Obstetrics Simplified - Diaa
Anatomical definition: It is a pelvis in which one or more of its diameters
is reduced below the normal by one or more centimeters.
Obstetric definition: It is a pelvis in which one or more of its diameters
is reduced so that it interferes with the normal mechanism of labour.
Factors influencing the size and shape of the pelvis
- Developmental factor: hereditary or congenital.
- Racial factor.
- Nutritional factor: malnutrition results in small pelvis.
- Sexual factor: as excessive androgen may produce android pelvis.
- Metabolic factor: as rickets and osteomalacia.
- Trauma, diseases or tumours of the bony pelvis, legs or spines.
Aetiology of Contracted Pelvis
Causes in the pelvis
- Developmental (congenital):
- Small gynaecoid pelvis (generally contracted pelvis).
- Small android pelvis.
- Small anthropoid pelvis.
- Small platypelloid pelvis (simple flat pelvis).
- Naegele’s pelvis: absence of one sacral ala.
- Robert’s pelvis: absence of both sacral alae.
- High assimilation pelvis: The sacrum is composed of 6 vertebrae.
- Low assimilation pelvis: The sacrum is composed of 4 vertebrae.
- Split pelvis: splitted symphysis pubis.
- Osteomalacia (triradiate pelvic brim).
- Traumatic: as fractures.
- Neoplastic: as osteoma.
Causes in the spine
- Lumbar kyphosis.
- Lumbar scoliosis.
- Spondylolisthesis: The 5th lumbar vertebra with the above
vertebral column is pushed forward while the promontory is pushed backwards
and the tip of the sacrum is pushed forwards leading to outlet contraction.
Causes in the lower limbs
- Dislocation of one or both femurs.
- Atrophy of one or both lower limbs.
N.B. oblique or asymmetric pelvis: one oblique diameter is obviously
shorter than the other. This can be found in:
- Naegele’s pelvis.
- Scoliotic pelvis.
- Diseases, fracture or tumours affecting one side.
Diagnosis of Contracted Pelvis
- Rickets: is expected if there is a history of delayed walking and
- Trauma or diseases: of the pelvis, spines or lower limbs.
- Bad obstetric history: e.g. prolonged labour ended by;
- difficult forceps,
- caesarean section or
- still birth.
- General examination:
- Gait: abnormal gait suggesting abnormalities in the pelvis,
spines or lower limbs.
- Stature: women with less than 150 cm height usually have contracted
- Spines and lower limbs: may have a disease or lesion.
- Manifestations of rickets as:
- square head,
- rosary beads in the costal ridges.
- pigeon chest,
- Harrison’s sulcus and bow legs.
- Dystocia dystrophia syndrome: the woman is
- has android pelvis and
- masculine hair distribution,
- with history of delayed menarche.
- This woman is more exposed to occipito-posterior position and
- Abdominal examination:
- Nonengagement of the head: in the last 3-4 weeks in primigravida.
- Pendulous abdomen: in a primigravida.
- Malpresentations: are more common.
It is assessment of the pelvic diameters and capacity done at 38-39 weeks.
- Clinical pelvimetry:
- Internal pelvimetry for:
- cavity, and
- External pelvimetry for:
- Imaging pelvimetry:
- Computerised tomography (CT).
- Magnetic resonance imaging (MRI) .
N.B. CT and MRI are recent and accurate but expensive and not always
available so they are not in common use.
Internal pelvimetry (is done through vaginal examination)
- The inlet:
- Palpation of the forepelvis (pelvic brim):
- The index and middle fingers are moved along the pelvic
brim. Note whether it is round or angulated, causing the fingers
to dip into a V-shaped depression behind the symphysis.
- Diagonal conjugate:
- Try to palpate the sacral promontory to measure the diagonal
conjugate. Normally, it is 12.5 cm and cannot be reached. If
it is felt the pelvis is considered contracted and the true
conjugate can be calculated by subtracting 1.5 cm from the diagonal
conjugate .This assessment is not done if the head is engaged.
- The cavity:
- Height, thickness and inclination of the symphysis.
- Shape and inclination of the sacrum.
- Side walls:
- To determine whether it is straight, convergent or divergent
starting from the pelvic brim down to the base of ischial spines
in the direction of the base of the ischial tuberosity. Then
relation between the index and middle finger of the base of
ischial spines and the thumb of the other hand on the ischial
tuberosity is detected. If the thumb is medial the side wall
is convergent and if lateral it is divergent.
- Ischial spines:
- Whether it is blunt (difficult to identify at all), prominent
(easily felt but not large) or very prominent (large and encroaching
on the mid-plane).
- The ischial spines can be located by following the sacrospinous
ligament to its lateral end.
- Interspinous diameter:
- By using the 2 examining fingers, if both spines can be
touched simultaneously, the interspinous diameter is £ 9.5 cm
i.e. inadequate for an average-sized baby.
- Sacrosciatic notch:
- If the sacrospinous ligament is two and half fingers, the
sacrosciatic notch is considered adequate.
- The outlet:
- Subpubic angle:
- Normally, it admits 2 fingers.
- Bituberous diameter:
- Normally, it admits the closed fist of the hand (4 knuckle).
- Mobility of the coccyx.
- by pressing firmly on it while an external hand on it can
determine its mobility.
- Anteroposterior diameter of the outlet:
- from the tip of the sacrum to the inferior edge of the symphysis.
FINDINGS INDICATING ADEQUATE PELVIS:
Forepelvis (pelvic brim)
Anterposterior diameter of outlet
³ 11.5 cm.
Average thickness, parallel to sacrum.
Hollow, average inclination.
³ 10.0 cm.
2.5 -3 finger - breadths.
2finger - breadths.
4 knuckles (> 8.0 cm).
³ 11.0 cm.
It is of little value as it measures diameters of the false pelvis.
Thom’s, Jarcho’s or crossing pelvimeter can be used for external pelvimetry.
- Interspinous diameter (25cm): between the anterior superior iliac
- Intercrestal diameter (28 cm): between the most far points on the
outer borders of the iliac crests.
- External conjugate (20 cm).
- Bituberous diameter: can be measured by pelvimeter.
In rickets, the interspinous equals or even exceeds the intercrestal
It is indicated mainly in borderline pelvic contraction.
- Lateral view: The patient stands with the X-ray tube on one side
and the film cassette on the opposite side.
- It is the most important view as it shows the anteroposterior
diameters of the pelvis, angle of inclination of the brim, width
of sacrosciatic notch, curvature of the sacrum and cephalo-pelvic
- Inlet view: The patient sits on the film cassette and leans backwards
so that the plane of the pelvic brim becomes parallel to the film.
- Outlet view: The patient sits on the film cassette and leans forwards.
N.B. The measurements can be identified by using a graduated scale or
Thom’s perforated grid, in which the perforations are 1cm apart, while taking
the X-ray film.The picture of the scale or grid on the X-ray film allows
- Ultrasonography: is the safe accurate and easy method and can detect:
- The biparietal diameter (BPD).
- The occipito-frontal diameter.
- The circumference of the head.
- Radiology (X-ray): is difficult to interpret.
Cephalopelvic disproportion tests
These are done to detect contracted inlet if the head is not engaged
in the last 3-4 weeks in a primigravida.
- (1) Pinard’s method:
- The patient evacuates her bladder and rectum.
- The patient is placed in semi-sitting position to bring the
foetal axis perpendicular to the brim.
- The left hand pushes the head downwards and backwards into the
pelvis while the fingers of the right hand are put on the symphysis
to detect disproportion.
- (2) Muller - Kerr’s method:
- It is more valuable in detection of the degree of disproportion.
- The patient evacuates her bladder and rectum.
- The patient is placed in the dorsal position.
- The left hand pushes the head into the pelvis and vaginal examination
is done by the right hand while its thumb is placed over the symphysis
to detect disproportion.
Degrees of Disproportion
- Minor disproportion:
- The anterior surface of the head is in line with the posterior
surface of the symphysis. During labour the head is engaged due to
moulding and vaginal delivery can be achieved.
- Moderate disproportion (1st degree disproportion):
- The anterior surface of the head is in line with the anterior
surface of the symphysis. Vaginal delivery may or may not occur.
- Marked disproportion (2nd degree disproportion):
- The head overrides the anterior surface of the symphysis. Vaginal
delivery cannot occur.
Degrees of Contracted Pelvis
- Minor degree: The true conjugate is 9-10 cm. It corresponds to minor
- Moderate degree: The true conjugate is 8-9 cm. It corresponds to
- Severe degree: The true conjugate is 6-8 cm. It corresponds to marked
- Extreme degree: The true conjugate is less than 6 cm. Vaginal delivery
is impossible even after craniotomy as the bimastoid diameter (7.5 cm)
is not crushed.
Mechanism of Labour in Contracted Pelvis
The Flat Rachitic Pelvis
- Inlet: reduced antero-posterior diameter.
- The pelvic inclination: is exaggerated due to increased lumbar
- The sacrum has the following characters:
- - The promontory is pushed forwards so the tip is pushed backwards.
- - Diminished or obliterated concavity.
- - Bent at the middle may be present.
- The outlet has the following characters:
- Increased antero-posterior diameter.
- Increased bituberous diameter.
- The interspinous equal the intercrestal diameter.
Mechanism of labour:
- Engagement: with the sagittal suture in the transverse diameter.
- Asynclitism with anterior parietal bone presentation so that the
shorter subparietal supraparietal diameter (9cm) is passed instead of
the biparietal (9.5cm) in the narrow true conjugate.
- Lateral displacement of the head so that the bitemporal diameter
is passed through the narrow true conjugate .
- Deflexion of the head as the descent of the occiput is resisted
by the lateral pelvic wall .
- Correction of the asynclitism and deflexion with further descent
of the head.
- Rotation of the occiput 2/8 circle anteriorly and the head is delivered
easily due to wide outlet.
Simple Flat Pelvis
- Reduced antero-posterior diameters of the inlet, cavity and outlet.
- No rachitic manifestations.
Mechanism of labour:
The process passes as flat rachitic pelvis till the mid cavity where
internal rotation and further descent cannot occur due to persistence of
flattening of the pelvis and contracted outlet. So deep transverse arrest
is common and vaginal delivery is obstructed.
Contracted Outlet (Funnel Pelvis)
- The pelvic capacity is diminished from the inlet to the outlet.
- Subpubic angle is acute.
- Convergent side walls.
- Bituberous diameter is 8 cm or less.
- Android pelvis.
- Anthropoid pelvis.
- High assimilation pelvis.
- Oblique pelvis.
- 20% of generally contracted pelvis.
Mechanism of labour:
- Normal descent and engagement as the pelvic inlet is normal.
- Extreme flexion and moulding of the head at the level of the jutting
- Because of the narrow subpubic angle, the head is pushed backwards
with more liability to perineal tears.
- In case of occipito-posterior, the funnel pelvis interferes with
long anterior rotation so persistent occipito-posterior and deep transverse
arrest are common. The face to pubis position is more favourable as
it brings the short bitemporal diameter in the narrow subpubic angle.
It depends on Thom’s dictum:
- If the sum of bituberous + posterior sagittal is >15 cm and bituberous
diameter is >8cm: vaginal delivery is allowed with episiotomy and low
- If the Thom’s dictum is <15 cm or the bituberous diameter is <8cm:
caesarean section is performed.
- Symphysiotomy: may be done in distant areas with no facilities for
C.S. and the foetus is living.
Management of Contracted Pelvis
It depends mainly on the degree of disproportion.
- Minor disproportion (minor degree of contracted pelvis):
- Moderate disproportion (moderate degree of contracted pelvis):
trial labour, if failed ® caesarean section.
- Marked disproportion (severe or extreme degree of contracted
pelvis): caesarean section.
Trial of Labour
It is a clinical test for the factors that cannot be determined before
start of labour as:
- Efficiency of uterine contractions.
- Moulding of the head.
- Yielding of the pelvis and soft tissues.
- Trial is carried out in a hospital where facilities for C.S
- Adequate analgesia.
- Nothing by mouth.
- Avoid premature rupture of membranes by:
- rest in bed,
- avoid high enema,
- minimise vaginal examinations.
- The patient is left for 2 hours in the 2nd stage with good uterine
contractions under close supervision to the mother and foetus.
Suitable cases for trial of labour:
- Young primigravida of good health.
- Moderate disproportion.
- Vertex presentation.
- No outlet contractions.
- Average sized baby.
Termination of trial of labour:
- Vaginal delivery:
- either spontaneously or by forceps if the head is engaged.
- Caesarean section if:
- failed trial of labour i.e. the head did not engage or
- complications occur during trial as foetal distress or
prolapsed pulsating cord before full cervical dilatation.
Indications of caesarean section in contracted pelvis
- Moderate disproportion if trial of labour is contraindicated or
- Marked disproportion.
- Extreme disproportion whether the foetus is living or dead.
- Contracted outlet.
- Contracted pelvis with other indications as;
- elderly primigravida,
- malpresentations, or
- placenta praevia.
Complications of Contracted Pelvis
- During pregnancy:
- Incarcerated retroverted gravid uterus.
- Pendulous abdomen.
- Pyelonephritis especially in high assimilation pelvis
due to more compression of the ureter.
- During labour:
- Inertia, slow cervical dilatation and prolonged labour.
- Premature rupture of membranes and cord prolapse.
- Obstructed labour and rupture uterus.
- Necrotic genito-urinary fistula.
- Injury to pelvic joints or nerves from difficult forceps
- Postpartum haemorrhage.
- Intracranial haemorrhage.
- Fracture skull.
- Nerve injuries.
- Intra-amniotic infection.
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Edited by Aldo Campana,