The mechanism of labour is not possible in which of the following presentation:

PGIMER Chandigarh NO - 2015

Obstetrics & Gynaecology

Medium

Appeared in: PGIMER Chandigarh NO - 2015

Explanation

A brow presentation occurs when the fetal head is in a state of partial extension, midway between a flexed vertex and a fully extended face presentation.
This attitude causes the mentovertical diameter, the largest anteroposterior diameter of the fetal skull (measuring approximately 13.5 cm), to engage in the pelvis.
This diameter is significantly larger than the dimensions of a normal maternal pelvis, creating an absolute cephalopelvic disproportion.
Consequently, if the brow presentation persists and does not convert to a vertex or face presentation, the mechanism of labor cannot proceed, and vaginal delivery is impossible.

Option A: In a face presentation, the head is fully extended. If the chin (mentum) is anterior, the presenting diameter is the submentobregmatic (9.5 cm), which is favorable for vaginal delivery.
Option B: Right Occipito-Posterior (ROP) is a malposition, not a malpresentation. It is a variation of the vertex presentation. Labor may be longer and more difficult, but a mechanism of labor exists, often involving internal rotation to an anterior position.
Option D: Breech presentation is a malpresentation where the buttocks or feet present first. It has its own distinct mechanism of labor. Although it carries higher risks and often results in a cesarean delivery, vaginal delivery is possible.

Clinical Relevance

Nursing practice connection: This is primarily an exam-oriented knowledge point with limited direct bedside application, so retain This question tests the knowledge of fetal presentations, attitudes, and the corresponding diameters of the fetal skull, specifically identifying which presentation leads to cephalopelvic disproportion and makes the mechanism of labor impossible as background academic context rather than a clinical decision trigger.
A brow presentation is often unstable and may spontaneously convert to a vertex (flexion) or face (extension) presentation during labor. Close monitoring of labor progress is essential.
If a brow presentation is diagnosed and persists, especially in a term fetus, it is an indication for cesarean delivery to prevent obstructed labor and associated maternal and fetal complications.
Nursing interventions include monitoring for signs of obstructed labor (e.g., failure to descend, abnormal contraction patterns), assessing fetal well-being, and preparing the patient for a potential cesarean section.

How to Approach the Question

First, understand the question is asking for a presentation where the natural process of labor is mechanically impossible.
Recall the common fetal presentations: vertex (occiput), face, brow, and breech.
For each cephalic presentation (vertex, face, brow), recall the attitude of the head (flexed, extended) and the resulting presenting diameter.
Compare the sizes of these diameters: suboccipitobregmatic (vertex, ~9.5 cm), submentobregmatic (face, ~9.5 cm), and mentovertical (brow, ~13.5 cm).
Identify that the mentovertical diameter is the largest and is generally too large to pass through the maternal pelvis.
Conclude that a persistent brow presentation makes the mechanism of labor impossible, making it the correct answer.

Concept Tested & Keywords

Concept Tested: This question tests the knowledge of fetal presentations, attitudes, and the corresponding diameters of the fetal skull, specifically identifying which presentation leads to cephalopelvic disproportion and makes the mechanism of labor impossible.
Stem keywords: mechanism of labour, presentation
Lead-in keywords: not possible
Negative lead-in flag: The question uses the negative phrase 'not possible', requiring the identification of the fetal presentation that absolutely obstructs labor.

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E6 Obstetrics Williams p. 38-56

E6 Midwifery Obstetrics and Gynecology Nursing Sandeep Kaur p. 43-51