ANATOMY OF THE FEMALE PELVIS AND THE FETAL SKULL Dr. Basima Al- Ghazali

Knowledge of the shape and dimension of the normal female pelvis and of fetal skull is essential for proper understanding of the mechanism of labor and its management. THE PELVIS TYPES 1. GYNAECOID PELVIS occurs in only 40 percent of white women. It is the normal female pelvis and the most favourable for labour. 2. ANDROID PELVIS, pelvis with male characteristic and is said to predispose to deep transverse arrest. 3. ANTHROPOID PELVIS, in which there is slight increase in the antero-posterior diameter of the pelvis. It encourages an occipitoposterior position. 4. PLATYPELLOID PELVIS, slightly flattened pelvis, is also associated with an increased risk of obstructed labour.

BONY PELVIS Hip bone (Ilium, ischium and pubis) Sacrum Coccyx Joined anteriorly by pubic symphysis Posteriorly by sacro -iliac joint

GYNAECOID PELVIS The pelvic brim; the pelvic brim lies in one plane bounded in front by the sympheses pubis , on each side by the upper margin of the pubic bone , the ilio -pectinial line and the ala of the sacrum and posteriorly by the promontory of the sacrum. The brim is oval in shape with the transverse diameter (13.5cm) slightly greater than the antero-posterior diameter (11cm).

The Pelvic Inlet (Brim):- Boundaries:- Sacral promontory, Ala of the sacrum, sacroiliac joints, iliopectineal lines, iliopubic eminencies, upper border of the superior pubic rami, pubic tubercles, pubic crests and upper border of symphysis pubis.

The pelvic cavity ; this is some time described in terms of an imaginary plane bounded in front by the middle of sympheses pubis , on each sides by the pubic bone , the obturater fascia and the inner aspect of the ischeal bone and posteriorly by the junction of the second and third peice of the sacrum . the ischeal spines lie slightly below this plane .The pudendal nerve passes behind and below the ischial spine on each side. In the gynecoid pelvis this cavity is circular and roomy because the sacrum is inclinated bach ward , is well curved and the sacrosciatic notches are wide . the cavity is almost round as the transverse and antro posterior diameter are similar at 12cm.

The pelvic outlet ; it is roughly diamond shaped and is bounded in front by the lower margin of the symphesis pubis , on each side by the desending ramus of the pubic bone ,the ischeal tuberosity and the sacrotuberous ligament and posteriorly by the last peice of the sacrum ( not the coccyx , which is mobile ). Unlike the brim, the outlet does not have boundaries which lie in a single plane, but an imaginary plane of the outlet is described ,passing from the lower margin of the symphesis pubis to the last peice of the sacrum .

The shape of the outlet is ovale with the long axis in the anteroposterior diameter is 13.5cm while the transverse diameter is 11cm. During pregnancy the ligaments of the sacroiliac joints and the symphesis pubis become softened and there is slightly increased mobility at these joints. The sacrococcygeal joint allows the coccyx to move freely backwards during delivery.

CLINICAL EXAMINATION OF THE PELVIS A pelvic examination is some time made early in pregnancy to; 1.Confirm the diagnosis of pregnancy and its duration. 2.To exclude abnormalities of the pelvic organs and 3.To assess the capacity of the pelvis. If the patient had miscarriage previously, it is wise to postpone any internal examination until later. In any case , a further pelvic examination is usually performed at about 36 weeks , when a better assessment of pelvic cavity can be made , since at that time the pelvic floor is more relaxed , and the size of the fetal head can also be related to that of the pelvic brim at this time .

VAGINAL ASSESSMENT OF THE PELVIS THE BRIM The sacral promontory can not be reached with an examining finger in a normal pelvis unless the patient is anesthetised. If it can be felt it is considerably reduced, and in such case it may be possible to estimate the diagonal conjugate with the exploring finger. This is distance between the promontory and the lower margin of the symphysis pubis. The true conjugate may be derived by subtracting 1.5 cm from the diagonal conjugate.

THE PELVIC CAVITY On vaginal examination a general idea of the capacity of the pelvic cavity can be gained; 1. The anterior surface of the sacrum is palpated from above downwards; noting weather it is straight or concave. 2. The position of the ischial spines may be assessed by palpation of the sacrospinous ligaments, which should be of a length that will accommodate three finger breadths. 3. the spines are some time unduly prominent but it is the distance between them rather than their prominence that matters .

THE PELVIC OUTLET The inter tuberous diameter can be determined by external palpation, but vaginal examination gives the best assessment of the width of the sub pubic arch and of the position of the sacrum.

THE PELVIC FLOOR Although this is not part of the bony pelvis it is mentioned here because it forms part of the birth canal and plays an important part in the mechanism of labour. The two levator ani muscles, with their fascia form a musculofascial gutter during the second stage of labour.