
- Clinical importance:
- Vaginal delivery possible if chin is anterior
- If mento-posterior → vaginal delivery not possible
D. Mento-vertical diameter
- Extent: From chin (mentum) to highest point of vertex
- Length: 13.5–14 cm
- Seen in: Brow presentation
- Clinical importance:
- Largest diameter of fetal skull
- Cannot engage in pelvis
- Vaginal delivery not possible → usually cesarean section
Quick comparison table
Diameter Length Presentation Vaginal Delivery Suboccipito-bregmatic 9.5 cm Vertex (flexed) Easy Suboccipito-frontal 10 cm Deflexed vertex Slightly difficult Submento-bregmatic 9.5 cm Face (complete extension) Possible if mento-anterior Mento-vertical 14 cm Brow Not possible
Key exam tip 🩺
- Best diameter for normal delivery: Suboccipito-bregmatic
- Worst / largest diameter: Mento-vertical
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Writer: Vandita Singh, Lucknow (GS India Nursing Group