Post on 07-May-2015
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Presenting Diameters of Fetal Head and it's Importance
Anatomy of Fetal Skull
Fetal cranium made of 5 bones- 2 parietal bones / 2 frontal bones and the occipital bone
These are held together by membranous structures called sutures- permit movements of bones during labour
There are 4 prominent sutures
1. Coronal suture : separates frontal bones from parietal bones
2. Sagittal suture : separates two parietal bones
3. Lamboid suture : separates occipital bone from parietal bones
4. Frontal suture : separates two frontal bones.
Fontanelle : When two or more sutures meet, there is an irregular membranous part between them called fontanelle
There are two important fontanelles
1. Anterior fontanelle ( bregma ) : Diamond shaped area at the junction of coronal and sagittal sutures. AP/transverse diameter 3 cm. Fused around ~18 months
2. Posterior fontanelle ( lambda ) : Small triangular area at the junction of sagittal and lamboid sutures (closes 2-3 months after birth )
Regions of Fetal Head
Occiput : The bony prominence that lies behind posterior fontanelle
Vertex : Diamond shaped area between anterior and posterior fontanelle
Sinciput : Area in front of anterior fontanelle. Includes forehead.
Brow – between bregma and root of nose lying below root of nose and supra orbital bridges
Diameters of Fetal Head
Five important diameters.
1. Suboccipito-bregmatic diameter
2. Suboccipitofrontal diameter
3. Occipitofrontal diameter
4. Mentovertical diameter
5. submentobregmatic diameter
Suboccipito-bregmatic diameter
The diameter is from suboccipital region to centre of the bregma.
Diameter = 9.5 cm Fetal head circumference is
smallest (32 cm ) Head well flexed Flexed vertex presentation
Suboccipito-frontal diameter
Diameter calculated from prominence at mid frontal bone to the under-surface of the occipital bone where it joins neck
The diameter is 10.5 cm Vertex is partially deflexed. Results in occipito-posterior
position.
Occipito-frontal diameter
Diameter extends from the prominent point of mid-frontal bone to the most prominent point of occipital bone
The diameter = 11.5 cm Fetal head circumference ~ 34.5
cm Vertex is deflexed Associated with Direct occipito-
posterior position.
Occipito-posterior position
It is a vertex presentation in which the occiput is placed posteriorly . It can be:-
1.Right occipto-posterior (the commonest)
2.Left occipto-posterior.
3.Direct occipto-posterior
Associated diameters are
1. Suboccipito frontal diameter
2. Occipitofrontal diameter
Incidence :Incidence :In 20% of cases the occiput is posterior at the beginning of
labour .
Causes :-Causes :-1. Pelvic Factors:- 50% of cases are associated with anthropoid
pelvis or android pelvis .
2.Fetal Factors:- Marked deflection of the fetal head due to high pelvic inclination or anterior wall placenta .
3.Uterine Factor:- Abnormal uterine contraction which may be the cause or effect .
DiagnosisDiagnosisAbdominal ExaminationFeatures suggesting the diagnosis include
-backache during labour.
-flattening of the abdomen below the umbilicus .
-the fetal limbs are more easily felt near the midline on both sides.
-The head not engaged and feels larger than usual
Problems associated with Occiput Posterior –
-the head faces the front of the mother's pelvis instead of turning toward the mother's back.
-delivered with the head facing the ceiling,which is often a more difficult way to deliver.
vaginal examination-Elongated bag of membrane which is likely to rupture early .
-High deflexed head with the anterior fontanelle in the centre of the pelvis .
- A large episiotomy may be required.
- OP may lead to dysfunctional labour (in primigravida).
- Contraction may be painful and accompanied by backache
Mechanism of Labour in OP positionFirst and second stage of labour usually prolonged .
-membrane usually rupture early with the hazards of cord prolapse and infection .
-In favorable circumstances (90% of cases) good uterine contraction result in good flexion of the head and the occipt rotates 3/8 of the circle (135c0 ) anteriorly and deliver as occipito-anterior position .
In unfavorable circumstances (10% of cases) the occiput
1. Fail to rotate and remain in the oblique diameter of the pelvis .
2. Rotate anteriorly 1/8th of circle (short rotation) and the head become arrested in the transverse diameter of the pelvis (deep transverse arrest) .
3. Rotate posterioly 1/8th of the circle to lie on the sacral hollow this called direct-occipto- posterior position .
And if the fetus is small & pelvis is Adequate spontenous delivery can occur as face to pubic.
ManagementManagement - Unless there is fetal hypoxia or other complication labour is
allowed to proceed with the following special instructions .
-Provide adequate analgesia (an epidural is ideal).
-Prevent dehydration with intravenous fluid.
-You may need to promote uterine contraction with oxytocin .
-Good monitoring for progress of labour ,fetal condition and maternal condition .
- In the majority of cases anterior rotation of the occipt is completed and the baby is delivered as occipto-anterior.
- In direct occipto-posterior delivery as face to pubis may occur ,The perineum should be protected by a generous episiotomy.
Persistent –occipto posterior and deep transverse arrest .- If the fetal head is not engaged caesarian section is the
treatment of choice .- If the fetal head is engaged the treatment will be one of
the following .
1) Manual rotation and delivery by forceps as occipto-anterior .
2) Rotation to occipto-anterior and extraction using kielland’s forceps .
3) Ventouse (vacuum extraction).
4) Caesarean section if the above lines of treatment fail or there is other complicating factor .
5) Craniotomy when the fetus is dead .
Mento-vertical diameter Diameter extending from the
chin to furthest point of vertex. Measures 13cm Largest antero-posterior
diameter Head is partially extended. Associated with brow
presentation
Brow presentation Incidence:
ranges from 1:1000 to 1:3500 Rarest malpresentation The presenting diameter 13.5cm
(mento-vertical ) Incompatible with vaginal delivery
Causes Prematurity Multiple pregnancies Goiter or hygroma
DiagnosisDiagnosis Abdominal examination-
suspect if both chin and occiput are palpable
head doesn't descend below ischeal spines Vaginal examination
Palpate supra orbital ridges/nose, anterior fontanelle.
Cant palpate chin.
ManagementManagement Watch and wait : may become face or vertex
presentation If progress is slow or the brow persists
caesarian section
Submento-bregmatic diameter
The diameter extending from just behind chin to the centre of bregma
Measures 9.5cm Head is fully extended Associated with face
presentation
Face presentation
Incidence :
1:600- 1:1500 Due to hyper-extension of fetal head Presenting diameter 9.5 cm
(submento-bregmatic diameter ) Engagement of fetal head late Progression of labour slow Probably due to lack of molding of
facial bones
CausesCauses Fetal anomalies.
The most common anomaly that causes face presentation is anencephaly. Anencephalic babies present face first because of the faulty development of the cranium. Tumors on the neck or back may also cause extension of the head.
Pelvic contractures or android pelvis. This is the major factor. It accounts for about 40% of face presentations.
Fetopelvic disproportion Multiparity
Preterm birth Polyhydramnios. When the membranes rupture the
rush of fluid may cause the head to extend as it descends.
Coils of umbilical cord around the neck.
DiagnosisDiagnosis Vaginal examination
The orbital ridges/nose/malar eminences/ mentum/ mouth and gums
Management In the chin-anteriorchin-anterior
position prolonged labor is common. Descent and delivery of the head by flexion may occur.
In the chin-posterior chin-posterior position, however, the fully extended head is blocked by the sacrum. This prevents descent and labour is arrested.
ManagementChin-Anterior PositionChin-Anterior Position
If the cervix is fully dilated: Allow to proceed with
normal childbirth; If there is slow progress
and no sign of obstruction, augment labor with oxytocin;
If descent is unsatisfactory, deliver by forceps.
If the cervix is not fully dilated and there are no signs of obstruction: augment labor with
oxytocin.
Chin-Posterior PositionChin-Posterior Position
If the cervix is fully dilated: Deliver by caesarean
section.
If the cervix is not fully dilated Monitor descent, rotation
and progress. If there are signs of obstruction, deliver by caesarean section.
*Do not perform vacuum Do not perform vacuum extraction for face extraction for face presentation.presentation.
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