The pelvis

• Pelvis


  A hard ring of bone that supports and protects the pelvic organs and the contents of the abdominal cavity
• Female pelvis


  • Wider and flatter shape than the male pelvis, adapted for childbearing
• Bones of the female pelvis

  • Ilium
  • Ischium
  • Pubic bones
  • Sacrum
  • Coccyx
• Iliac crest


  The upper border of the ilium
• Anterior superior iliac spine


  A bony protuberance at the front of the iliac crest
• Ischial spines


  Inward projections of the ischium, used to estimate fetal head descent in labour
• Pubic symphysis


  The joint where the pubic bones meet in the middle
• Sacrum


  A tapered, wedge-shaped bone at the back of the pelvis, consisting of fused vertebrae
• Sacral promontory


  The upper border of the first sacral vertebra, an important landmark for labour and delivery
• Pelvic canal


  The bony passage through which the baby must pass during childbirth
• Gynaecoid pelvis


  • A broad, round pelvis well-adapted for childbirth
• Pelvic inlet


  The roughly circular space where the baby's head enters the pelvis
• The pelvic inlet is 13 cm wide and 12 cm from top to bottom on average
• Pelvic outlet


  The roughly circular space where the baby's head emerges from the pelvis
• The pelvic outlet is only 11 cm wide on average, narrower than the pelvic inlet
• The fetus has to rotate to fit through the pelvic outlet, as it is narrower than the pelvic inlet
• Bones of the fetal skull

  • Frontal bone
  • Parietal bones
  • Occipital bone
  • Temporal bones
• Sutures


  Joints between the bones of the fetal skull that can 'give' under pressure during childbirth
• Occipital bone


  Forms the back of the skull and part of its base. Joins with the cervical vertebrae (neck bones in the spinal column, or backbone)
• Temporal bones


  Two bones, one on each side of the head, closest to the ear
• Understanding the landmarks and measurements of the fetal skull will help you to recognise normal and abnormal presentations of the fetus during antenatal examinations, labour and delivery
• Sutures


  Joints between the bones of the skull. In the fetus they can 'give' a little under the pressure on the baby's head as it passes down the birth canal. During early childhood, these sutures harden and the skull bones can no longer move relative to one another
• Lambdoid suture


  Forms the junction between the occipital and the frontal bone
• Sagittal suture


  Joins the two parietal bones together
• Coronal suture


  Joins the frontal bone to the two parietal bones
• Frontal suture


  Joins the two frontal bones together
• At its widest part, the fetal skull is (on average) 9.5 cm wide. This is 3.5 cm less than the widest diameter of the pelvic inlet, and 1.5 cm less than the widest diameter of the pelvic outlet. Thus, if the mother's pelvis and the fetal skull are the average size, there is just sufficient room for the baby's head to pass through the pelvic canal if the head rotates to present to the widest dimension of the pelvis
• Fontanels


  Spaces created by the joining of two or more sutures. Covered by thick membranes and the skin on the baby's head, protecting the brain underneath
• Anterior fontanel


  Diamond-shaped space towards the front of the baby's head, at the junction of the sagittal, coronal and frontal sutures. Very soft and you can feel the fetal heart beat by placing your fingers gently on the fontanel
• Posterior fontanel


  Triangular shape, found towards the back of the fetal skull. Formed by the junction of the lambdoid and sagittal sutures
• Vertex


  The area midway between the anterior fontanel, the two parietal bones and the posterior fontanel. A vertex presentation occurs when this part of the fetal skull is leading the way. This is the normal and the safest presentation for a vaginal delivery
• Brow


  The area of skull which extends from the anterior fontanel to the upper border of the eye. A brow presentation is a significant risk for the mother and the baby
• Face


  Extends from the upper ridge of the eye to the nose and chin (lower jaw). A face presentation is also a significant risk for the mother and baby
• Occiput


  The area between the base of the skull and the posterior fontanel. It is unusual and very risky for the occiput to be the presenting part