Pelvis and fetal skull are main entities to learn mechanism of labour, having clear concepts regarding the pelvic diameters, there importance, and engaging diameters of fetal skull helps to learn and manage labour process in a better way.

1. Pelvis and fetal skull
Dr. Apurva Maheshwari

2. CONTENTS
● Maternal pelvis-
Parts
Relevent diameters
Clinical pelvimetry
● Caldwell-Moloy classification- types of pelvis
● Abdomino-vaginal method of assessing CPD
● Review of Fetal skull-
Parts
AP and transverse diameters
Engaging diameters

3. Maternal pelvis ● Bones
2 innominate bones (ilium, ischium, pubis)
Sacrum
Coccyx
● Joints
2 sacroiliac joint
Sacrococcygeal joint
Symphysis pubis
PELVIS WORKS AS A SINGLE UNIT
● True pelvis
Inlet- at the level of pelvic brim
Mid cavity -at the level of ischial spines
Outlet- at the level of ischial tuberosities
● False pelvis

4. Boundaries of pelvic brim / inlet-
1. Sacral promontory
2. Alae of the sacrum
3. Sacroiliac joints
4. Iliopectineal lines
5. Iliopectineal eminencies
6. Upper border of the
superior pubic rami
7. Pubic tubercles
8. Pubic crests and
9. Upper border of
symphysis pubis

5. Pelvic inlet
● Shape- most commonly gynecoid with AP diameter being shortest
● Inclination- in erect posture, the pelvis is tilted forward. The plane makes an angle of 55
degree with the horizontal and is called angle of inclination.
● Sacral angle- formed between true conjugate and first 2 pieces of sacrum
Sacralisation of lumbar vertebra-
Increased angle of inclination (high inclination)-
1. There is delay in engagement of head because the uterine axis
fails to coincide with the inlet.
2. It favours occipito-posterior position
3. There is difficulty in descent of head due to long birth canal and flat
sacrum interfering with internal rotation
Lumbarisation of first piece of sacral vertebra-
Low inclination and has got no obstetrical significance

6. Pelvic inlet- Diameters
11cm- Oblique diameter (extends from
sacroiliac joint of one side to opposite
iliopectineal eminence)
12 cm - AP diameter ( from pubic symphysis to
sacral promontory)
13cm- Transverse diameter( distance between
farthest points on iliopectineal line)
● It is the mid-perpendicular line drawn to
the plane of inlet.
● Direction- downward and backward
● When extended the line passes through
the umbilicus to coccyx.
● The uterine axis should coincide with the
axis of inlet so that force of the uterine
contractions will be spread in the right
direction to force the fetus pass through
the brim.
Axis

7. Antero-posterior Diameters of pelvis inlet
True conjugate-
●Measures-11cm
●Cannot be estimated directly
●Estimated by subtracting 1.2cm from diagonal
conjugate, thus allowing for the inclination, height and
thickness of pubic symphysis.
Obstetric conjugate- Distance between the mid point of
the sacral promontory to prominent bony projection in the
midline on the inner surface of PS (10cm)
Diagonal conjugate- Distance between lower border of
PS to sacral promontory (12cm)

8. Measurement of diagonal conjugate-
● Obstetric conjugate is obtained by
subtracting 1.5cm-2cm from the diagonal
conjugate
● How to measure?
-Fingers are reached till sacral promontory
-For clinical practice, if middle finger fails to
reach the sacral promontory or touches it
with difficulty, it is likely that the conjugate
is adequate for an average size head to
pass through

9. Transverse diameter
● Distance between the two farthest points
on the pelvic brim over the ilio-pectineal
lines(13cm)
● This diameter lies slightly closer to sacral
promontory and divides the brim into
anterior and posterior segment.
● The head negotiates the brim through a
diameter called the available or
obstetrical transverse (at the junction of
antero-posterior diameter and transverse
diameter), may be equal to or less than
transverse diameter.
● Extends from one sacro-iliac joint to
opposite iliopubic eminence
Oblique diameter

10. Cavity
● Segment of pelvis bounded above by the inlet and below by the plane of least pelvic dimensions
● Shape- round
● Plane – extends from the midpoint of posterior surface of pubic symphysis to the junction of the 2nd
and 3rd sacral vertebrae (called as plane of greatest pelvic dimensions)
Most roomy plane of pelvis
● Axis- it is the mid- perpendicular line drawn to the plane of the cavity

11. Mid pelvis- plane of least pelvic dimensions
●Extends from lower margin of symphysis-pubis to ischial spines, sacrospinous ligaments to 4th and 5th sacral vertebrae
AP diameter - 11.5cm
Transverse diameter - between 2 ischial spines - Inter- spinous diameter (10.5cm)
Posterior Sagittal diameter (6cm)-
between tip of sacrum and the midpoint of the bispinous diameter
Importance –
● Cardinal movements of fetal head (internal rotation occurs here)
● External os of cervix lies at this level
● Ischial spines presents at 0 station of fetal head
● Marks the beginning of forward curve of pelvic axis
● Pudendal nerve block is given at this level
● It is the narrowest plane in the pelvis
● Corresponds roughly to the origin of levator ani muscle

12. Pelvic axis – Curve of Carus  Anatomical pelvic axis is formed by joining
the axes of the inlet, cavity and outlet.
 It is uniformly curved with convexity fitting
into concavity of the sacrum
 It is through this axis that the fetus
negotiates the pelvis.
 It is uniformly not curved
 Its direction is first downward and
backward upto the level of ischial spines
and then directed abruptly forwards

13. Pelvic Outlet Shape- diamond-shaped
AP diameter - 12cm
Transverse diameter - between 2 ischial
tuberosities - Inter- tuberous diameter (10.5cm)
Posterior Sagittal diameter- 8.5cm

14. Sub-pubic angle
Formed by meeting of 2 descending pubic rami
Measures 85-90 degrees
Waste space of Morris-
● Distance between the pubic symphysis and edge of a round disc around- 9.5cm
(fetal head size) placed under sub-pubic arch.(should not exceed more than 1cm)
● Narrow sub-pubic angle - increase in waste space of morris-feta head accommodate
posterior sagittal diameter, resulting in more chances of perineal trauma

15. Caldwell-Moloy classification- types of pelvis

17. Disorders affecting pelvis

19. Asymmetrically
contracted pelvis

24. Fetal skull
Compressible to some extent and made of thin pliable tabular (flat) bones
forming the vault. Anchored by rigid and incompressible base

25. Landmarks of obstetrical significance

26. Sutures and fontanels
Sutures
Permits gliding movement of one bone
over the other during molding of the head
Digital palpation of sagittal suture during
internal examination in labor gives an idea
of the manner of the engagement of the
head (asynclitism / synclitism), grade of
molding and internal rotation

27. Fontanels
● Wide gap in the suture line
● Anterior fontanel
Diamond in shape
Ossified at 18 months after birth
Importance-
 Its palpation through vaginal examination denotes the degree of flexion of head
 It facilitates molding of head
 As it remains membranous long after birth, it helps in accommodating the marked brain growth
 Palpation of the floor reflects intracranial status- depressed in dehydration, elevated in raised
intracranial tension
 Collection of blood/ exchange transfusions via sup sagittal sinus can be done
● Posterior fontanel
Triangular in shape
Floor becomes bony at term
It denotes position of head in relation to maternal pelvis

28. AP Diameters of Fetal skull
The engaging
diameters depends on
the degree of the
flexion present .

31. Molding
●It is the alteration of the shape of the forecoming head while passing through the resistant birth passage during
labor
●Mechanism:
There is compression of engaging diameter of the head with corresponding elongation of the diameter at right
angle to it.
In well flexed head of the anterior vertex presentation,the engaging suboccipitobregmatic diameter is compressed
with elongation of the head and there is over-riding of parietal bones one over the other.
Molding disappears within few hours of birth
●Grading:
GRADE-1- The bones touching but not overlapping
GRADE-2- Overlapping but easily separable
GRADE-3- fixed over-lapping
Molding
Slight molding is inevitable and beneficial
Extreme molding as met in disproportion
may produce severe intracranial
disturbance in the form of tearing of
tentorium cerebelli or subdural hemorrhage.

32. Caput succedaneum
● It is the formation of swelling due to stagnation of
fluid in the layers of the scalp beneath the girdle of
contact
(The girdle of contact is either bony or the dilating cervix
or vulval ring)
● The swelling is diffuse, boggy and is no limited to
suture line
● May be confused with cephal-hematoma
● It disappears spontaneously within 24 hrs of birth
Mechanism
Head descend to press over the dialating
cervix or vulval ring
The overlying scalp is free from pressure ,but
the tissue in contact with full circumference
of the girdle of contact is compressed
This interferes with venous return and
lymphatic drainage from the unsupported
area of scalp
Stagnation of fluid and appearence of a
swelling in the scalp
Caput succedaneum
Signifies the static position of the head for
long time
Location of caput gives an idea about the
position of head occupied in the pelvis and
degree of flexion achieved.

33. Take home message
Maternal pelvis is a dynamic structure -
main role in mechanism of labour
Clinical pelvimetry
Fetal head –
The engaging diameters depends on the
degree of the flexion present .

34. Thank you...