Importance of the fetal skull
- Largest part of the fetal body.
- Most frequent [resenting part of the fetus.
- Least compressible of all fetal parts.
Anatomy of the Fetal Skull
Cranial Bones
The fetal skull is made up of six cranial bones which are the following:
- Sphenoid
- Ethmoid
- Temporal
- Frontal
- Occipital
- Parietal
The frontal, occipital and the parietal cranial bones could either be fetal presenting part if the presentation is vertex.
Membrane Spaces
During birth, bones move and overlap with each other to allow the fetal head to fit through the birth canal which is a process termed as molding. Molding is made possible because of the presence of the suture lines. Without these structures a fetus’ head cannot pass through the birth canal. There are different types of sutures:
- Sagittal suture line – joins the two parietal bones.
- Coronal suture line – joins the frontal and the parietal bones.
- Lambdoid suture line – joins the occiput and the parietal bones.
Fontanelles
Fontanelle is a membrane-covered space at the junction of a main suture line.
Types of Fontanelles:
- Anterior fontanelle – diamond-shaped fontanelle. This fontanelle closes at about 12-18 months and is larger than the other.
- Posterior fontanelle – triangular-shaped fontanelle. This fontanelle closes between 2-3 months of age and is smaller.
Measurements of the fetal skull
Transverse diameters of the fetal skull
- Biparietal – 9.25 cm
- Bitemporal – 8 cm
- Bimastoid – 7 cm
Anteroposterior (AP) diameter
- Suboocipitobregmatic – 9.5 cm (the narrowest AP diameter). This measurement is taken from below the occiput to the anterior fontanelle.
- Occipitofrontal – 12 cm (from the occiput to the mid-frontal bone)
- Occipitomental – 13.5 cm (the widest AP diameter). This measurement is taken from the occiput to the chin.
Which one of these diameters is presented at the birth canal depends on the degree of flexion, which is known as the ATTITUDE, the fetal head assumes prior to delivery.
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