Female pelvis and Fetal skull

University of Baghdad

College of Medicine
Obstetrics & Gynecology Dept

Dr. Najma Mahmood

Female pelvis & Fetal skull

The Pelvis :-The pelvic brim or inlet :-The pelvis is sometime divided into the true & false pelvis which are separated by pelvic brim ,the plane of pelvic brim is bounded infront by symphysis pubis , on each side by the upper margin of pubic bone ,the ileopectineal line & the ala of sacrum posteriorly by promontary of sacrum

The normal transverse diameter of this plane is 13.5 cm, while anterior-posterior diameter is 11cm , the angle of the brim is normally 60degree to the horizontal in erect position.



The pelvic cavity:-

The cavity of the pelvis is almost round as the transverse & anterior diameters are similar at 12 cmthe ischial spine are palpable vaginally & are used as landmarks to assess the descent of the head (station).

The pelvic axis describe an imaginary curved line which shows the path that the centre of the fetal head takes during its passage through the pelvis The pelvic floor:- This is formed by the 2 levator ani muscles which with their fascia form amusculofascial gutter during the second stage of labour. during its passage through the pelvis .
The pelvic outlet :- This is bounded infront by the lower margin of symphysis pubis, on each side by the descending ramus of pubic bone , ischial tuberosity & the sacrotuberous ligament,posteriorly by The last piece of sacrum, the A-P diameter of pelvic outlet is 13.5 cm & transverse diameter 11 cm.



Types of female pelvis:-
1) Gynecoid pelvis:- this is suitable for vaginal delivery.
2) Android pelvis:- associated with deep transverse arrest.
3) Anthropoid pelvis :- associated with persistant OP position .
4) Plattypeloid pelvis :- associated with obstructed labour.

Fetal skull :-

The fetal skull made up of the vault , face , & the base. At the time of labour only unossified membrane at the sutures join the bones of the vault, unlike the face & the base which are firmly united.
The bones that form the vault are the parietal & parts of the occipital , frontal & temporal bones. Between these bones there are 4 membranous sutures : sagittal , frontal, coronal & lambdoidal sutures.

Fontanelles are the junction of the various sutures .

The anterior fontanelle or bregma (diamond shaped) is at the junction of the sagittal, frontal & coronal sutures . The posterior fontanelle (triangular shaped) lies at the junction of the sagittal suture & the lamdoidal suture between the 2 parietal bones & the occipital bone.

The area of the fetal skull bounded by the 2 parietal eminences & the anterior and posterior fontanelles is termed the vertex.

The diameters of the skull :-

The fetal head is ovoid in shape. There are different longitudinal diameters that may present in labour depending on the attitude the fetal head.
The longitudinal diameter that presents in a well flexed head (vertex presentation) is the suboccipito-bregmatic diameter, this is usually 9.5 cm & is measured from suboccipital region to the centre of anterior fontanelle ( bregma).
The longitudinal diameter that present in less well
Flexed head , such as found in occipito-posterior
(OP) position ,is the suboccipito- frontal diameter
and is measured from the suboccipital region to the prominence of the forehead, it measures10 cm.
With further extension of the head the occipito- frontal diameter presents , this is measured from the root of the nose to the posterior fontanelle & is 11. 5 CM.
The greatest longitudinal diameter that may present is the mento- vertical which is taken from the chin to the furthest point of the vertex & measure 13 cm ,this is known as brow presentation & it is usually too large to pass through the normal pelvis .


Extension of the fetal head beyond this point result in a smaller diameter presenting . The submento-bregmatic diameter is measured from below the chin to the anterior fontanelle & measure 9.5 cm , this is clinically a face presentation.