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Instructions for dictation by the trainers for maternal pelvis, foetal skull and
mechanism of labour in occipito – anterior and posterior positions in vertex, face
and breech presentations.
Maternal pelvis
Pelvis is anatomically divided into false and true pelvis boundary line being brim of pelvis,
its bony landmarks from anterior to posterior are upper border of pubic symphysis, pubic
crest, pubic tubercle, pectineal line, ilio pubic eminence, ilio pectineal line, sacro-iliac
articulation, anterior border of ala of sacrum and sacral promontory
1. Hold the pelvis with a forward tilt so that the brim makes an angle of 55˚ with the
horizontal plane.
Anterior superior iliac spine and pubic symphysis should lie in the same vertical plane
Upper border of pubic symphysis and tip of the coccyx should be in the same horizontal
plane.
2. To understand the mechanism of labour in occipito – anterior and posterior
positions mark the important diameters in the pelvis with the right index finger.
3. True pelvis is divided into inlet, cavity and outlet.
In the plane of inlet show the antero-posterior diameter, the anatomical conjugate
extends from upper border of pubic symphysis to midpoint of sacral promontory
and is 12cm, the obstetric conjugate from mid-point of sacral promontory to the
nearest point on the posterior surface of symphysis pubis. It is 10.5cm.
Clinically it can be indirectly measured by placing the tip of the middle finger on the
sacral promontory and the base of the index finger on the inferior border of
symphysis pubis. This is the diagonal conjugate and is about 11.5 – 12cm. If one
subtracts 1.5 – 2cm from this measurement one can indirectly get to know the
obstetric conjugate.
Show the transverse diameter of the inlet which is the distance between the farthest points
on the ilio –pectineal line and is 13cm
Show the oblique diameter of the inlet from one sacro –iliac joint to the ilio – pectineal
eminence of the opposite side. The side is designated according to the sacro – iliac joint as
the right or left. It is 12.5cm
In the pelvic cavity the least pelvic diameter is between the two ischial spines and is 10cm.
internal rotation takes place at this level.
In theoutlet, the transverse diameter of the outlet referred to as TDO lies between the inner
border of ischial tuberosities, which is 11cm. It is the shortest diameter of the outlet. It is
measured by placing 4 knuckles between the two tuberosities. Pubic arch formed by
descending pubic rami of both sides measured clinically by placing 3 fingers side by side
Fetal skull
Hold the fetal skull and show vertex. Place two index fingers in front and thumbs
behind. Tilt the head towards the observers. Take out the right finger and show the
boundaries with the right index finger. Anteriorly, coronal sutures and bregma i.e. anterior
fontanelle. Posteriorly lambdoid sutures and lambda i.e.posterior fontanelle. Laterally by
the line passing through the parietal eminence.
Show the denominator in vertex presentation, the occiput. This enters the pelvic brim first.
Brow is bounded on one side by anterior fontanelle and coronal sutures and on other side
by root of nose and supraorbital ridges on either side
Face is bounded on one side by root of nose and supraorbital ridges and on other side by
junction of floor of the mouth with neck.
Place the skull with the occiput in front and the base facing upwards. Put the left
middle finger in foramen magnum. Place the index and ring fingers by the side. Hold the
chin with the thumb. Show flexion attitude by bending the middle finger, keep chin closer
to the palm. Show the sub – occipito bregmatic diameter landmark with the right index
finger—from the nape of the neck to the centre of bregma. This is the engaging AP diameter
in well flexed head and is 9.5cm
Show the biparietal diameter between the two parietal eminences, this is 9.5cm and
this transverse diameter crosses the pelvic brim first and is referred to as diameter of
engagement.
Mechanism of labour in LOA position
Demonstrate mechanism of labour in vertex presentation.
Anterior position is common – 70% of cases of vertex presentation. (LOA > ROA)
To show LOA position, hold the skull in left hand and pelvis in right hand. Place the
denominator, occiput against left iliopectineal eminence. Keep sagittal suture in right
oblique diameter and biparietal diameter in left oblique diameter.
In many cases occiput lies in LOT position where sagittal suture is in transverse diameter of
pelvis.
Quite often it does not stay exactly over the transverse diameter of pelvic inlet. It inclines
posteriorly towards sacrum, where the posterior parietal bone is the leading portion
referred to as posterior asynclitism. Posterior asynclitism / Litzman’s obliquity is more
common in primigravidae. In Anterior asynclitism/ Naegle’s obliquity anterior parietal
bone is the leading bone, it is more common in multigravidae. Mild to moderate asynclitism
is accepted as normal.
Now show the first cardinal movement, the engagement. Biparietal diameter should have
crossed the pelvic brim and occiput should have reached the ischial spine level.
Engagement is not a part of labour. It occurs 2-3 weeks earlier in primigravidae and it
occurs with onset of labour in multigravidae.
As the head descends there will be increased flexion. Once occiput reaches ischial spine
level it rotates through 45˚ (1/8thof acircle) to hitch against symphysis pubis.
At the obstetric outlet the sub occipital region lies under pubic arch. BPD stretches the
vulval outlet.
Now show the delivery of the head. By the process of extension successive parts of the
head, the vertex, bregma, fore head, nose, mouth and chin are born. Head drops out. Chin
lies close to the maternal anal opening.
Once the head is born, show the next step, restitution. Here the neck untwists and
restitutes back through 45˚(1/8th of a circle) to the original position in the pelvis.
Now show the external rotation. Rotation of the head by further 45˚(1/8th of a circle) in the
same direction as in restitution. Occiput lies towards the maternal left thigh and the face
points to right thigh. This external movement of the head corresponds to the internal
rotation of shoulder in the pelvic cavity. Now show the delivery of the shoulders.
Bisacromial diameter of the shoulder comes to lie in AP diameter of the outlet. Anterior
shoulder hitches behind symphysis pubis; posterior shoulder sweeps over the perineum
and delivers first by lateral flexion. The rest of trunk is delivered by lateral flexion.
Mechanism of labour in ROP position
To demonstrate occipito –posterior position keep the right thumb over the occiput. When
the head is partially deflexed sub occipito frontal diameter is the engaging diameter, 10cm,
it extends from sub – occiput (nape of neck) to midpoint of sinciput. When the head is
completely deflexed it is referred as military position, occipito – frontal diameter is the
diameter of engagement i.e from the occiput to the glabella (root of the nose). It is longer,
11.5cm.
Place the occiput against right sacro – iliac joint, BPD in the left oblique diameter to show
engagement. In majority of cases with full flexion occiput rotates anterior. First, 45˚(1/8 th
of a circle) to ROT position. In some cases arrest can take place at this level. In others it
rotates further 45˚(1/8th of a circle) total 90˚(2/8th of a circle) to become ROA position. At
this juncture change your thumb to the chin for easy demonstration. Show further anterior
rotation of 45˚(1/8th of a circle)total 135˚(3/8th of a circle), occiput hitches behind the
symphysis pubis.
Delivery of the head is just like LOA. The only difference is the restitution and external
rotation. They occur to the right side. Shoulder and trunk deliver in a similar manner as in
LOA.
Face to pubis delivery
Now demonstrate face to pubis delivery. In 10% of cases in ROP the occiput rotates
45˚(1/8th of a circle) posteriorly to the sacrum.
In partially deflexed head the middle of sinciput i.e. the forehead hitches against symphysis
pubis. By process of flexion, bregma, vertex and occiput are delivered. Then by process of
extension the brow face and chin are delivered. Occiput lies close to maternal anus.
In a complete deflexed head the root of the nose, glabella hitches against the symphysis
pubis. By the process of flexion forehead, bregma, vertex and occiput get delivered and
then by process of extension face and chin are delivered.
After delivery of head, restitution, external rotation, delivery of shoulders and trunk are in
the similar manner as anterior position.
Mechanism of labour in LMA position
Show the denominator in face presentation, the mentum. This enters the pelvic brim
first.
Place the skull with the mentum in front and the base facing upwards. Put the left
middle finger in foramen magnum. Place the index and ring fingers by the side. Place the
thumb over the occiput. Show extension attitude by bending the middle finger, keep
occiput closer to the palm. Show the sub – mento bregmatic diameter landmark with the
right index finger—from the floor of the mouth to the centre of bregma. This is the
engaging AP diameter in completely extended head and is 9.5cm
Show the Biparietal diameter between the two parietal eminences, this is 9.5cm and
this transverse diameter crosses the pelvic brim first and is referred to as diameter of
engagement.
Anterior position is common – 70% of cases of face presentation. {LMA > RMA}
To show LMA position, hold the skull in left hand and pelvis in right hand. Place the
denominator, mentum against left iliopubic eminence. Keep sagittal suture in right oblique
diameter and biparietal diameter in left oblique diameter.
In many cases mentum lies in LOT position where sagittal suture is in transverse diameter
of pelvis.
Now show the first cardinal movement, the engagement. Biparietal diameter should have
crossed the pelvic brim and mentum should have reached the ischial spine level.
As the head descends there will be increased extension. Once mentum reaches ischial spine
level it rotates through 45˚(1/8th of a circle ) to hitch against symphysis pubis.
At the obstetric outlet the mentum lies under pubic arch. BPD stretches vulval outlet.
Now show the delivery of the head. By the process of flexion successive parts of the head,
the mouth, nose, forehead, bregma, vertexocciput and chin are born. Head drops out.
Occiput lies close to the maternal anal opening.
Once the head is born, show the next step, restitution. Here the neck untwists and
restitutes back through 45˚(1/8th of a circle) to the original position in the pelvis.
Now show the external rotation. Rotation of the head by further 45˚(1/8th of a circle) in the
same direction as in restitution. Face lies towards the maternal left thigh and the occiput
points to right thigh. This external movement of the head corresponds to the internal
rotation of shoulder in the pelvic cavity. Now show the delivery of the shoulders.
Bisacromial diameter of the shoulder comes to lie in AP diameter of the outlet. Anterior
shoulder hitches behind symphysis pubis; posterior shoulder sweeps over the perineum
and delivers first by lateral flexion. The rest of trunk is delivered by lateral flexion.
Mechanism of labour in LMP position
30% of face presentations are posterior. 70% rotate anteriorly. The flexed counterpart of
posterior face is the anterior occiput and thus LMP flexes to ROA and RMP to LOA.
Persistent posterior face presentations become arrested because they cannot deliver
spontaneously. This is because the relatively short neck cannot clear off the total length of
sacrum and as such the thorax is thrust in, resulting inthe sternobregmatic diameter
(18cm) to occupy the pelvis. As a result, the labour becomes inevitably obstructed.
Mechanism of labour in brow presentation
No mechanism of labour as presenting diameter is mentovertical 13.5cm.