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The bony pelvis and fetal skull

C. Savona-Ventura

Trainee Obstetrics Programme - 2007St. Luke’s Teaching Hospital

Introduction

Knowledge of the shape and dimensions of the normal female pelvis is essential for a proper understanding of the second stage of labour and its abnormalities since the body pelvis is an important component which determines the birth canal structure. The human female pelvis shows adaptations that are of obstetric advantage and relate also to the relative “big” head of the foetus. These adaptations develop chiefly in childhood and puberty.

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Comparative anatomy

Human Gynaecoid Orangutan

General anatomy

Pelvic bone is made up of various sections:For obstetrical purposes, the pelvis is divided by the pelvic brim into two parts:

– The False Pelvis– The True Pelvis

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General anatomy

The False Pelvis is that portion above the pelvic brim. It does not take part in the mechanism of delivery and is of no obstetric interest.In the past attempts were made to form a judgement of the size of the true pelvis by measuring the width of the false pelvis. The information thus obtained was often inaccurate

Intercristal diameter [IC ~29 cm]: widest point on lateral aspect of iliac crestInterspinous diameter [IS ~26 cm]: distance between the lateral tips of the anterior superior iliac spinesExternal conjugate [AP] diameter [EC ~20 cm]: distance between apex of spine of 5th lumbar vertebra and centre of the superior border of symphysis pubis.

Martin’s pelvinometer

General anatomy

The True Pelvis is that portion below the pelvic brim. It determines the size and shape of the birth canal.

Brim: formed by the upper margins of pubic bones, the ilio-pectineal lines and the anterior upper margin of the sacrum.Cavity: formed by the pubic bones, ischium, ilium, and sacrumOutlet: diamond-shaped made up of the pubic bones, ischium, ischial tuberosities, sacrotuberous ligament, and 5th segment of sacrum.

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General anatomy

12.513.111.3Anteroposterior

11.813.112.5Oblique

11.812.513.1Transverse

Outlet CavityBrim

Inclination of the Pelvic brim: ~1200

General anatomyFour different types of pelvises, but frequently mixed types.

Gynaecoid Android

Anthrapoid

Platypelloid

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Gynaecoid pelvis

Ideal pelvis favouring a normal delivery; 50.6% of women.

Brim slightly oval transversely but almost roundedSacrum curvedIschial spines not prominentShort-cone pelvisObtuse greater sciatic notchTriangular obturatorforamenSub-pubic arch rounded [Roman arch] angle at least 900

Android pelvis

Male-type pelvis favouring OP positions and apt to cause deep transverse arrest of head; 22.4% of women.

Brim heart-shapedSacrum curvedIschial spines prominentLong-cone funnel pelvisAcute greater sciatic notchOval obturator foramenSub-pubic arch very narrow [Gothic arch]

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Anthrapoid pelvis

Ape-like pelvis favouring OP positions often requiring operative vaginal deliveries; 22.7% of women.

Brim AP ovalSacrum very slightly curvedIschial spines prominentLong-cone funnel pelvis with straight sidewallsObtuse greater sciatic notchOval obturator foramenSub-pubic arch narrow

Platypelloid pelvis

Often leads to cephalo-pelvic disproportion; 4.4% of women.

Brim oval transverselySacrum very slightly curvedIschial spines prominentShort-cone shallow pelvisAcute greater sciatic notchTriangular obturatorforamenWide arch narrow

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Mixed variety pelvises

Asymmetrical pelvises

Split pelvis

Robert’s pelvis

Naegele’s pelvisCoxalgic pelvis

Scoliotic pelvisOsteomalacic pelvis

Abnormality of lower limbAbnormality of pelvic girdleAbnormality of vertebral column

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Clinical AssessmentBody build

Gynaecoid Anthrapoid Android

Clinical Assessmentfoetal head as pelvimeter

Fifths palpable above symphysis pubis

Engagement defined as the point when the engaging diameter [BPD = ~10 cm] goes past the pelvic brim. Five fingers = 10 cm.

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Clinical Assessmentfoetal head as pelvimeter

Station of the head in relation to ischial spines

In Gynaecoid & Android pelvis distance between ischial spine to brim is ~5 cm.

In Anthropoid pelvis distance is ~7 cm

In Platypelloid pelvis distance is ~3 cm

5 cm: Station 0

7 cm: Station 0-2

3 cm: Station 0+2

Head 3/5

Clinical Assessmentfoetal head as pelvimeter

Munro Kerr’s method of assessing for engagement

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Clinical Assessmentvaginal examination

Measurement of AP conjugates•Diagonal conjugate ~12.0 cm•True conjugate ~11.0 cm•AP outlet ~12.5 cm

Assess shape of sacrum

Clinical Assessmentvaginal examination

Assess mobility of sacro-coccygeal joint

Assess interspinousdiameter ~12.0 cm

Assess intertuberousdiameter ~11.8 cm

Assess spino-tuberous distance ~4.5 cm

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Clinical Assessmentvaginal examination

Assessment of pubic arch angle

Effect of sub-pubic angle on pelvic depth

Types of pubic arches

Effects of a narrow pubic arch•Thrusts foetal head further posteriorly•Prevents extension of head

Gynaecoid Android

Anthropoid Platypelloid

Clinical Assessmentradiological examination

1. True AP Conjugate2. Obstetric Conjugate3. Mid-cavity AP Conjugate4. Outlet AP conjugate5. Angle Greater Sciatic notch6. Angle of inclination of pelvic brim7. Angle of inclination of sacrum8. Ischial spine9. Ischio-tuberous distance10. Foetal head lie, position, engagement

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Clinical Assessmentradiological examination

Antero-posterior view of pelvis

Thom’s Supero-inferior view of pelvis

Outlet view of pelvis

Relationship of foetal skull to pelvisAxis of birth canal

900 rotation for Occipito-transverse when engaging diameter is at the brim Occiptio-oblique in mid-cavity Occipito-anterior at ischial spines

Axis of inlet

Axis of cavity

Axis of outlet

50-600

Outlet

Promontory

Symphysispubis

Horizontal plane

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Foetal skull

1. Future Coronal Suture2. Anterior Fontanel3. Anterolateral Fontanel4. Future Squamosal Suture5. Posterolateral Fontanel6. Future Lamdoidal Suture7. External Acoustic Meatus8. Future Sagittal Suture9. Posterior Fontanel

Foetal skull

1. Bones: 2 parietals, 2 frontals, 2 temporals, occipital

2. Sutures: sagital, frontal, lamboidal, coronal, temporal

3. Fontanelles: anterior, posterior, 2 anterior temporals, 2 posterior temporals.

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Foetal skull

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1. Suboccipitobregamatic: ~9.5 cc Vertex2. Suboccipitofrontal: ~10.0 cm Sinciput3. Occipitofrontal:~11.24 cm persistent OP 4. Mentovertical: ~13.8 cm brow5. Submentobregmatic: ~9.5 cm Face6. Submentovertical: ~11.25 cm incompletely

extended face7. Biparietal diameter: ~9.5 cm8. Bitemporal diameter: ~8.0 cm9. Bimastoid diameter: 7.5 cm

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Foetal presentations

Suboccipitofrontal~10.0 cm

Submentobregmatic~9.5 cm

Suboccipitobregmatic~9.5 cm

Mentovertical~13.8 cm

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Foetal skull moulding in labour

Bones of base of skull are incompressibleBones of vault are compressible

– Parietal bones override occiptial and frontal– Anterior parietal bone overrides its posterior fellow– Moulding can decrease biparietal diameter by ~1cm

Normal vertex position

Persistent OP position

Brow presentation

Face presentation

Malpositions

Occiptio-posterior position 1 in 5 deliveries [generally left because of dextrorotation and descending colon]

Face presentation 1 in 500 deliveries Brow presentation 1 in 1000 deliveriesBreech presentation 1-2 in 50 deliveriesUnstable lie 1 in 350 deliveries