What Is The Diameter Of The Skull?

The measurements of the skull are important because the nurse needs a practical understanding of the relationship between the fetal head and the mother's pelvis. It will become clear that some diameters are more favorable for easy passage through the pelvis canal and this will depend on the attitude of the head.

There are two transverse diameters:

1. Biparietal diameter: 9.5 cm – between the parietal eminences.

2. Bitemporal diameter: 8.2 cm – between the furtherest points of the coronal sutures at the temples.

The remaining diameters are described below:
3. Sub occipito bregmatic: 9.5 cm – from below the occipital protuberance to the centre of the anterior frontal or Bregma.

4. Sub occipito frontal: 10 cm – from below the occipital protuberance to the centre of the frontal suture.

5. Occipito frontal: 11.5 cm – from the occipital protuberance to the glabella.

6. Mento vertical: 13.5 cm – from the point of the chin to the highest point on the vertex, slightly nearer to the posterior than to the anterior fontanel.

7. Sub mento vertical: 11.5 cm – from the point where the chin joins the neck to the centre of the Bregma.

8. Sub mento bregmatic: 9.5 cm – from the point where the chin joins the neck to the centre of the Bregma.

The attitude of the head determines which diameter will present in labor and therefore influences the outcome.

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