ANATOMY OF PELVIS IN RELATION TO

OBSTETRICS AND FETAL SKULL

BY: KHAIRUN NASIRIN

Pelvis is made up of : • 2 hip bone ( ilium,

pubis, ischium)• Sacrum• Coccyx

Articulation of bones :• 2 sacroiliac joints :

sacrum & iliac• Symphysis pubis :

2pubic bones• Sacrococcygeal

joint : sacrum & coccyx

FALSE PELVIS Above brim support enlarged uterus

during pregnancy Guide fetus into true

pelvis

TRUE PELVIS Below brim/inlet Bony canal which fetus

passes through during birth

FALSE PELVISBOUNDARIES• Ant : lower part

of abd wall

• Post : lumbar vertebrae

• Lat : Iliac fossae

TRUE PELVIS Inlet mid pelvis outlet

PELVIC INLET / PELVIC BRIM

Boundaries: Anterior

Pubic bone, symphysis pubis

Lateral Ileopectineal

lines Posterior

sacral promontory

INLET DIAMETERS• AP diameter (11 cm) :

sup margin of symphysis pubis to sacral promontory

• Transverse diameter(13.5cm)

widest distance btw iliopectineal lines

• Diagonal conjugate(12.5cm):subpubic angle to middle of sacral promontory

Diagonal conjugate

PELVIC CAVITYBOUNDARIES• Ant : pubic bones• Post : curve of sacrum• Lat : inner aspect of ischial bones & spines

Ischial spines

•Ref point for designation of station of presenting part

•Fetus’ head assumed to be engaged at this point

PELVIC OUTLET

Boundaries:- Anterior – lower margin of

symphysis pubis

Lateral – descending ramus of pubic bone, ischial tuberosity and sacrotuberous ligament

Posterior – last piece of sacrum

• AP Diameter (Anatomical outlet) (13.5cm)inferior margin of symphysis pubis to tip of coccyx

• Transverse diameter (11cm)distance btw inner surfaces of ischial tuberosities

OUTLET DIAMETERS

BRIM CAVITY OUTLET

AP 11 12 13.5

TRANSVERSE 13.5 12 11

PELVIC FLOOR Formed by levator ani muscles

Puborectalis Pubococcygeus Iliococcygeus

PELVIC INCLINATION The lateral view of the pelvis

indicates that the pelvic brim makes an angle of 40° to 50° with the horizontal plane

This is called angle of inclination

Increase in angle may delay the head entering pelvis in labour

The inclination lessens as the birth canal is descended, being 30° in the mid pelvic & 10° at the outlet

Gynaecoid pelvis

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

Brim slightly oval transversely but almost rounded

Sacrum curved Ischial spines notprominent Short-cone pelvis Obtuse greater sciatic notch Triangular obturator foramen Sub-pubic arch rounded[Roman arch]

angle at least 90 degree

Android pelvis Male-type pelvis favouring OP

positions and apt to cause deep transverse arrest of head; 22.4% of women.

Brim heart-shaped Sacrum curved Ischial spines prominent Long-cone funnel pelvis Acute greater sciatic notch Oval obturator foramen Sub-pubic arch very

narrow[Gothic arch]

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

Brim AP oval Sacrum very slightly curved Ischial spines prominent Long-cone funnel pelvis with

straight sidewalls Obtuse greater sciatic notch Oval obturator foramen Sub-pubic arch narrow

Anthrapoid pelvis

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

Brim oval transversely Sacrum very slightly curved Ischial spines prominent Short-cone shallow pelvis Acute greater sciatic notch Triangular obturator foramen Wide arch narrow

Platypelloid pelvis

Cranium comprise 2 frontals 2 parietals 2 temporals 1 occipital

Sutures Frontal Coronal Sagittal Lambdoidal

Frontal suture

AREAS OF THE SKULLArea btw ant

& post fontanelles &

2 parietal eminences

Behind post

fontanelle

In front ant

fontanelle

Sutures are important for labour Allows bones to move together & even to

overlap Moulding occurs - reduces diameter of fetal

skull & encourages progress through bony pelvis

Severe moulding can be a sign of cephalo-pelvic disproportion

DIAMETERS OF THE SKULL Suboccipito-bregmatic

9.5 cm Nape of neck to bregma Complete flexion Vertex presentation

Suboccipito-frontal 10cm Nape of neck to centre of

sinciput Incomplete flexion Vertex presentation

DIAMETERS OF THE SKULL Occipito-frontal

11.5 cm Occipital to root of nose Marked deflexion Vertex presentation

Mento-vertical From midpoint of chin to

highest point of sagittal suture 14 cm Partial extension Brow presentation

DIAMETERS OF THE SKULL

Submento-bregmatic From junction of floor of

mouth and neck to centre of bregma

9.5 cm Complete extension Face presentation

Relationship of foetal skull to pelvis Axis of birth canal

90 degree rotation for Occipito-transverse when engagingdiameter is at the brim

Occiptio-oblique in midcavity

Occipito-anterior at ischial spines