27.     The fetal skull.  

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MCQ Paper 1

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a. moulding in labour only affects the vault True
b. the anterior fontanelle is at the junction of the sagittal and coronal sutures True
c. the anterior fontanelle closes in the first six months of infancy False
d. the vertex is delineated by the anterior and posterior fontanelles and the frontal eminences False
e. the smallest AP diameter is the sub-occipito-frontal False
f. the greatest diameter is the mento-vertical, typical of brow  presentation True

The fetal skull has three parts, the face, the base and the vault.

Only the vault moulds.

It takes up to two years for the anterior fontanelle to close.

The vertex is the area between the fontanelles and the parietal eminences.

The smallest diameter is the sub-occipito-bregmatic.

It is worth getting out a skull model and rehearsing the basic anatomy.

Understanding big and small diameters and explaining them to patients can be made easy by using a couple of simple analogies.

The first shows the adverse effects of a fetal head deflexing away from the ideal vertex presentation.

I ask the woman to imagine taking a hen’s egg in one hand.

I then ask her to form a circle with the thumb and first finger of the other hand.

Then she has to imagine how she would pass the egg through the circle.

She will automatically present one or other end of the egg.

You explain that this is like the vertex.

If she “deflexes” the egg, she can easily see that wider and wider diameters present and that the egg won’t pass unless she has hands like a gorilla.

Another analogy is for those who regularly dress small children.

Ask a mother, “how do you pull a tight sweater or T-shirt over a child’s head?”.

Experience will have shown them that this is most easily done by applying the opening in the garment to the child’s vertex.

Applying it to the top of the head, or worse, the brow, will lead to trauma and tears.

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