The largest diameter of the fetal skull is:
A. Mento-vertical
B. Occipito-frontal
C. Submento-vertical
D. Submento-vertical
Correct Answer: A. Mento-vertical
Explanation
The largest diameter of the fetal skull is the mento-vertical diameter.
Explanation
- The mento-vertical diameter extends from the chin (mentum) to the highest point of the vertex (mid-parietal region).
- Its length is about 13.5–14 cm, making it the largest anteroposterior diameter of the fetal skull.
Clinical significance
- This diameter presents when the fetal head is in brow presentation.
- Because it is very large, it cannot pass through the normal maternal pelvis, so vaginal delivery is usually impossible unless the presentation changes (to face or vertex).
Comparison with other diameters
- Occipito-frontal: ~11.5 cm
- Submento-vertical: ~11.5 cm
- Suboccipito-bregmatic: ~9.5 cm
✅ Therefore, the correct answer is: Mento-vertical diameter.
Other Options Details
These terms refer to antero-posterior diameters of the fetal skull, important in obstetrics for assessing the mechanism of labour.
Below is the explanation of each option:
A. Mento-vertical diameter
- Extent: From the chin (mentum) to the highest point of the vertex
- Length: ≈ 13.5–14 cm
- Presenting head attitude: Brow presentation (partial extension)
- Obstetric importance:
- This is the largest AP diameter
- Vaginal delivery is usually impossible
B. Occipito-frontal diameter
- Extent: From the glabella (root of nose) to the external occipital protuberance
- Length: ≈ 11.5 cm
- Presenting head attitude: Deflexed vertex
- Obstetric importance:
- Larger than suboccipito-bregmatic
- Labour is more difficult but may still be possible
C. Submento-vertical diameter
- Extent: From the junction of neck and chin (submentum) to the highest point of the vertex
- Length: ≈ 11.5 cm
- Presenting head attitude: Incomplete extension of head
- Obstetric importance:
- Seen in brow → face transition
- Vaginal delivery is usually difficult
D. Submento-vertical diameter
- This is the same as option C
- Likely a repetition/printing error
- Features and significance remain identical
Quick comparison table
Diameter Length Head attitude Delivery Mento-vertical ~14 cm Brow Impossible Occipito-frontal ~11.5 cm Deflexed Difficult Submento-vertical ~11.5 cm Partial extension Difficult
I hope that you liked this article.
Thanks!! 🙏 😊
Writer: Vandita Singh, Lucknow (GS India Nursing Group