CEPHALOPELVIC DISPROPORTION

C

ephalopelvic disproportion is an anomaly where there is a mismatch between the head of the foetus and the maternal pelvis.

This happens when the foetal head is large or the female pelvis is small in size.

Normal Foetal Skull Anatomy

Foetal skull is large as compared to the body of the foetus, which later on shrinks to the size of the normal size in comparison to the whole body.

  

Fig 1.0 Foetal head and body proportion.

This Figure shows the relative proportion of the Head to Body of the foetus and its size in comparison to the body. The head progressively gets into the proportion ratio as the foetus grows.

The foetal skull is short and wide, immature and has the tendency to adjust according during the delivery.

This adjustment where the skull bones move on top of each other when it comes out of the vaginal canal is called as moulding of the skull.

Fig 1.1 The movement of skull bones so as to come out of vaginal canal during delivery.

The foetal skull has three major parts –

1.    Roof

2.    Face

3.    Base

Fig 1.2 Parts of foetal skull

Vault is the superior (top) part of the skull which is large and bulging above the imaginary line passing through the orbital ridges. 

Face is made up of small 14 bones those are firmly attached and non-compressible.

Base is formed by all the bones that join together to protect the medulla.

The foetal skull has the following bones –

1.    2 Frontal bones

2.    2 Parietal bones

3.    2 Temporal bones

4.    1 Occipital bone

Fig 1.3 Bones and Fontanel of skull.

The above figure shows a lateral view of the skull with the bones. But as you can see there are a few gaps or openings in the skull. These are called as Fontanels. These are present because the skull bones can accommodate the growing brain inside and these later on fuse together to completely close the vault.

The fontanels present are –

1.    Anterior fontanel

2.    Posterior fontanel

3.    Sphenoidal fontanel

4.    Occipital fontanel

Frontal Bone forms the sinciput or the forehead. It has 2 parts and they are fused in the midline by a suture called as Metopic suture. Metopic suture can be sometimes seen in adults too, but majority of the times it is fused and becomes completely vanished.

The 2 frontal bones are completely ossified by the age of 8 years to form one frontal bone.

The two parietal bones are present on either side of the sull and are joined together at the centre forming the sagittal suture. This sagittal suture becomes the important landmark as the sagittal sinus of the brain runs directly underneath this suture. There is a opening in between the frontal bone and parietal bones and this is where the anterior fontanel is located. The anterior fontanel closes by 8 – 24 months and thus the frontal and parietal bones are joined by another suture called as Coronal Suture.

The two parietal bones grow and start to ossify from the parietal eminence.

Occipital bone is present at the back of the skull and forms the base of the skull. It has the opening called as Foramen magnum which acts as the exit for the medulla oblongata from the brain towards the lower back, where the medulla continues as the spinal cord. Occipital bone fuses from the part called as occipital protuberance. The occipital bone joins the parietal bone by a suture called as Lambdoid suture. There is a small fontanel present in this suture which is called as Posterior fontanel, which closes by 6 – 12 weeks of age.

Anterior Fontanel (BREGMA)	Posterior Fontanel (LAMBDA)
Large Diamond shape	Small Triangular in shape
Situated at the junction of Sagittal and coronal suture, i.e. between frontal bones and parietal bones.	Situation at the junction of lambdoid sutures, i.e. between parietal and occipital bones.
Closed by 24 months.	Closed at 6 – 12 weeks.
Soft membrane floor.	Hard bony floor.

Table 1.0 Shows the difference between the anterior and posterior fontanels.

Also, there are a few diameters to be kept in mind and these diameters are –

1.    Bi-parietal diameter – 9.2 cm

2.    Sub-parietal diameter – 9 cm

3.    Bi-temporal diameter – 8.2 cm

4.    Bi-mastoid diameter – 7.5 cm

Bi – parietal diameter is between the two parietal eminences of the skull.

Sub-parietal diameter is when the line passes from above the parietal eminence of one side and below the parietal eminence of other side.

Bi-temporal diameter is the diameter between the between the 2 farthest points on the temporal aspect of coronal suture.

Bi-mastoid diameter the diameter between the two mastoid fontanels

 

Fig 1.4 Showing the sutures and fontanels of the skull.

Norma female pelvis anatomy

The female pelvis is divided into two parts-

a.     True pelvis

b.    False pelvis

The false pelvis is located above the pelvic brim and has no obstetric importance, where as the true pelvis is related to child birth and is thus has an obstetric importance.

Fig 1.5 shows female pelvis

The bones in pelvis are –

i.                Sacrum

ii.              Coccyx

iii.            Ilium

iv.            Ischium

v.              Pubis

These bones make up the pelvis.

The two Iliac bones are joined posteriorly by the sacral wings (ala) forming the sacroiliac joints, anteriorly they are joined by the cartilaginous joint called as pubic symphysis. The female pelvis is different from male pelvis in the fact that it is wider and shorter with wide pelvic inlet which creates room for the foetus to pass down while delivery.

The table 1.1 shows difference between male and female pelvis.

                         

                                  

Fig 1.6 Shows the male and female pelvis.

Male Pelvis	Female Pelvis
Bones are heavier and thicker.	Bones are lighter and thinner.
False pelvis is deep.	False pelvis is shallow.
Pelvic cavity is narrow and deep.	Pelvic cavity is shallow and wide.
Pelvic outlet is small.	Pelvic outlet is big.
Pelvic inlet is heart shaped.	Pelvic inlet is round/oval.
Sub-pubic angle is acute.	Sub-pubic angle is obtuse.
Coccyx is less flexible, curved.	Coccyx is flexible and straight
Ischial tuberosity lies more medially and are long.	Short ischial tuberosities and lie laterally.

Table 1.1 Difference between male and female pelvis.

Boundaries of pelvic inlet –

a.     Sacral promontory

b.    Alae of sacrum

c.     Sacroiliac joints

d.    Iliopectineal lines

e.     Iliopectineal eminences

f.      Upper border of superior pubic rami

g.    Pubic tubercles

h.    Pubic crests

i.      Upper border of symphysis pubis

There are certain diameters which define the female pelvis and are of utmost importance when it comes to the delivery of the foetus as it helps us to identify and determine the type of pelvis of the female.

The diameters are –

a.     Anteroposterior diameters

b.    Transverse diameters

c.     Oblique diameters

 

Anteroposterior diameters –

1.    Anatomical anteroposterior diameter (true conjugate) = 11 cm

2.    Obstetric conjugate = 10.5 cm

3.    Diagonal conjugate = 12.5 cm

4.    External conjugate = 20 cm

Transverse diameters –

1.    Anatomical transverse = 13 cm

2.    Obstetric transverse

Oblique diameters –

1.    Right oblique = 12 cm

2.    Left oblique = 12 cm

3.    Sacro-cotyloid diameter = 9-9.5 cm

   

Fig 1.7 Showing the diameters of the pelvis at the pelvic inlet, mid pelvis and outlet.

Diameters of pelvic outlet –

a.     Anteroposterior = 13 cm (obstetrical, anatomical is 11 cm)

b.    Bi-tuberous = 11 cm

c.     Bi-spinous = 10.5 cm

According to these diameters and measurements there are 4 different types of female pelvises.

Caldwell – Moloy classification helps us to know these following 4 types of pelvises.

1.    Gynaecoid pelvis (50%)

2.    Anthropoid pelvis (25%)

3.    Android pelvis (20%)

4.    Platypelloid pelvis (5%)

Fig 1.8 Shows the different types of female pelvises.

The gynaecoid pelvis is the normal female pelvis, where the sub-pubic angle is 90 – 100 degrees.

Anthropoid pelvis is ape-like pelvis, characterised by short transverse diameters and long anteroposterior diameters. Sub-pubic angle is narrow.

Android pelvis is male type of pelvis with inlet shaped slightly triangular with narrow apex and sub-pubic angle is less than 90 degrees.

Platypelloid pelvis is the rarest type of pelvis; characterised by short anteroposterior diameters and long transverse diameters with wide sub-pubic angle.

Cephalopelvic Disproportion

Causes of CPD/CephaloPelvic Disproportion are –

1.    Large baby due to diabetes, multiparity (not the 1^st child), post- maturity (still pregnant even after the due date is passed).

2.    Abnormal foetal positions.

3.    Contracted pelvis.

4.    Abnormally shaped pelvis.

Contracted pelvis is a pelvis where one or more of the anatomical diameters are reduced by 1 or more centimetres or the size of the pelvis is not sufficient for the foetal head to pass through the vagina while normal delivery.

Pelvic factors for CPD –

1.    Developmental factors

2.    Racial factors

3.    Nutritional factors

4.    Sexual factors (more of testosterone)

5.    Metabolic factors

6.    Trauma

Congenital abnormalities of Pelvis –

1.    Naegele’s pelvis – absence of one sacral ala.

2.    Robert’s pelvis – absence of both sacral alae.

3.    High assimilation pelvis – sacrum has 6 vertebrae.

4.    Low assimilation pelvis – sacrum has 4 vertebrae.

5.    Split pelvis – split symphysis pubis.

Causes of contracted Pelvis –

1.    Trauma

2.    Rickets

3.    Osteomalacia

4.    Tuberculosis

5.    Lumbar kyphosis

6.    Lumbar scoliosis

Degrees of contracted pelvis –

1.    Minor degree – true conjugate = 9-10 cm

2.    Moderate degree – true conjugate = 8-9 cm

3.    Severe degree – true conjugate = 6-8 cm

4.    Extreme degree – true conjugate is less than 6 cm

Vaginal delivery is not possible in extreme degree.

Management of Contracted pelvis –

1.    Minor degree – Vaginal birth

2.    Moderate degree – trial of birth or caesarean.

3.    Severe degree – caesarean.

Degrees of disproportion in CPD –

1.    Severe disproportion – obstetric conjugate is less than 7.5 cm.

2.    Borderline disproportion – obstetric conjugate is between 9.5 cm to 10 cm.

Investigations –

1.    Pelvimetry

2.    Cephalometry

3.    CT pelvis

4.    MRI

5.    USG

Management according to degrees –

1.    Borderline degree = Vaginal delivery

2.    Severe degree = Caesarean or trial of labour.

Trial of Labour – It is conduction of spontaneous labour in moderate degree of disproportion in an institution hoping for vaginal delivery.

The progress of labour is assessed by plotting partograph, where progressive descend of head is noted along with progressive dilation of cervix.

A trial is said to be successful if the foetus is born vaginally or using forceps or ventose and the mother is in good condition.

A trial is said to be failed if the foetus is born using caesarean or delivery of dead baby by craniotomy.

The following factors help in favour of trial of labour –

1.    Flat pelvis better than android.

2.    Vertex presentation.

3.    Minor degree of contraction

4.    Intact membranes till full dilation

5.    Good uterine contraction.

6.    Emotional stability of woman,

Contraindications for trial of labour –

1.    Primigravida.

2.    Mal-presentation.

3.    Post maturity.

4.    Post caesarean pregnancy.

5.    Diabetes.

6.    Pre-eclampsia.

7.    Lack of facilities for caesarean.

8.    Abnormal uterine contractions.

9.    Cervical dilation less than 1cm/hr.

10.  Foetal distress.

11.  Arrest of cervical dilation and non-descent of foetal head in spite of oxytocin therapy.

The methods of termination of trial of delivery is by –

1.    Spontaneous delivery (30%)

2.    Symphysiotomy followed by forceps delivery or vacuum delivery (30%)

3.    Caesarean (30%)

4.    Craniotomy (if foetus is dead)