6. Anatomy of the Female Pelvis and Fetal Skull



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6.3  The fetal skull


The fetal skull is the most difficult part of the baby to pass through the mother’s pelvic canal, due to the hard bony nature of the skull. Understanding the anatomy of the fetal skull and its diameter will help you recognise how a labour is progressing, and whether the baby’s head is ‘presenting’ correctly as it comes down the birth canal. This will give you a better understanding of whether a normal vaginal delivery is likely, or if the mother needs referral because the descent of the baby’s head is not making sufficient progress.

6.3.1  Fetal skull bones


The skull bones encase and protect the brain, which is very delicate and subjected to pressure when the fetal head passes down the birth canal. Correct presentation of the smallest diameter of the fetal skull to the largest diameter of the mother’s bony pelvis is essential if delivery is to proceed normally. But if the presenting diameter of the fetal skull is larger than the maternal pelvic diameter, it needs very close attention for the baby to go through a normal vaginal delivery.

You can locate the main skull bones in Figure 6.5.



Figure 6.5  Bones of the fetal skull — side view facing left.

The fetal skull bones are as follows:


  • The frontal bone, which forms the forehead. In the fetus, the frontal bone is in two halves, which fuse (join) into a single bone after the age of eight years.

  • The two parietal bones, which lie on either side of the skull and occupy most of the skull.

Parietal is pronounced ‘parr eye ett al’. Occipital is pronounced ‘ox ipp itt al’.

  • The occipital bone, which forms the back of the skull and part of its base. It joins with the cervical vertebrae (neck bones in the spinal column, or backbone).

  • The two temporal 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.

6.3.2  Sutures


Sutures are 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, as they can to a small extent in the fetus and newborn. It is traditional for their names and locations to be taught in midwifery courses. You may be able to tell the angle of the baby’s head as it ‘presents’ in the birth canal by feeling for the position of the main sutures with your examining fingers. You can see the position of the sutures in the fetal skull in Figure 6.6, and also the diameters at two points.

Suture is pronounced ‘soo tyor’.



Figure 6.6  Regions and landmarks in the fetal skull facing to the left, as seen from above. Notice the average diameters in red.



  • The lambdoid suture forms the junction between the occipital and the frontal bone.

Lambdoid is pronounced ‘lamm doyd’. Sagittal is ‘saj itt al’ and coronal is ‘korr oh nal’.

  • The sagittal suture joins the two parietal bones together.

  • The coronal suture joins the frontal bone to the two parietal bones.

  • The frontal suture joins the two frontal bones together.

Question


What do you notice about the diameters given in Figure 6.6, relative to the dimensions of the pelvic canal (Figures 6.3 and 6.4)?

Answer


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.

End of answer

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.

6.3.3  Fontanels


A fontanel is the space created by the joining of two or more sutures. It is covered by thick membranes and the skin on the baby’s head, protecting the brain underneath the fontanel from contact with the outside world. Identification of the two large fontanels on the top of the fetal skull helps you to locate the angle at which the baby’s head is presenting during labour and delivery. The fontanels are shown in Figures 6.5 and 6.6. They are:

  • The anterior fontanel (also known as the bregma) is a diamond-shaped space towards the front of the baby’s head, at the junction of the sagittal, coronal and frontal sutures. It is very soft and you can feel the fetal heart beat by placing your fingers gently on the fontanel. The skin over the fontanel can be seen ‘pulsing’ in a newborn or young baby.

  • The posterior fontanel (or lambda) has a triangular shape, and is found towards the back of the fetal skull. It is formed by the junction of the lambdoid and sagittal sutures.

6.3.4  Regions and landmarks in the fetal skull


Figures 6.5 and 6.6 allow you to identify certain regions and landmarks in the fetal skull, which have particular importance for obstetric care because they may form the so-called presenting part of the fetus — that is, the part leading the way down the birth canal.

  • The vertex is 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.

  • The brow is 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.

  • The 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.

  • The occiput is 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.

When you study the next Module on Labour and Delivery Care, you will learn about other presentations, including ‘breech’ (the baby is head-up and its feet or bottom is the presenting part), and ‘shoulder’ first.

Now that you know all the major anatomical features of the female reproductive system, the female pelvis and the fetal skull, we move on in Study Session 7 to consider the major physiological changes that take place in a woman’s body during pregnancy.


Summary of Study Session 6


In Study Session 6, you have learned that:

  1. The bony pelvis is composed of the ilium, ischium, pubic bones and sacrum.

  2. The size and shape of the bony pelvis can affect the ease or difficulty of labour and delivery; a broad pelvis gives less difficulty than a narrow one, which may obstruct the descent of the baby down the birth canal.

  3. Certain landmarks in the anatomy of the pelvis are commonly used to estimate the descent of the baby during labour and delivery. The two most important landmarks are the ischial spines and the sacral promontory, which can be felt with the fingers during a vaginal examination.

  4. The pelvic inlet is the space where the baby’s head enters the pelvis; it is larger than the pelvic outlet, where the baby’s head emerges from the pelvis. In order to get through the widest diameter of the inlet and the outlet, the baby has to rotate as it passes through the pelvic canal.

  5. The skull is formed by several bones joined tightly together by joints called sutures. In the fetus and newborn, spaces called fontanels exist between some of the skull bones on the top of the baby’s head. The position of the sutures and the fontanels can tell you about the angle at which the baby’s head is presenting during labour and delivery.

  6. The vertex presentation (where the top of the baby’s head is the presenting part) is the most common and the safest presentation for a normal vaginal delivery. Other presentations carry a much higher risk for the mother and baby.

Self-Assessment Questions (SAQs) for Study Session 6


Now that you have completed this study session, you can assess how well you have achieved its Learning Outcomes by answering these questions. Write your answers in your Study Diary and discuss them with your Tutor at the next Study Support Meeting. You can check your answers with the Notes on the Self-Assessment Questions at the end of this Module.

SAQ 6.1 (tests Learning Outcomes 6.1, 6.2 and 6.3)


Match each anatomical name with the correct description.

Matching quiz

Options


Fused vertebrae at the back of the bony pelivs

Hip bone in the pelvis

Joint between the parietal bones in the fetal skull

Paired bones forming the front of the skull

The top of the fetal skull between the two fontanels


Matches


Frontal bones

Ilium


Sacrum

Sagittal suture

Vertex


Solution

Pair 1


Hip bone in the pelvis

Ilium



Pair 2


Paired bones forming the front of the skull

Frontal bones




Pair 3


Joint between the parietal bones in the fetal skull

Sagittal suture




Pair 4


Fused vertebrae at the back of the bony pelivs

Sacrum



Pair 5


The top of the fetal skull between the two fontanels

Vertex



SAQ 6.2 (tests Learning Outcomes 6.1, 6.2 and 6.3)


Which of the following statements is false? In each case, say why it is incorrect.

A  The female bony pelvis is broader and flatter than the male pelvis.

B  The pelvic inlet is narrower than the pelvic outlet.

C  The iliac crest is an important landmark in measuring the progress of the fetus down the birth canal.

D  The sutures in the fetal skull are strong hard joints that hold the skull bones rigidly in place.

E  A newborn baby’s pulse can be seen beating in the anterior fontanel.


Answer


A is true. The female bony pelvis is broader and flatter than the male pelvis.

B is false. The pelvic inlet is wider (not narrower) than the pelvic outlet.

C is false. The iliac crest is the protuberance at the front of each hip bone; it is not important in measuring the progress of the fetus down the birth canal.

D is false. The sutures in the fetal skull ‘give’ a little under the pressure in the birth canal, allowing the skull bones to move to a small extent. This makes it easier for the baby’s head to pass through the mother’s bony pelvis.

E is true. A newborn baby’s pulse can be seen beating in the anterior fontanel.

End of answer


SAQ 6.3 (tests Learning Outcomes 6.1, 6.2 and 6.3)


List four possible features of the maternal bony pelvis and/or the fetal skull that may result in a difficult labour and delivery.

Answer


The possible features of the maternal bony pelvis and/or the fetal skull that may result in a difficult labour and delivery include (you only had to suggest four):

  • A narrow or deformed pelvis

  • Abnormal growth of tissue in the pelvic cavity

  • A large fetal skull

  • A brow, face, breech or shoulder presentation of the fetus

  • A fetus that does not present the widest part of its skull to the widest part of the pelvic inlet, and then rotate to do the same in the pelvic outlet.

End of answer


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