Bony Pelvis: Description, Structure & Functions

General description of bony pelvis

The bony pelvis is a strong bony ring linked to a tilted basin which protects and contains the female reproductive organs. The following are found within the pelvis: 

  • Uterus
  • Fallopian tube
  • Ovaries
  • Vagina
  • Bladder
  • Urethra
  • Pelvic colon
  • Rectum
  • Anal canal

It is through these route that a baby passes through during delivery.

Structure of the pelvis

The bony pelvis is made up of 4 bones these are:

  • Two innominate bones
  • One sacrum
  • One coccyx


These forms the anterior and lateral walls of the pelvis. Each innominate bones consists of:

  • Ilium
  • Ischium
  • Pubic bone


This is the largest flat plate of bone above and part of the acetabulum below.


  • The external aspect of the ilium is gently curved and has roughed surface to which are attached to gluteal muscle
  • The greater part of the inner aspect is smooth and concave and is known as iliac fossa.
  • The ridge which somount these two surfaces is called the iliac crest. It is shaped like enlongated letter S and serves for attachment of muscle of abdominal wall.

Anteriorly: the iliac crest ends in the anterior superior iliac spine

Posteriorly: it ends in the posterior superior iliac spine.

  • Below the anterior superior iliac spine is another bony 
  • prominence called anterior inferior iliac spine
  • At the lowest part of the ilium forms the ⅖ acetabulum where it fuses with the ischium and pubic
  • Behind the acetabulum, the ilium forms the upper part of a largest notch called the greater sciatic notch
  • On the inner aspect of the bone, the iliac fossa is bounded below by a prominate ridge called ilio pectineal line at a lower level, the ilium enters into the formation of the side wall of the pelvis where it forms the floor of the acetabulum anteriorly. The ilio pectineal line swells into a bony prominence at the point where the ilium fuses with the superior ramus of the pubis to form the ilio pectineal eminance.


This is the lowest of the constituent bone of the innominate bones. The shape is C curved.


  • The head forms the lowest 5th of the acetabulum where it fuses with the ilium and pubic
  • Below the acetabulum a thick butress of bones passes downwards and terminates at the ischia tuberlosity. It is this part of the pelvis that carries the weight of the body in the sitting position
  • Passing upwards and inwards forms ischia tuberlosity, from the ischia tuberosity a small shaft of the ischium becomes continuous with the ischia ramus of the pubic forming the pubic arch
  • The ischium forms the lowest boundary of the large foramen, the obturator foramen on the inner aspect. The ischium forms the side wall of the true pelvis protruding inwards from its posterior edge, above the tuberosity is a projection known as the ischia spine. Which separates the greater sciatic notche from the less sciatic notche. The ischia spines are important landmark in an obstetric since over projection of it obstruct the passage of the baby at the pelvis outlet.


This is the smallest of the innominate bones and it is the only one that articulate with it’s fellow on the opposite side.


  • It consists of a body and two wings known as superior and inferior rami, it is roughly V shaped.
  • The body of the pubis is square shaped with it’s median aspect articulating with the body of the femur forming a cartilaginous joint known as Symphysis pubis. The upper surface of the body forms a crest known as the pubic crest which terminates laterally to form pubic tubercle
  • The superior ramus passes upward and laterally forming ⅕ of the acetabulum where it fuses with the ilium and ischium. It’s junction with the ilium passes into the ilio pectineal eminance, the superior ramus complete the upper boundary of the obturator foramen
  • Below the body of the pubis, the inferior ramus passes downwards and outwards to join the ischium to form the upper part of the pubic arch.
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Is situated at the posterior part of the pelvis, therefore it forms the posterior wall of the pelvis. It is waged shaped


  • It consists of 5 sacral bones (vertebrae). Fused together, the anterior surface is smooth and concave from above downward and from side to side forming the hallow of the sacrum 
  • The first sacral vertebral over hanged the hallow of the circle and the central point of each upper projecting magin is known as Sacral promontory. This is a very important landmark in obstetric because it encoaches on anterior posterior diameter of the pelvic inlet. And in some cases may prevent the fetal head from entering the brim
  • The sacrum has 8 foramen situated on either side of the hollow of the sacrum. For the passage of sacral nerves
  • The posterior surface of the sacrum is rough and irregular for the attachment of muscles and ligaments of the back
  • Passing longitudinally through the center of the bone but near it posterior surface is the sacral canal which contains the sacral and coccygeal nerve derives from the spinal cord. These nerves are paralyzed when caudal block is been introduced during labour
  • On the upper lateral aspect of the sacrum are flat masses of bones known as the Alae of the sacrum
  • The lower end of the sacrum articulates with coccyx below.


This is a small bone which consists of 4 fused coccygeal vertebrae. It is trianglar in shape with it base at the upper most


  • The first coccygeal vertebral articulates with the lower end of the vertebra
  • The remaining three vertebrae are rudimentary noodles of bones. They are smooth at the inner surface where they support the rectum


There are 4 in number 

  1. Two sacro illiac joint
  2. One symphysis pubis joint
  3. Sacro coccygeal joint

The sacroilliac  Joint

The joint is formed at the articulates of the sacrum with the ilium in normal condition, very slight movement occurs at this joint. But during cyesis( pregnancy) and labour. This movement increases in range when the ligament becomes softened under the influence of the hormone relaxing

The symphysis pubis Joint

This joint is formed at the junction of the two pubic bones, which are United by a pad of cartilage. This joint widely appreciably during the later month of pregnancy and the degree of movement permit may give rise to pain on walking

The sacrococcygeal Joint

This joint is formed where the base of coccyx articulates with the tip of the sacrum and allows the coccyx to bend backwards during the actual birth of the head of the baby


There are 6 pelvic ligaments which are of important to midwife.

  • Supporting ligament
  • Sacro tuberous ligament
  • Sacro spineous ligament
  • Inguinal ligament
  • Lacuna ligament
  • Obturator membrane

Supporting Ligament

This is the strongest ligament in the whole body, they binds with the sacrum and ilium at the Sacro Illiac Joint

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Sacro Tuberous Ligament

It is a strong ligament passing from the posterior superior iliac spine and the lateral border of the sacrum and the coccyx to the ischial tuberosity. It bridges across the greater and lesser sciatic notches.

Note: the Sacrotuberous and spineous ligament forms the posterior wall of the pelvic outlet.

Sacrospineous Ligament

This pass from the sides of the sacrum and coccyx across the greater sciatic notch to the ischia spines. It is a strong ligament and lies in front of the Sacro tuberous ligament.

 Inguinal Ligament

This ligament runs from the anterior superior iliac spine to the pubic tubercle.

Lucuna Ligament

This is a small ligament which occupies the angle under the inner end of the inguinal ligament.

Obturator Membrane

This ligament closes the obturator foramen with the exception of small area in its upper part which transmit the obturator vessels and nerves and also contains the obturator lymphatic glands.


Obstetrically the bony pelvis is divided into two as follows:

  • False Pelvis
  • True  Pelvis

The False Pelvis

This is the part above the brim which consists mainly the flared out iliac bones. It has little obstetric importance.

The True Pelvis

This is the curved bony canal through which the fetus most pass during birth. It consists of the brim, the cavity and the outlet.




This is the first part of the Pelvic gland, it is rounded accept where sacral promontory encroaches into it. The promontory and the wings of the sacrum form its posterior border (alae of the sacrum). The iliac bone forms it’s lateral bounder while the pubic bone forms it’s anterior bounder. Obstetrically, the boundaries of the brim also known as its landmarks, are very important to midwife and these includes: 

  • Sacral promontory 
  • Alae of the sacrum
  • Sacroilliac Joint
  • Iliopectineal line
  • Iliopectineal eminance
  • Inner upper border of the superior ramus
  • Inner upper border of the pubic bone
  • Inner upper border of the symphysis pubis.


This is circular in shape and it’s anterior walls form by the posterior pubic bones and the symphysis pubis. It measures 4cm, the posterior walls measures 12cm and is formed by the curve of the sacrum. Laterally are the greater sciatic notch,body of the sacrum and obturator foramen covered by the obturator internus muscles.


This is described into two parts

  • The anatomical outlet
  • The obstetrical outlet

The Anatomical outlet

  • Is bounded by the lower bounder of the symphysis pubis
  • The pubic arch
  • The lower bounder of the ischial tuberosity
  • The Sacro tuberous ligament
  • The tip of the coccyx.

The Obstetrical outlet

  • Is bounded by the lower inner bounder of the symphysis pubis
  • The ischia spines
  • The Sacro spineous ligament
  • The lower bounder of the sacrum.

The obstetric outlet is of greater important to midwife because it is the areas in which the fetus must negotiate as passes through the birth canal rather than the wide anatomical outlet.


A diameter is a distance between two points. The study of diameter is of greater important to midwives for the skillful management of labour.


Four diameters are measured and they includes: 

  • Anteroposterior diameter (AP): this is measured from the centre of sacral promontory to the inner upper bounder of the symphysis pubis and is measures 11 cm.
  • The transverse diameter: this is a line between the two points farthest apart on the ilio pectineal lines and is measures 13cm. It crosses the anterior posterior diameter in its middle third.
  • The oblique diameter: this is measured from Sacro Illiac Joint on one side to the ilio pectineal eminance on the opposite side and is measures 12 cm.
  • The sacrocondyloid diameter: this is measured from the centre of sacral promontory to the Illiac pectineal eminance on each side and is measures 9 – 9.5cm.
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the sacrocondyloid is the smallest diameter of the brim and it is improtance is concerned with posterior position of the occiput when the pariental eminance of the fetal head may become trapped.


There are 3 diameters of the cavity

  • The antero posterior diameter: measured from the meet point of the Symphysis Pubis to the junctions of second and third sacral bones. It measures 12 cm
  • Transverse diameter: measured between the point farther most apart under the lateral walls. It measures 12 cm
  • The oblique diameter: measured parallel to oblique diameter of the brim and is 12 cm because the cavity is circular in shape all the diameters are considered equal.


3 diameters are obtained at the outlet and these includes: 

  • Anteroposterior diameter: taking from the lower inner bounder of the symphysis pubis to the lower bounder of the sacrum or the sacrum coccygeal joint. It measures 13cm
  • Transverse diameter: is measured between the two ischial spines and is 11cm.
  • The oblique diameter: has no fixed point for measurement but it said to be parallel to the other oblique diameter and is 12 cm.


  • Anatomical Conjugate: this is measured from the sacral promontory to the submount of symphysis pubis and is measured 12cm.
  • Obstetrical Conjugate: measured from the sacral promontory to about 1.25cm lower on the posterior upper bounder of the symphysis pubis. It measures 11 cm, the obstetric conjugate is available for the passage of fetus at birth.
  • Diagonal Conjugate: taking from the sacral promontory to the lower bounder of symphysis pubis and measures 13cm. The importance of the diagonal conjugate is that through it, the obstetrical conjugate is assessed.


The planes of the pelvis are imaginary flats surface drawn at various levels of the pelvis canal that is: 

  • The brim
  • The cavity
  • The outlet

When a person is standing upright the plan makes an angle of 60 degree to the floor. Because the Pelvis is curved canal. The angles of the brim, cavity and outlet must be different too. Therefore the angle of the plane of the Pelvic brim to the floor is 60 degree. The angle of the plane of the cavity is 30 degree while that of the outlet to the floor is 15 degree.


This is an imaginary line drawn through the centre or at the right angle to each plane and will trace a curve known as curve of the carus. It marked the direction the fetus moves during birth, the midwife need to be familiar with these concepts in other to make accurate observation of vaginal examination and to facilitate the birth of the baby.

Functions of bony pelvis

  • Allows movement of the body
  • Takes the weight of the body in the sitting position
  • Gives protects to the pelvic organs
  • In female, it is adapted for child bearing.