The female pelvis : types and diameter

 The Female pelvis and diameter 

            The bony ring of the pelvis is made up of two symmetrical innominate bones and the sacrum. Each innominate bone is made up of the ilium, ischium, and the pubis, which are joined anteriorly at the symphysis pubis and posteriorly to the sacrum at the sacroiliac joints. 

The female pelvis has evolved for giving birth, and differs from the male pelvis in the following ways:

• The female pelvis is broader, and the bones more slender than those of the male. 
• The male pelvis brim heart-shaped and widest towards the back. Whereas the female pelvis brim is oval-shaped transversely and widest further forwards; the sacral promontory is less prominent. 
• The female pelvic cavity is more spacious and has a wider outlet than the male pelvis. 
• The subpubic angle is rounded in a female pelvis (like a Roman arch) and more acute in the male pelvis (like a Gothic arch). 

Pelvis muscles ligaments 

The pelvis gains its strength and stability through numerous muscles and ligaments. The inner aspect of the pelvic bones is covered by muscles. Above the pelvic brim are the iliacus and psoas muscles; the obturator internus and its fascia occupies the side walls; the posterior wall is covered by the pyriformis; and the levator ani and coccygeus, with their opposite counterparts, constitute the pelvic floor. 

Pelvic ring stability is provided by the following ligaments : 

• Sacrospinous ligament: extending from the lateral margin of the sacrum and coccyx to the ischial spine. 
• Sacrotuberous ligament: extending from the sacrum to the ischial tuberosity. 
• Iliolumbar ligament: extending from the spine to the iliac crest at the back of the pelvis. 
• Dorsal sacroiliac ligament: a heavy band passing from the ilium to the scrum posterior to the sacroiliac joint. 
• Ventral sacroiliac ligament: bridging the sacroiliac joint anteriorly, and is an important stabilizing structure of the joint. 
• Inferior and superior public ligament: a band across the lower and upper part of the symphysis respectively, providing further strength to the joint. 
• Inguinal ligament: running from the anterior superior iliac spine of the ilium to the pubic tubercle of the pubic bone. 

The remaining ligaments that surrounded the pelvis are ligaments that do not provide stabilization of the pelvis. 

Pelvic boundaries 

The pelvis is divided by an oblique plane passing through the prominence of the sacrum, the arcuate, and pectineal lines, and the upper margin of the symphysis pubis, into the greater and the lesser pelvis. The circumference of this plane above from the true pelvis below. The plane of the pelvis is at an angle of 55° to the horizontal. 

Pelvic shapes

There are four basic shapes of the female pelvis:

• Gynaecoid: the classical female pelvis with the inlet transversely oval and a roomier pelvic cavity. 
• Anthropoid: a long, narrow and oval-shaped pelvis due to the assimilation of the sacral body to the fifth lumbar vertebra. 
• Android: the inlet is heart-shaped and the cavity is funnel-shaped with a contracted outlet. 
• Platypolloid: a wide pelvis flattened at the brim with the sacral promontory pushed forward. 

Diameters of the female pelvis 

The female bony pelvis is not distensible, and only very minor degree of movement are possible at the symphysis pubis and the sacroiliac joints. Its dimensions are hence, critical for normal childbirth. 

The diameters of the female pelvis vary at different parts of the pelvis. 

• The true pelvis is bound anteriorly by the symphysis pubis (3.5cm long) and posteriorly by the sacrum (12cm long)
• The superior circumference of the true pelvis is the pelvic inlet and the inferior circumference is the outlet.

The true pelvis has four planes.

 

Plane of pelvic inlet

• This is bound anteriorly by the upper border of the pubis,  laterally by the iliopectineal line, and posteriorly by the sacral promontory. 
• The average transverse diameter is 13.5 cm and the average anteroposterior diameter is 11cm ( obstetrics conjugate diameter ) (transversely oblong)
• It is not possible to measure these diameters clinically, and the only diameter at the pelvic inlet amenable to clinical assessment is the distance from the inferior margin of the pubic symphysis to the midpoint of the sacral promontory ( the diagonal conjugate ), which is 1.5cm greater than the obstetric conjugate diameter. 

Plane of greatest pelvic dimension/cavity 

• This is the roomiest part of the pelvis and has little significance 
• It is almost round in shape with an average transverse diameter of 13.5 cm and an average anteroposterior diameter of 12.5cm.

Plane of least pelvic dimensions /mid-pelvis (circular in shape)

• This is bound anteriorly by the apex of the pubic arch, laterally by the ischial spines, and posteriorly by the tip of the sacrum. 
• The interspinous diameter is the narrowest space in the pelvis (10cm) and represents the level at which impaction of the fetal head is most likely to occur.

Plane of pelvic outlet 

• This is bound anteriorly by the pubic arch, which should have a desired angle of >90°, posterolaterally by the sacrotuberous ligaments and ischial tuberosities leading to the coccyx posteriorly (anteroposteriorly oblong)
• The average intertuberous diameter in 11cm.

Assessment of pelvic adequacy 

Examination of pelvis before labour does not accurately discriminate between those who will achieve vaginal birth and those who will not. Even computed Tomography (CT) or magnetic resonance imaging (MRI) scanning, together with ultrasound of the fetal head,  is not helpful, unless there is a gross abnormality, which will be evident from the history or gait. This is because of the dynamic nature of labour, when the head 'moduls' ( reducing the head circumference by a few centimetres ) and the joints of the pelvis can move, increasing the pelvic dimensions slightly. 

The ideal female pelvis has the following features :

• Oval brim
• Shallow cavity 
• Non-prominent ischial spines
• Curved sacrum with large sciatic notches (>90∞)
• Sacrospinous ligament >3.5cm long.
• Rounded subpubic arch >90∞.
• Intertuberous distance of at least 10 cm.

Diagonal conjugate diameter of at least 12cm.