MATERNAL & FETAL PELVIS
THE BONY PELVIS
RH1
Presenter.SAMUEL MUTURI N
BSC.CLINICIAN-NAKURU PROVINCIAL GENERAL HOSPITAL- RH AMD INTERNAL MEDICINE.
BSC.CLINMEDICINE/SURGERY & COMMUNITY HEALTH-M.K.U
M.P.H (EPIDMIOLOGY AND INFECTIOUS DS CONTROL)-M.K.U
A.CHEM-UON
ACLS & BLS-NHCPS
PAIN MANAGEMENT-KPA
CVDS-TOT KCS
SPIROMETRY-RESOK
RAPID RESPONSE,TRACING AND DATA MANAGEMENT- W.H.O
INFECTIOUS DS CONTROL AND CRITICAL CARE MX FOR COVID -19-W.H.O
COVID MX AND RESPONSE TEAM-NAKURU PGH
LEARNING OUTCOMES
• By the end of the lesson, the student should be able ;
 to describe the bony pelvis
 identify the important features for obstetric care
 Describe types of pelvis
Identify the pelvic inlets in relation to delivery.
To identify and perform pelvic assessment.
Describe the fetal skull bones, sutures and fontanelles and their
importance for labor and delivery
Relate the significance of the bony pelvis, the fetal skull and labor
outcome.
Introduction
• We will learn about the bony structures with the most importance for
the pregnant woman and the baby she will give birth to.
• The skeleton bones have 2 main functions;
•
supporting our body weight
•
acting as attachment points for our muscles.
• Our focus will be understanding the female pelvis for purposes of
clinical practice, labor process, appreciate mechanisms of sexual
function, reproduction and gynecological pathologies
Introduction
• The female pelvis supports the major load of the pregnant uterus,
and allows the fetal skull to pass through the woman’s pelvis when
giving birth.
• There are certain key landmarks in the anatomy of the female pelvis
and the fetal skull that will be shown in this study session.
• Knowing these landmarks will enable you as a clinician to estimate
the progress of labour, by identifying changes in their relative
positions as the baby passes down the birth canal.
Introduction
• Normal or Obstructed labour depends on the pelvis, and the presenting
diameters of a normal fetal head.
• This can be determined by knowledge, understanding and abilities to
perform objective pelvic assessment.
• The topic aims to summarize the important aspects of the anatomy of the
pelvis and the fetal skull
• This should be known to any health care provider or skilled service
provider dealing with maternal health.
• Many of the investigations and treatments we will order on a daily basis
require good anatomical knowledge in order to be properly understood.
Introduction
• The bony pelvis forms the basis for pelvic assessment and projecting
labour outcomes in relation to the fetal skull.
• The pelvis is a ring composed of the two innominate or hip bones
which are joined anteriorly at the symphysis pubis and posteriorly to
the sacrum and coccyx
• The pubic symphysis is a unique joint consisting of a fibrocartilaginous
disc sandwiched between the articular surfaces of the pubic bones
THE BONY PELVIS
• The pelvis is a hard ring OR GIRDLE of bones formed by the sacrum
and 2 innominate bones
FUNCTIONS
1. Supports and protects the pelvic organs(bladder, uterus, ovaries,
lower bowel )and the contents of the abdominal cavity(intestines
etc).
2. Support the weight of the upper body
3. transmits stresses weight bearing to the lower limbs through the
acetabulae
4. Provides firm attachment of several muscles- muscles of the legs,
back and abdomen are attached to the pelvis including the sphincters
of the bladder and anus.
Their strength and power keeps the body upright and enable it to bend
and twist at the waist, and to walk and run.
5. It forms the bony margins of the birth canal,accommodating passage
of the fetus during labour
Bony Pelvis
• Is composed of pairs of bones fused together so tightly that the joints
are difficult to see.
• It consists of two hip bones/ innominate, sacrum and coccyx
BONY PELVIS
• the innominate(each consists of the ileum , ischium and pubis fused
together) joined together by the sacrum posteriorly and the
symphysis pubis anteriorly.
• Laterally is the acetabulum that articulates with head of femur
• The pubis and ischium forms the obturator foramen
• The coccyx lies on the inferior aspect of the sacrum.
• The Pelvic canal through which the fetus must pass during child birth
consists of the brim, cavity and outlet.
• Female pelvis is adapted for child bearing, and is a wider and flatter
shape than the male pelvis
The Ilium
• The major portion of the pelvis is composed of two bones, each called
the ilium — one on either side of the backbone (or spinal column)
and curving towards the front of the body.
• When you place your hand on either hip, your hand rests on the iliac
crest, which is the upper border of the ilium on that side.
• At the front of the iliac crest, you can feel the bony protuberance
called the anterior superior iliac spine (a ‘protuberance’ is something
that sticks out, like a little hill or knob).
• Anterior tells you that the iliac spines are at the front of the body, and
superior tells you that they are above the main portion of the ilium on
each side
Ischium
• It is the thick lower part of the pelvis formed from two fused bones —
one on either side.
• When a woman is in labour, the descent of the fetal head as it moves
down the birth canal is estimated in relation to the ischial spines.
• Ischial spines are inward projections of the ischium on each side.
• They are smaller and rounder in shape in the woman’s pelvis than in
that of the man. That means they are flat.
• The cervix is at the level of the ischial spines.
• Clinical IMPORTANCE .. During pelvic assessment palpate them
determine whether flat or sharp
• Clinical significance of ischial spines
• When performing digital examination, while assessing the adequacy
of the pelvis at the mid pelvic cavity, turn your fingers on the sides
lateral to the cervix in order to determine whether they are flat or
sharp.
• Flat ischial spines roughly mean the antero-positerior diameter and
tranverse diameter are equal and adequate
Pubic bones and the symphysis pubis
• The pubic bones on either side form the front part of the pelvis.
• Two pubic bones meet in the middle at the pubic symphysis- a very
strong bony joint.
• The pubic symphysis is immediately below the hair-covered pubic
mound that protects the woman’s external genitalia
Pubic bones and the symphysis pubis
• Clinical importance .
• When you examine the abdomen of a pregnant woman, feeling for
the top of the pubic symphysis with your fingers is a very important
landmark.
• It is a landmark to measure fundal height from the pubic symphysis
to the fundus This measurement enables you to estimate the
gestational age of the fetus, i.e. how many weeks of the pregnancy
have passed, and whether the fetus is growing at the normal rate in
relation to gestational age by dates.
Pubic bones and the symphysis pubis
• at the level of the pubic symphysis is 12 weeks
• at the umbilicus is 24
• at the xyphisternum is 36 weeks
• Below is 40 weeks( after lightening)
Pubic bones and the symphysis pubis
• Descend – we measure using finger to determine how many parts of
the presenting part are felt above the pelvic brim
• whole head above=5/5
• no head felt above=0/5 meaning the whole head is inside the pelvis
read to be delivered
Pubic bones and the symphysis pubis
• Clinical importance
• We determine engagement and descend in reference to the pubic
symphysis
• Engagement- When the head is fixed into the pelvis; may occur two
weeks prior to labour depending on the type of pelvis and fetal size
• Descend- downward progression of the presenting part normally the
head into the pelvis that occurs during labour under the influence of
strength of contractions
• Expressed in fifths(1/5,2/5…5/5 and used to determine progress of
labour
Sacrum.
• Is a tapered, wedge-shaped bone at the back of the pelvis
• Consisting of five fused vertebrae (the small bones that make up the
spinal column or backbone).
• Bound to ilium via sacroiliac joints
• At the bottom of the sacrum is a tail-like bony projection called the
coccyx. The upper border of the first vertebra in the sacrum sticks
out, and points towards the front of the body; this protuberance is
the sacral promontory
• sacral promontory — an important landmark for labour and delivery.
• The sacral promontory is used as a landmark for assessing the pelvic
inlet, the anteroposterior diameter.
• Ideally it should not be easily tipped
• . Each individual should be able to determine the length as there is
variation due to our differences in height hence the length.
coccyx
• it articulates with the sacrum inferiorly in a movable articulation.
• it is mobile
• Physiologic mobility of the coccyx is important for sexual and
reproductive reasons, as it allows higher mobility and elasticity of
the levator ani, partially inserted to the coccyx.
• When rigid or lesioned, it may contribute to chronic pelvic
pain and dyspareunia.
• Physiologic retropulsion of the coccyx increases the diameter of the
lower pelvic and gives more room to permit the passage of the fetus
during labour
The pelvic canal
• Is the roughly circular space enclosed by the pubic bones at the front,
and the ischium on either side and sacrum at the back
• the bony passage through which the baby must pass.
• The canal has a curved shape because of the difference in size
between the anterior (front) and posterior (back) borders of the
space created by the pelvic bones. You can see it from the side view
in figure below
The pelvic canal
• The pelvic canal
• It has two portions namely;
1. False pelvis / pelvic minor/upper pelvis
2. True pelvis / pelvic major / lower
• False pelvis or pelvic minor supports abdominal organs.
• It consists of the space bounded by ;
2 iliac fossae laterally
lower parts of the lumbar spines and sacral spines posteriorly
iliopectineal line inferiorly
• True pelvis / pelvic major or lower portion
• It lies below the iliopectineal line.
Superiorly is iliopectineal line.
Anterior boundary are the pubic bones,
laterally the ischium
posterior is the coccyx.
• The baby passes through the true pelvis during delivery.
• The pelvis has pelvic brim/pelvic inlet, a mid cavity and an outlet
• the pelvic brim is the lower parts of the innominate bones and the
sacrum
• it is outlined by sacral promontory, the sacral alae, the iliopectineal
lines and the pubis anteriorly
The pelvic inlet
• The pelvic inlet is formed by the pelvic brim bounded at the back by
the promontory and the alae of the sacrum, and infront by the pubic
bones.
• The pelvic brim is rounded
• Except where the sacral promontory and the ischial spines project
into it.
• The dimensions in centimeters (cm) of the pelvic inlet are
top to bottom known as antero-posterior(AP ) diameter;
 transverse diameter from side to side.
• Averagely AP is 13 cm wide and 12 cm transverse diameter.
• When you look at the figure, imagine that you are a baby in the headdown position, looking down on the pelvis from above, at the space
you must squeeze through!
Pelvic landmarks
1 - Sacral Promotory
2 - Sacral ala (wing)
3 - Sacral iliac joint
4 - Illiopectineal
line5- Illiopectineal
eminence
6- Superior pubic
ramus
7- Body of pubic
bone
8- Symphysis pubis
Pelvic inlet
• A plane drawn between the sacral promontory and the superior
aspect of the symphysis pubis marks the pelvic inlet.
• The two most important diameters of the pelvic brim are:
• 1. The antero-posterior (AP) diameter from the upper border of the
symphysis pubis to the sacral promontory.
• The normal AP diameter measures 11- 12 cm.
• There are 3 conjugates recognized at the pelvic inlet important in
pelvic assessment:
(i) the true conjugate- measured from the promontory to the top of
the symphysis
(ii) the diagonal conjugate- measured from the promontory to the
bottom of the symphysis.
(iii) the obstetric conjugate- measured from the promontory to the mid
of symphysis
• The true conjugate(measured from the promontory to the top of the
symphysis on vaginal/pelvic assessment)
• 2. The transverse diameter is the widest part of the brim. It measures
13cm from the side to side
• 3. The oblique diameter
Diameter of the pelvic inlet
Diameters
Inlet
Cavity
Outlet anatomical
Anteroposterior
11
12
13
Oblique
12
12
12
Transverse
13
12
11
The Pelvic cavity/mid cavity:
• The pelvic cavity is a curved canal between the inlet and the outlet.
• In the normal female pelvis, the cavity is circular in shape and curves
forwards.
• All diameters measure approximately 12 cm( equal)
Inlet cavity
The Pelvic Outlet
• The pelvic outlet is formed by the lower border of the pubic bones at
the front and the lower border of the sacrum at the back.
• The ischial spines point into this space on both sides.
• It is diamond shaped
• Bounded anteriorly by the pubic arch which in the normal female
pelvis forms an angle of 900.
• It faces downward and slightly backward, is very irregular.
• Beginning anteriorly, it is bounded by;
• (1) the arcuate ligament of the pubis (in the midline)
• (2) the ischiopubic arch
• (3) the ischial tuberosity
• (4) the sacrotuberous ligament
• (5) the coccyx (in midline).
• Laterally the pelvic outlet is bounded by the ischial tuberosities.
DIAMETERS FROM BELOW
• Figure below shows the dimensions of the space that the fetus must
pass through as it emerges from the mother’s pelvis.
• As you look at the figure, imagine that you are the birth attendant
who is looking up the birth canal, waiting to see the fetal head
emerging.
• Its anteroposterior(AP) diameter is drawn from the lower border of
the symphysis pubis to the tip of the coccyx.
• The transverse diameter passes between the medial surfaces of the
ischial tuberosities.
• The smallest diameter is between the two ischial spines which
project into the outlet.
• The posterior landmarks of the pelvis are the coccyx and the
sacrotuberous ligaments.
• During birth the coccyx bends backwards to increase the diameter of
the pelvic outlet.
• The most important diameters of the pelvic outlet
• The transverse diameter measured between the two ischial
tuberosities
• Is normally 10.5 -11 cm (accommodates 4 knuckles on V.E)
• The antero posterior diameter measured from the apex of the pubic
arch to the sacrococcygeal joint .
• Is normally approximately 13cm
Compare the pelvic inlet and outlet dimensions
• The narrowest diameter for the fetus to pass through is the pelvic outlet,
which is only 11 cm wide in the average female pelvis.
• The fetus has to rotate in order to get through the pelvic canal.
• This is because the pelvic inlet is 13 cm wide, whereas the pelvic outlet is
only 11 cm wide.
• In order to fit through the pelvic outlet at its widest dimension (12.5 cm
from top to bottom), the fetus must rotate so it ‘presents’ its head to the
widest dimension of the pelvic cavity at every point as it passes through.
• The largest part of the fetus is the skull, so the baby’s head rotates first,
and the shoulders and the rest of the body follow.
• Clinically the ischial spine is important as it can be felt vaginally just
lateral to the level of the cervix, and progress in labour can be
measured using it as a landmark in the mid cavity or pelvic cavity.
• ischial spine is an insertion point of the sacrospinous ligament which
also attaches to the lower lateral part of the sacrum.
• Together with the sacrotuberous ligament and the bony pelvis, it
forms the borders of the greater sciatic foramen (through which the
sciatic nerve passes) and the lesser sciatic foramen (through which
the pudental nerve enters the pelvis).
Pubic arch
• Is located immediately inferior to the pubic symphysis and is formed
by the paired ischiopubic rami.
• The breadth of the pubic arch is measured as the subpubic angle.
FEMALE
•
•
•
•
•
•
•
•
Bones are lighter, thinner
False pelvis is shallow
Pelvic cavity is wide & shallow
Pelvic inlet round/oval
Pelvic outlet comparatively large
Subpubic angle large
Coccyx more flexible, straighter
Ischial tuberosities more everted
MALE
•
•
•
•
•
•
•
•
Bones heavier, thicker
False pelvis is deep
Pelvic cavity is narrow & deep
Pelvic inlet heart-shaped + smaller
Pelvic outlet comparatively small
Subpubic angle more acute
Coccyx less flexible, more curved
Ischial tuberosities longer, face
more medially
variation of pelvic shape
 Female pelvis shapes may be subdivided as follows
 1. Normal and its variants
- Gynaecoid – most common type , suited for delivery
- Android – the male type of pelvis
- Platypelloid – flat pelvis; short AP diameter & wide transverse
diameter
- Anthropoid – resembling that of anthropoid ape, AP diameter is greater
than the transverse
2. Symmetrically contracted pelvis
- That of a small women but with a symmetrical shape
3. Rachitic pelvis
- This deformity is caused by rickets (due to Vit D
deficiency)
- Sacrum is rotated so that the sacral promontory
projects forward and coccyx tips backward
- AP diameter of inlet is therefore narrowed but
the outlet is increased
4. Asymmetrical pelvis
- Asymmetry pelvis can be due to variety of
causes such as scoliosis, poliomyelitis, pelvic
fracture, congenital abnormality due to
thalidomide etc
Rachitic pelvis
Asymmetrical pelvis
Pelvic
Variations and
Abnormalities
Pelvic Types
Gynaecoid pelvis

Is a typical female pelvis. Ideal for vaginal delivery
 Found in 80 % of Asian women; 50-70 % white women

Rounded or slightly oval inlet

Straight pelvic sidewalls with roomy pelvic cavity

Good sacral curve

Subpubic arch is wide 90 degree
Android pelvis




Present in most male and also in few females
Heart shaped (or triangular) pelvic inlet - due to prominent sacrum
The problem in delivery head occiptoposterior most common
Narrow sub-pubic angle less than 90
Anthropoid pelvis

Present in some males and females
 15% in Asian women; 15-30% in white women
 Pelvic inlet is long oval
 AP diameter > transverse diameter
 Long & narrow sacrum
 Women with this type tend to be tall.
Less labor complications
Platypelloid pelvis


Uncommon in both sexes
Pelvic inlet appears slightly flattened (kidney shape)
 Transverse diameter is greater than AP diameter

Sacral promontory pushed forwards
Normal Pelvic Variants
Feature
Gynaecoid
Android
Brim
Round
Heart-shaped /
triangular
Long oval
Flat (kidney)
For pelvis
Generous
Narrow
Narrow
Wide
Side walls
Straight
Convergent
Divergent
Divergent
Ischial spine
Not prominent
(blunt)
Prominent
Not prominent
Not prominent
90
Subpubic
angle
Incidence in 50 %
Asian women
Less 90
20%
Anthropoid Platypelloid
>90
25%
> 90
5%
FETAL SKULL
The skull is formed from the face , the vault & the base
The bones that form the skull are : two frontal bones, two
parietal bones, two temporal bones wings of the sphenoid
& occipital bone
The bones of the face & base are heavy & fused
The bones of the vault are 2 frontal ,2 parietal & occipital
The bones of the vault are not joined thus changes in the
shape of the fetal head during labor can occur due to
molding
FETAL SKULL DEFINITIONS
Bregma 
• Ant fontanelle
Brow 
• lies between bregma &root of the nose
Face 
• lies between root of the nose & suborbital ridges
Occiput 
• boney prominence behind post fontanelle
Vertex 
• diamond shaped area between ant & post
fontanelles & parietal eminences
FETAL SKULL SUTURES
• Frontal suture 
between 2 frontal bones
• Sagittal suture 
between 2 parietal bones
• Coronal suture 
between parietal & frontal
• Lambdoid suture 
between parietal & occipital
• Temporal suture 
between inferior margin of the parietal &
temporal
FETAL SKULL FONTANELLES
• Anterior fontanelle 
diamond shaped space between
coronal & sagittal suture 3 * 3 cm ,
ossifies at 18 m
• Post font (lambda) 
triangle shaped space between sagittal
&
lambdoid suture
FETAL SKULL DIAMETERS
• Biparietal diameter 9.5 cm. 
between parietal eminences The greatest transverse
diameter
• Suboccipitobregmatic 9.5 cm. 
middle of the bregma to undersurface of the occipital
bone at the neck
The presenting diameter of the well flexed head in labour
• Suboccipitofrontal 10.5 cm 
root of the nose to undersurface of the occipital bone at
the neck
The presenting diameter of the partially flexed head
FETAL SKULL DIAMETERS
• Occipitofrontal 11.5 cm 
Root of the noose to the most prominent point of the
occiput
A defelexed head presents with this diameter
• Mentovertical 13 cm 
Chin to most prominent point of the occiput
The presenting diameter in brow presentation
The largest diameter of the fetal head
• Submentobregmatic 9.5 cm 
Chin to middle of bregma
The presenting diameter in face presentation
MOULDING OF THE HEAD
• Occurs with descent of the fetal head into the pelvis to reduce the
head circumference
• Frontal bones slip under parietal bones
• Parietal bones override each other
• Parietal bones slip under the occipital bone
MOULDING OF THE HEAD
DEGREE OF MOULDING
Assessed vaginally
• 0
suture lines are separate
• +1 
suture lines meet
• +2 
suture lines overlap but can be reduced by gentle
digital pressure
• +3 
overlap irreducible
End