ANATOMY OF THE PELVIS AND FETAL SKULL

•	The Pelvis.
•	Clinical Pelvimetry.
•	The Fetus.

The Pelvis

1.	The pelvis is divided into the true and false pelvis, which are separated by the pelvic brim or inlet.

2.	Pelvic brim:

a.	The plane of the pelvic brim is bounded:

i.	in front: by the symphysis pubis.

ii.	on each side: by the upper margin of the pubic bone, the ileopectineal line and the ala of the sacrum.

iii.	posteriorly: by the promontory of the sacrum.

b.	Normal transverse diameter: 13.5cm.

c.	Anterior-posterior diameter: 11cm.

d.	The angle of the brim is normally 60 degrees to the horizontal in the erect position.

3.	Pelvic mid-cavity

a.	The plane of the pelvic mid-cavity is bounded:

i.	in front: by the middle of the symphysis pubis.

ii.	on each side: by the pubic bone, the obturator fascia and the inner aspect of the ischial bone and spines.

iii.	posteriorly: by the junction of the second and third pieces of the sacrum.

b.	The cavity is almost round as the transverse and anterior diameters are similar at 12cm.

c.	The ischial spines are palpable vaginally and are used as landmarks to assess the descent of the head on vaginal examination (station).

d.	They are also used as landmarks for providing an anaesthetic block to the pudendal nerve.

4.	Pelvic outlet:

a.	The pelvic outlet is bounded:

i.	in front: by the lower margin of the symphysis pubis.

ii.	on each side: by the descending ramus of the pubic bone, the ischial tuberosity and the sacrotuberous ligament.

iii.	posteriorly: by the last piece of the sacrum.

b.	Anterior-posterior diameter: 13.5cm.

c.	Transverse diameter: 11cm.

d.	The pelvic floor is formed by the two levator ani muscles, which with their fascia form a musculofascial gutter during the second stage of labor.

5.	Pelvic shapes:

a.	Gynaecoid: most favorable for labor and the most common.

b.	Android: deep transverse arrest.

c.	Anthropoid: persistent occipito-posterior position.

d.	Platypelloid: obstructed labor.

Clinical Pelvimetry

1.	Usually done at 36 weeks of gestation.

2.	To assess pelvic suitability for vaginal delivery routine for:

a.	Primigravida.

b.	Others who have not delivered vaginally before.

3.	Procedure:

a.	Assess brim:

i.	palpate sacral promontory: usually cannot be felt once you cannot palpate it.

ii.	estimation of diagonal conjugate: true conjugate = diagonal – 1.5cm.

b.	Assess cavity:

i.	assess sacral curvature.

ii.	assess interspinous distance: usually more than 10cm.

c.	Assess outlet:

i.	assess greater sciatic notch distance: usually 2 fingers can fit into notch.

ii.	assess cavity contours by opening up 2 fingers: usually parallel or diverging.

iii.	estimate suprapubic angle: should be more than 90 degrees.

iv.	estimate intertuberous distance by palpation: should fit a fist accommodating 4 knuckles, i.e. 7.5 – 8cm.

d.	Conjugate values:

i.	true conjugate (TC) = AP diameter of brim = 11cm.

ii.	diagonal conjugate (DC) = TC + 1.5cm = 12.5cm.

iii.	occipital conjugate (OC) = available AP diameter for fetal engagement.

iv.	TC is not equal to OC if fibroids are present. 

The Fetus

1.	The bones, sutures and fontanelles

a.	The fetal skull is made up of the vault, face and the base.

b.	Bones forming the vault: parietal bones and parts of the occipital, frontal and temporal bones.

c.	Between these bones are four membranous sutures: the sagittal, frontal, coronal and lambdoidal sutures.

d.	Fontanelles are the junction of the various sutures:

i.	anterior fontanelle or bregma (diamond shaped): at the junction of the sagittal, frontal and coronal sutures.

ii.	posterior fontanelle (triangular shaped): at the junction of the sagittal suture and the lambdoidal sutures between the two parietal bones and the occipital bone. 

e.	The area of the fetal skull bounded by the two parietal eminences and the anteiror and posterior fontanelles is termed the vertex.

2.	The diameters of the skull

a.	The fetal head is ovoid in shape.

b.	Suboccipito-bregmatic diameter:

i.	usually 9.5cm.

ii.	longitudinal diameter that presents in a well-flexed fetal head (vertex presentation).

iii.	measured from the suboccipital region to the center of the anterior fontanelle.

iv.	found in occipito-anterior (OA) position.

c.	Suboccipito-frontal diameter:

i.	usually 10cm.

ii.	measured from the suboccipital region to the prominence of the forehead.

iii.	found in occipito-posterior position (OP).

d.	Occipito-frontal diameter:

i.	usually 11.5cm.

ii.	measured from the root of the nose to the posterior fontanelle.

e.	Mento-vertical:

i.	usually 13cm.

ii.	measured from the chin to the furthest point of the vertex.

iii.	known as a brow presentation: usually too large to pass through the normal pelvis.

f.	Submento-bregmatic diameter:

i.	usually 9.5cm.

ii.	measured from below the chin to the anterior fontanelle.

iii.	clinically a face presentation. 

g.	Transverse diameter:

i.	biparietal diameter (cephalic presentation): 9.5cm.

ii.	bitrochanteric diameter (breech presentation): 10cm.