1. ANATOMY OF NORMAL PELVIS & FETAL SKULL

2. Knowlage of the anatomy of normal female pelvis, fetal skull & soft tissues is essential to understand mechanism of labour.

3. THE PELVIS Normal female pelvis is the rounded gynaecoid pelvis occurs only in 40% of white women. There are three other types: Android, Anthropoid & Plattypelloid pelvis.

4. THE PELVIC BRIM OR INLET The pelvis is divided into true & false pelvis which are separated by pelvic brim. The plane of pelvic brim is bounded in front by the symphysis pubis [the joint separating the two pubic bones] on each side by the upper margin of pubic bone, iliopectineal line & ala of sacrum. Posteriorly by promontary of the sacrum. The anteroposterior diameter of brim [true conjugate] is 11cm & the transverse diameter is 13.5cm.

7. THE PELVIC MID CAVITY Can be described as an area bounded in front by the middle of symphysis pubis,on each side by pubic bone, obturator fascia & inner aspect of ischial bone & spines. Posteriorly bounded by the junction of second & third pieces of sacrum. The cavity is roomy circular with anterioposterior & transverse diameters both measure 12cm.

9. THE PELVIC OUTLET Is roughly diamond shaped & bounded infront by the lower margin of symphysis pubis, on each side by the descending ramus of pubic bone, ischial tuberosity & sacrotuberous ligament & posteriorly by the last piece of the sacrum. In gynaecoid pelvis the subpubic arch is wide & tuberosities are far apart. The anterior-posterior diameter is 13.5cm & the transverse diameter is 11cm.

12. THE PELVIC FLOOR It forms part of birth canal, it is formed by the two levator ani muscles which with their fascia form amusculofascial gutter during the second stage of labour with the opening of the vagina looking forward between sides of the gutter

13. THE PELVIC AXIS Is an imaginary curve line shows the path which the centre of fetal head follows during its passage through the pelvis, it is obtained by taking several anteroposterior diameters of the pelvis & joining their centers.

15. PELVIC INCLINATION Is the angle that any pelvic plane makes with the horizontal. In the erect position the brim is normally inclined at 60 degrees. Pelvic outlet is inclined at about 25 degrees. On vaginal assessment sacral promontary cannot be reached with the examining finger in normal pelvis.

17. It is possible to estimate the diagonal conjugate vaginally which the distance between the promontory &lower margin of symphysis pubis is 12.5 cm. The true conjugate between the promontory &upper margin of symphysis pubis is 11cm.

18. Android pelvis It had many characteristics of male pelvis, the brim is heart-shaped so the widest transverse diameter is much nearer to the sacrum, the side walls tend to converge, the ischial spines are prominent, the sacrum is straight & the subpubic arch is generally narrow with an angle of 70 or less.

20. Both the anteroposterior & transverse diameters of the outlet tend to be reduced. This type of pelvis is funnel-shaped with diameters decrease from above downwards so disproportion become worse as labour proceeds.

21. Anthropoid pelvis The anteroposterior diameter of the brim exceeds the transverse diameter. It tends to be deep & the sacrum has six segments instead of five this is known as a high assimilation pelvis. Sacrum & axis of pelvic cavity are less curved than in gynecoid pelvis & subpubic may be little narrow, but the sacrosciatic notches are wide & anteroposterior diameter of the outlet is large.

23. Platypelloid pelvis Is described as the simple [non-rachitic] flat pelvis. The brim is elliptical with a wide transverse diameter, the subpubic arch is wide & rounded. Except in case of android pelvis, these variations have little effect on normal mechanism of labour unless there is considerable reduction in the size of pelvis.

25. Gynecoid pelvis

26. Anthropoid pelvis

27. Android pelvis

28. Platypelloid pelvis

29. THE FETAL SKULL The bones,sutures & fontanelles Fetal skull is made of the vault, face & base. By the time of birth the bones of face & base are firmly united but the bones of the vault are not well ossified being joined by unossified membranes at the sutures.

30. The bones which form the vault are the parietal bones, parts of occipital, frontal & temporal bones. Three sutures are of obstetric importance: SAGITTAL SUTURE lies between the superior borders of the parietal bones. FRONTAL SUTURE is a forward continuation of the sagittal suture, lies between the two parts of frontal bone. CORONAL SUTURE lies between the anterior borders of the parietal bones & the posterior borders of frontal bones.

32. FONTANELLES Are the junctions of various sutures; ANTERIOR FONTANELLE OR BREGMA: Lies where the sagittal, frontal & coronal sutures meet, is diamond shaped is present at birth & takes about 20 months to close. POSTERIOR FONTANELLE: Lies at the posterior end of the sagittal suture between the two parietal bones & occipital bone. Is triangular in shape &it closed soon after birth.

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

34. DIAMETERS OF FETAL SKULL Is divided into vertical, longitudinal & transverse diameters. The fetal head is ovoid in shape, there are different longitudinal diameters that may present in labour depending on the attitude of fetal head.

35. The longitudinal diameter that present in a well flexed head [vertex presentation] is suboccipito- bregmatic diameter. Ii usually 9.5 cm from suboccipital region to the centre of the anterior fontanelle. If the head is less well flexed the suboccipitofrontal diameter is involved, is taken from the suboccipital region to the prominence of the forehead & measures 10cm.

36. With further extention of the head the occipitofrontal diameter presents which is measured from the root of the nose to the posterior fontanelle & is 11.5. The greatest longitudinal diameter that may present is the mento-vertical, which is taken from chin to the furthest point of vertex & measures 13cm.This is known as a brow presentation & is too large to pass through normal pelvis.

37. Extention of the fetal head beyond this point results in a smaller diameter presenting. The submento-bregmatic diameter is measured from below the chin to the anterior fontanelle &measures 9.5 cm, this is clinically a face presentation. TRANSVERSE DIAMETER is the biparietal diameter is measured from one parietal eminence to the other, is 9.5cm.