Anatomy of the female pelvis and the fetus relevant to labour
Dr . sarab salihThe pelvis:-
The pelvic brim or inlet:The pelvic brim is the inlet of the pelvis andis bounded in front by the symphysis pubis (the joint separating the two pubic bones), on each side by the upper margin of the pubic bone, the ileopectineal line and the ala of the sacrum, and posteriorly by the promontory of the sacrum (Fig 1). The normal transverse diameter in this plane is 13.5 cm and is wider than the anterior-posterior (AP) diameter, which is normally 11 cm (Fig 2). The angle of the inlet is normally 60 to the horizontal in the erect position, but in Afro-Caribbean women this angle may be as much as 90 (Fig 3). This increased angle may delay the head entering the pelvis in labour.
The pelvic mid-cavity:
The pelvic mid-cavity can be described as an area bounded in front by the middle of the symphysis pubis, on each side by the pubic bone, the obturator fascia and the inner aspect of the ischial bone and spines, and posteriorly by the junction of the second and third sections of the sacrum. The cavity is almost round, as the transverse and anterior diameters are similar at 12 cm. The ischial spines are palpable vaginally and are used as landmarks to assess the descent of the head on vaginal examination (station). They are also used as landmarks for providing an anaesthetic block to the pudendal nerve. The pudendal nerve passes behind and below the ischial spine on each side. The pelvic axis describes an imaginary curved line, which shows the path that the centre of the fetal head takes during its passage through the pelvis.The pelvic outlet:
The pelvic outlet is bounded in front by the lower margin of the symphysis pubis, on each side by the descending ramus of the pubic bone, the ischial tuberosity and the sacrotuberous ligament, and posteriorly by the last piece of the sacrum. The AP diameter of the pelvic outlet is 13.5 cm and the transverse diameter is 11 cm (Fig. 4). Therefore, the transverse is the widest diameter at the inlet, but at the outlet it is the AP. Recognizing this is crucial to the understanding of the mechanism of labour.The pelvic measurements given here are obviously average values and relate to bony points. Maternal stature, previous pelvic fractures and metabolic bone disease such as rickets may all be associated with measurements less than these population means. Furthermore, as the pelvic ligaments at the pubic ramus and the sacroiliac joints loosen towards the end of the third trimester, the pelvis often becomes more flexible and these diameters may increase during labour.
Pelvic shapes :
A variety of pelvic shapes has been described, and these may contribute to difficulties encountered in labour. The gynaecoid pelvis is the most favourable for labour, and the most common (Fig.5). Other pelvic shapes are shown in (Figures 6 to 8). An android-type pelvis is said to predispose to deep transverse arrest and the anthropoid shape encourages an occipito-posterior (OP) position (see below). A platypelloid pelvis also is associated with an increased risk of obstructed labour. The final obstacle to be negotiated by the fetus during labour is the perineum.
The pelvic floor :
This is formed by the two levator ani muscles which, with their fascia, form a musculofascial gutter during the second stage of labour (Fig 9). The perineal body is a condensation of fibrous and muscular tissue lying between the vagina and the anus (Fig 10). It receives attachments of the posterior ends of the bulbo-cavernous muscles, the medial ends of the superficial and deep transverse perineal muscles, and the anterior fibres of the external anal sphincter. It is always involved in a second-degree perineal tear and an episiotomy.
Fetal skull :-
The bones, sutures and fontanelles:
The fetal skull is made up of the vault, the face and the base. The sutures are the lines formed where the individual bony plates of the skull meet one another.
At the time of labour, the sutures joining the bones of the vault are soft, unossified membranes, whereas the sutures of the face and the skull base are firmly united (Fig.11).
The vault of the skull is formed by the parietal bones and parts of the occipital, frontal and temporal bones. Between these bones there are four membra nous sutures: the sagittal, frontal, coronal and lambdoidal sutures.
Fontanelles
are the junctions of the various sutures. The anterior fontanelle, or bregma (diamond shaped), is at the junction of the sagittal, frontal and coronal sutures. The posterior fontanelle (triangular shaped) lies at the junction of the sagittal suture and the lambdoidal sutures between the two parietal bones and the occipital bone(Fig. 12). The fact that these sutures are not united is important for labour. It allows these bones to move together and even to overlap. The parietal bones usually tend to slide over the frontal and occipital bones. Furthermore, the bones themselves are compressible. Together, these characteristics of the fetal skull allow a process called moulding to occur, which effectively reduces the diameters of the fetal skull and encourages progress through the bony pelvis, without harming the underlying brain (Fig 13). However, severe moulding can be a sign of cephalopelvic dispro portion (CPD ).
The area of the fetal skull bounded by the two parietal eminences and the anterior and posterior fontanelles is termed the vertex.
The fetal skull diameters
The fetal head is ovoid in shape. The attitude of the fetal head refers to the degree of flexion and extension at the upper cervical spine. Different longitudinal diameters are presented to the pelvis in labour depending on the attitude of the fetal head (Figs14 and 15).
The longitudinal diameter that presents in a well-flexed fetal head (vertex presentation) is the suboccipitobregmatic diameter. This is usually 9.5 cm and is measured from the suboccipital region to the centre of the anterior fontanelle (bregma). The longitudinal diameter that presents in a less well-flexed head, such as is found in the OP position, is the suboccipito-frontal diameter, and is measured from the suboccipital region to the prominence of the forehead. It measures 10 cm.