Repeat the thing. For the mother, fetus left occipito anterior and right occipito anterior are normal. That is, here, the lie is longitudinal, the attitude is flexed, complete flexion. The position is LOA and ROA. The presentation is vertex, and presenting part is posterior part of the parietal bone. The denominator is occiput. The engaging diameter is subpubic diameter is 9.5, and transverse diameter is 13 centimeter. In LOA and ROA, first one is engagement with the diameter, and descent and flexion, and internal rotation of the head, one by eighth of the circle it is rotating. That is, head will turns to anterior side, and the shoulder will turn zero by eight. And you see the next step is crowning. After that, extension of the head and restitution of the head by one by eighth of the circle, and internal rotation of the shoulder one by eighth of the circle, and external rotation of the head one by eighth of the head. Anterior shoulder delivers by posterior flexion and posterior shoulder delivers by anterior flexion. This is for LOA and ROA. Like that, you give vertex presentation for LOL, LOP, ROL, ROP, and face presentation, LMAP, and brow presentation, LABP, and shoulder presentation, breech presentation, compound presentation, and cord presentation. How you will deliver? Same this order I need for all the six positions of the denominator for all presentation, vertex, face, shoulder etc. If it is not possible to deliver only LscS solution means, give the title and how the baby will deliver in one word. In detail, if anything wrong in above statement means kindly correct and i getting confusing that when head rotates what happened to shoulder, because some time 1/8 th sometimes 2/8 th you teach from loa and cover all presentation, break down and teach me, start
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I now have excellent textbook content from Roberts & Hedges and Tintinalli's to build this comprehensive answer. Let me now write the full, detailed breakdown.
Complete Mechanism of Labour - All Presentations: Corrected & Detailed
First, let me correct the errors in your statement, then teach each position systematically with the head-shoulder rotation relationship made crystal clear.
CORRECTIONS TO YOUR STATEMENT
| Your Statement | Correction |
|---|---|
| "Subpubic diameter is 9.5 cm is the engaging diameter" | The engaging (entering) diameter is the suboccipitobregmatic (SOB) = 9.5 cm. Subpubic diameter is a maternal pelvic diameter. The transverse engaging diameter is the biparietal diameter (BPD) = 9.5 cm, not 13 cm. |
| "Transverse diameter is 13 cm" | The engaging transverse diameter is the BPD = 9.5 cm. The 13 cm is the bitemporal + parietal width - actually the occipitomental or it refers to the transverse diameter of the pelvic inlet (13 cm). In vertex, the biparietal (9.5 cm) engages, not 13 cm. |
| "Presenting part is posterior part of the parietal bone" | In vertex (LOA/ROA), the presenting part is the posterior part of the anterior parietal bone - more precisely it is the vertex region (area bounded by the two parietal eminences, anterior and posterior fontanelles). |
| "Anterior shoulder delivers by posterior flexion" | Anterior shoulder delivers by lateral flexion of the trunk downward (toward the floor), and posterior shoulder delivers by lateral flexion upward. The terms "posterior flexion" and "anterior flexion" are not standard - use lateral flexion. |
| "Internal rotation of head 1/8 circle, shoulder turns 0/8" | CORRECT for LOA/ROA - see detailed explanation below. |
THE HEAD-SHOULDER ROTATION RULE (Your Confusion Cleared)
This is the most confusing part. Here is the logic:
The head and shoulders are connected by the neck. The fetal body does NOT rotate during descent - only the head rotates to pass through the pelvis.
At the time of internal rotation of the head:
- The head rotates to fit the pelvic outlet (antero-posterior)
- The shoulders remain in their original oblique diameter of the inlet
- So head rotates but shoulders do NOT rotate yet = shoulders rotate 0/8
At the time of restitution + external rotation:
- After the head is born, it "untwists" the neck
- The shoulders NOW internally rotate to bring the shoulder diameter into the AP diameter of the outlet
- This is why head does external rotation and shoulders do internal rotation together = 1/8 each
Summary of the rule:
| Step | Head | Shoulders |
|---|---|---|
| Internal rotation (before birth) | Rotates 1/8 (LOA/ROA) or 2/8 (LOL/ROL) or 3/8 (LOP/ROP) | 0/8 - stays put |
| Restitution | Rotates back 1/8 (undoes the neck twist) | 0/8 |
| External rotation | Rotates another 1/8 | Internally rotates 1/8 (now aligns with AP outlet) |
Why different degrees for different positions?
The head must ALWAYS end up with the occiput under the symphysis pubis (occipito-anterior) to deliver by extension. The degrees of rotation depend on how far the occiput starts from the anterior position:
- LOA/ROA - occiput is already 1/8 away from anterior → rotates 1/8
- LOL/ROL - occiput is 2/8 (transverse) away → rotates 2/8
- LOP/ROP - occiput is 3/8 (posterior) away → if it rotates forward it goes 3/8, if it does long rotation posteriorly it's considered a malposition
VERTEX PRESENTATION - ALL 6 POSITIONS
Fetal Lie, Attitude, Denominator (Common to All Vertex)
- Lie: Longitudinal
- Attitude: Complete flexion
- Presentation: Vertex
- Presenting part: Posterior part of the anterior parietal bone (vertex area)
- Denominator: Occiput
- Engaging diameter: Suboccipitobregmatic (SOB) = 9.5 cm (AP), Biparietal = 9.5 cm (transverse)
1. LOA - Left Occipito-Anterior (NORMAL)
Setup: Occiput points to left iliopectineal eminence. Head enters the left oblique diameter of the pelvis.
| Step | Head | Shoulders | Detail |
|---|---|---|---|
| 1. Engagement | Enters left oblique diameter | In right oblique diameter | BPD (9.5 cm) passes pelvic brim |
| 2. Flexion | Chin touches chest | No change | SOB (9.5 cm) becomes presenting diameter |
| 3. Descent | Descends into pelvis | Follow passively | Due to contractions + bearing down |
| 4. Internal rotation | Rotates 1/8 anticlockwise → occiput now under symphysis | 0/8 - no rotation | Occiput moves from left anterior → directly anterior |
| 5. Crowning | Head visible at introitus without recession between contractions | - | Perineum distends |
| 6. Extension | Head extends around symphysis pubis | - | Face sweeps perineum; vertex, brow, face, chin born |
| 7. Restitution | Head rotates 1/8 clockwise back to left | 0/8 | Undoes neck twist; occiput returns to face left |
| 8. External rotation of head | Rotates another 1/8 clockwise (total 2/8 from midline) | Internally rotates 1/8 anticlockwise | Shoulders come into AP diameter of outlet |
| 9. Delivery of anterior (left) shoulder | - | Delivered by lateral flexion of trunk downward | Anterior shoulder sweeps under symphysis |
| 10. Delivery of posterior (right) shoulder | - | Delivered by lateral flexion of trunk upward | Posterior shoulder over perineum |
| 11. Delivery of trunk & legs | By lateral flexion | - | Spontaneous expulsion |
Net head rotation before birth: 1/8
Net shoulder rotation: 1/8 (at external rotation step)
2. ROA - Right Occipito-Anterior (NORMAL)
Mirror image of LOA.
| Step | Head | Shoulders |
|---|---|---|
| Engagement | Enters right oblique diameter | In left oblique diameter |
| Flexion | Complete | - |
| Descent | Downward | - |
| Internal rotation | 1/8 clockwise → occiput anterior | 0/8 |
| Crowning | At introitus | - |
| Extension | Head born | - |
| Restitution | 1/8 anticlockwise back to right | 0/8 |
| External rotation | 1/8 anticlockwise | Internally rotates 1/8 clockwise |
| Anterior (right) shoulder | Lateral flexion downward | - |
| Posterior (left) shoulder | Lateral flexion upward | - |
Net head rotation before birth: 1/8
3. LOL - Left Occipito-Lateral (TRANSVERSE)
Setup: Occiput points directly to the left iliac fossa. Head enters the transverse diameter.
| Step | Head | Shoulders |
|---|---|---|
| Engagement | Enters transverse diameter | In AP diameter |
| Flexion | Complete | - |
| Descent | Downward | - |
| Internal rotation | 2/8 anticlockwise → occiput anterior | 0/8 |
| Crowning | At introitus | - |
| Extension | Head born | - |
| Restitution | 1/8 clockwise | 0/8 |
| External rotation | 1/8 clockwise (occiput faces left) | Internally rotates 1/8 |
| Anterior shoulder | Lateral flexion downward | - |
| Posterior shoulder | Lateral flexion upward | - |
Net head rotation before birth: 2/8
Restitution is always 1/8 (just undoing the neck twist from the LAST 1/8 of internal rotation)
4. ROL - Right Occipito-Lateral (TRANSVERSE)
Mirror image of LOL.
| Step | Head | Shoulders |
|---|---|---|
| Engagement | Enters transverse diameter | In AP diameter |
| Internal rotation | 2/8 clockwise → occiput anterior | 0/8 |
| Extension | Head born | - |
| Restitution | 1/8 anticlockwise | 0/8 |
| External rotation | 1/8 anticlockwise | Rotates 1/8 |
| Shoulder delivery | As above | - |
Net head rotation before birth: 2/8
5. LOP - Left Occipito-Posterior (MALPOSITION)
Setup: Occiput faces the left sacroiliac joint. This is a malposition. Two routes possible:
Route A: Long rotation (forward rotation - favourable, more common)
| Step | Head | Shoulders |
|---|---|---|
| Engagement | Left oblique diameter | Right oblique |
| Flexion | Complete (essential) | - |
| Descent | Downward | - |
| Internal rotation | 3/8 anticlockwise → occiput rotates all the way to anterior | 0/8 |
| Extension | Head born face down (occiput anterior) | - |
| Restitution | 1/8 clockwise | 0/8 |
| External rotation | 1/8 clockwise | 1/8 |
| Shoulders | As normal | - |
Net head rotation before birth: 3/8
Route B: Short rotation (posterior - unfavourable)
| Step | Head | Shoulders |
|---|---|---|
| Engagement + Descent | As above | - |
| Internal rotation | 1/8 clockwise → occiput directly posterior (under sacrum) | 0/8 |
| Delivery | By increased flexion then extension - face to pubis delivery | - |
| Restitution | 1/8 anticlockwise | 0/8 |
| External rotation | 1/8 anticlockwise | 1/8 |
Net head rotation before birth: 1/8 (short rotation)
Associated with persistent occipitoposterior (POP) - prolonged labour, deep transverse arrest, instrumental delivery
6. ROP - Right Occipito-Posterior (MALPOSITION)
Mirror image of LOP.
Long rotation (favourable):
- Internal rotation: 3/8 clockwise → anterior
- Restitution: 1/8 anticlockwise
- External rotation: 1/8 anticlockwise + shoulders 1/8
Short rotation (face to pubis):
- Internal rotation: 1/8 anticlockwise → directly posterior
- Delivery face to pubis
FACE PRESENTATION
- Lie: Longitudinal
- Attitude: Complete EXTENSION (opposite of vertex)
- Presentation: Face
- Presenting part: Face (between chin and orbital ridges)
- Denominator: Mentum (chin)
- Engaging diameter: Submentobregmatic = 9.5 cm (when fully extended)
- Normal positions: LMAP (Left Mento-Anterior Posterior - actually LMA, RMA are favourable), LMP/RMP are unfavourable
LMAP - Left Mento-Anterior (Normal, deliverable)
| Step | Head | Shoulders |
|---|---|---|
| Engagement | Enters oblique diameter, fully extended | - |
| Descent | With full extension maintained | - |
| Internal rotation | Chin (mentum) rotates anteriorly to under symphysis (1/8 or 2/8 depending on position) | 0/8 |
| Delivery | By flexion (opposite to vertex - head flexes around symphysis after chin is out) | - |
| Restitution | 1/8 | 0/8 |
| External rotation | 1/8 | 1/8 |
| Shoulders | As normal vertex | - |
Key difference from vertex: In vertex, delivery is by extension. In face, delivery is by flexion - because the chin (denominator) is anterior under the pubis, the head sweeps down and flexes.
Mento-Posterior (LMP/RMP) - CANNOT DELIVER VAGINALLY
- If chin is posterior, the forehead hits the sacrum → cannot flex further → impacted
- Management: Caesarean section (LSCS)
BROW PRESENTATION
- Lie: Longitudinal
- Attitude: Partial extension (between vertex and face)
- Presentation: Brow
- Presenting part: Area between orbital ridge and anterior fontanelle
- Denominator: Frontum (frontal bone) - sometimes called Mentoverticalis presentation
- Engaging diameter: Mentovertical = 13.5 cm (LARGEST - too big for normal pelvis)
- Position stated: LABP (Left Antero-Brow Presentation)
Mechanism
Brow presentation is the most unfavorable of all cephalic presentations because the mentovertical diameter (13.5 cm) is too large to pass through a normal pelvis (pelvic inlet ~13 cm transverse, ~11 cm AP).
| Outcome | Condition |
|---|---|
| Converts to vertex | Increased flexion (most common, favourable) |
| Converts to face | Increased extension (deliverable if mento-anterior) |
| Persists as brow | LSCS - cannot deliver vaginally in most cases |
LABP (Left Antero-Brow Position) delivery: In a large pelvis or small baby, rare spontaneous delivery possible by a complex mechanism similar to face, but standard management = LSCS for persistent brow.
SHOULDER PRESENTATION (TRANSVERSE LIE)
- Lie: Transverse
- Attitude: Variable
- Presentation: Shoulder
- Presenting part: Shoulder, arm, or trunk
- Denominator: Acromion process
- Engaging diameter: No single diameter engages normally
Mechanism of Delivery
There is NO mechanism of normal vaginal delivery for shoulder presentation at term.
| Method | Indication |
|---|---|
| LSCS | Standard at term |
| Spontaneous version | Very rare, premature/macerated fetus only |
| Internal podalic version + breech extraction | Only in second twin in transverse lie, under anaesthesia |
| Neglected shoulder presentation | Impacted → LSCS even then; old teaching of "conduplicato corpore" only in macerated fetus |
In one word: LSCS
BREECH PRESENTATION
- Lie: Longitudinal
- Attitude: Variable (flexed = complete breech; extended legs = frank breech; footling = incomplete)
- Presentation: Breech
- Presenting part: Buttocks (complete/frank), foot (footling)
- Denominator: Sacrum
- Engaging diameter: Bitrochanteric = 10 cm (for buttocks)
Types and Mechanism
Frank Breech (Extended Legs)
| Step | Buttocks/Trunk | Head (aftercoming) |
|---|---|---|
| Engagement | Bitrochanteric (10 cm) enters oblique | - |
| Descent | Buttocks descend | - |
| Internal rotation | Anterior hip under symphysis | - |
| Lateral flexion | Posterior hip born first, then anterior hip | - |
| Legs birth | Legs spring free | - |
| Shoulders | Engage in oblique, rotate to AP | - |
| Head (last) | Aftercoming head - most dangerous | Enters pelvis last; flexed |
| Delivery of head | By Burn-Marshall method or Mauriceau-Smellie-Veit manoeuvre | - |
Key danger: The aftercoming head (largest part) must pass through a pelvis already distended by the trunk. No time for moulding → risk of head entrapment.
Standard at term: LSCS (based on Term Breech Trial)
Vaginal breech delivery: Only in selected cases (frank breech, adequate pelvis, experienced operator)
Complete Breech
Same as frank but legs are flexed - slightly more mobile.
Footling/Incomplete Breech
LSCS - cord prolapse risk very high.
COMPOUND PRESENTATION
- Definition: Two parts present simultaneously (e.g., head + hand, head + arm)
- Commonest: Head with prolapsed hand
- Denominator: Same as the leading part (occiput if vertex + hand)
Mechanism
| Outcome | Management |
|---|---|
| Hand replaces itself spontaneously | Observe - most common with head |
| Hand persists | Gently push hand up during contraction |
| Arm/shoulder compounded | LSCS |
| Cord prolapse (high risk in compound) | Emergency LSCS |
Simple compound (head + hand): Often delivers vaginally after repositioning the hand.
Complex compound (trunk parts): LSCS
CORD PRESENTATION / PROLAPSE
- Definition: Umbilical cord lies in front of the presenting part
- Cord presentation: Membranes intact, cord felt anterior to presenting part
- Cord prolapse: Membranes ruptured, cord descends past presenting part into vagina or outside
Management
This is an obstetric emergency - cord compression → fetal hypoxia → death within minutes.
| Step | Action |
|---|---|
| 1 | Call for help immediately |
| 2 | Knee-chest position (Trendelenburg or exaggerated Sims) |
| 3 | Elevate presenting part manually (two fingers in vagina) |
| 4 | Do NOT replace cord - wrap in warm saline-soaked cloth if outside |
| 5 | Emergency LSCS - fastest route to delivery |
| 6 | If cervix fully dilated + vertex → immediate forceps/ventouse |
| 7 | Bladder filling (500 mL saline) to elevate presenting part while preparing for LSCS |
Delivery: Emergency LSCS (in one word, if vaginal delivery not immediately possible)
MASTER SUMMARY TABLE
| Presentation | Position | Denominator | Engaging Diameter | Internal Rotation | Restitution | External Rotation | Shoulder Rotation | Delivery Mode |
|---|---|---|---|---|---|---|---|---|
| Vertex | LOA | Occiput | SOB 9.5 cm | 1/8 ant | 1/8 | 1/8 | 1/8 | SVD |
| Vertex | ROA | Occiput | SOB 9.5 cm | 1/8 ant | 1/8 | 1/8 | 1/8 | SVD |
| Vertex | LOL | Occiput | SOB 9.5 cm | 2/8 ant | 1/8 | 1/8 | 1/8 | SVD (may need assistance) |
| Vertex | ROL | Occiput | SOB 9.5 cm | 2/8 ant | 1/8 | 1/8 | 1/8 | SVD (may need assistance) |
| Vertex | LOP | Occiput | SOB 9.5 cm | 3/8 ant (long) or 1/8 post (short) | 1/8 | 1/8 | 1/8 | SVD (long) / Instrumental / LSCS |
| Vertex | ROP | Occiput | SOB 9.5 cm | 3/8 ant (long) or 1/8 post (short) | 1/8 | 1/8 | 1/8 | SVD (long) / Instrumental / LSCS |
| Face | LMA/RMA | Mentum | Submentobregmatic 9.5 cm | To anterior | 1/8 | 1/8 | 1/8 | SVD (by flexion) |
| Face | LMP/RMP | Mentum | Submentobregmatic 9.5 cm | Cannot rotate anteriorly | - | - | - | LSCS |
| Brow | Any | Frontum | Mentovertical 13.5 cm | Usually cannot | - | - | - | LSCS (or conversion) |
| Shoulder | Any | Acromion | None | None | None | None | None | LSCS |
| Breech | LSA/RSA etc. | Sacrum | Bitrochanteric 10 cm | 1/8 (buttocks) | - | - | Head last | LSCS / Assisted breech |
| Compound | Head+hand | Occiput | SOB 9.5 cm | As vertex | As vertex | As vertex | 1/8 | SVD if hand retracts / LSCS if complex |
| Cord prolapse | Any | - | - | - | - | - | - | Emergency LSCS |
THE GOLDEN RULE SIMPLIFIED
Head rotates ALONE during internal rotation. Shoulders rotate ONLY at external rotation step - always 1/8, regardless of position. Restitution is always 1/8 (just undoing the neck twist from the final 1/8 of internal rotation). The variable part is INTERNAL ROTATION of the head: 1/8 (anterior), 2/8 (transverse), 3/8 (posterior).
Sources: Roberts & Hedges' Clinical Procedures in Emergency Medicine, p. 1351; Tintinalli's Emergency Medicine, p. 2011-2014
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