Fetal skull and its obstetric significance
The head of a term infant is the densest and largest part, paving the way through the birth canal. During labor, the physician relies on landmarks — sutures and fontanelles.
The skull consists of two frontal, two parietal, two temporal, occipital, sphenoid, and ethmoid bones. The main sutures lie between them:
- Sagittal (longitudinal) suture: connects the right and left parietal bones.
- Frontal suture: lies between the frontal bones (not yet fused in the fetus).
- Coronal suture: runs perpendicular to the sagittal suture, connecting the frontal bones to the parietals.
- Lambdoid (occipital) suture: connects the occipital bone to the parietals.
Fontanelles are located at the intersection of the sutures. The anterior (large) fontanelle is diamond-shaped, where four sutures converge (sagittal, frontal, and the two ends of the coronal). The posterior (small) fontanelle is triangular, representing a small depression where three sutures converge (the sagittal meets the lambdoid). In a term infant, the small fontanelle is closed by a fibrous plate or bone. Palpatory differentiation of these structures allows the physician to accurately determine the engagement of the head.
The presence of sutures and fontanelles ensures fetal head moulding — the ability of skull bones to shift and overlap. This gives the skull the plasticity required to overcome spatial obstacles in the maternal pelvis.
Dimensions of the head and trunk
Each presentation option and stage of the labor mechanism corresponds to a specific dimension with which the head passes through the pelvic planes.
Oblique diameters:
- Suboccipito-bregmatic: from the suboccipital fossa to the anterior angle of the anterior fontanelle. Length — 9.5 cm, circumference — 32 cm (the smallest of all).
- Suboccipito-frontal: from the suboccipital fossa to the hairline of the forehead. Length — 10.5 cm, circumference — 33 cm.
- Occipito-frontal: from the chin to the most distant point of the occiput. Length — 13.5 cm, circumference — 40 cm (the largest).
Other important head dimensions:
- Direct (straight) diameter: from the nasal bridge to the occipital protuberance (12 cm, circumference 34 cm).
- Vertical diameter: from the vertex to the hyoid bone (9.5 cm, circumference 32 cm).
- Biparietal diameter: greatest distance between the parietal eminences (9.5 cm).
- Bitemporal diameter: distance between the furthest points of the coronal suture (8 cm).
In obstetrics, the head is conventionally divided into segments. The large segment is the largest circumference passing through the pelvis in a specific presentation. For example, in occipital presentation (flexed position) this is the circumference of the suboccipito-bregmatic diameter, while in face presentation (maximum extension) it is the vertical diameter. Any segment smaller than the large segment is called the small segment.
The trunk also has its own parameters: the biacromial diameter is 12 cm (circumference 35 cm), and the bitrochanteric diameter is 9–9.5 cm (circumference 27–28 cm).
Obstetric topography of the fetus
For managing labor, it is critically important to know the spatial arrangement of the infant in the uterus. It is described by several basic terms.
Fetal attitude (habitus) — the relation of the limbs and head to the trunk. Normally, the infant is curled into an ovoid 25–26 cm long: the trunk is flexed, the chin is pressed against the chest, legs are flexed and pressed to the abdomen, arms are crossed. The wide part (pelvic end) is in the uterine fundus, while the narrow part (occiput) is directed toward the pelvic inlet.
Fetal lie (situs) — the relation of the long axis of the fetus to the long axis of the uterus. In 99.5% of cases it is longitudinal (axes coincide). If the axes intersect at a right angle, it is a transverse lie; if at an acute angle, it is an oblique lie. Transverse and oblique lies are considered pathological.
Fetal position (positio) — the relation of the fetal back to the lateral walls of the uterus. If the back faces left, it is the first position (more common due to the rotation of the uterus with its left side anterior); if right, the second position. In transverse and oblique lies, the position is determined by the head rather than the back.
Variety of position (visus) — the relation of the back to the anterior (anterior variety) or posterior (posterior variety) wall of the uterus.
Presentation (praesentatio) — the relation of a major part to the pelvic inlet. It can be cephalic (occipital, sinciput, brow, face) and pelvic/breech (frank breech, complete breech, footling breech). The lowest-lying part that passes first through the birth canal is called the presenting part (pars praevia).
How to remember the fontanelles: Anterior — like a four-way intersection (4 sutures, rhombus), Posterior — like a three-way fork (3 sutures, triangle).
Class and exam questions
How to distinguish an oblique fetal lie from a transverse lie?
Differential diagnosis is performed by the relation of the major fetal parts to the iliac crests of the mother. In an oblique lie, one of the major parts (head or pelvis) lies below the iliac crest.
What is head moulding and why is it needed?
This is the ability of the fetal skull bones to shift and overlap due to the presence of sutures and fontanelles. Moulding gives the head plasticity, which is necessary to overcome spatial obstacles in the birth canal.
How to determine fetal position in transverse or oblique lies?
In these cases, the position is determined by the location of the head rather than the back. If the head is on the left, it is the first position; if on the right, the second.
What else to study on this topic
- Shapes of the female pelvis (gynecoid, android, anthropoid, platypelloid)
- Stages of labor: latent, active, and deceleration phases
- Biomechanism of the second stage of labor (moulding and crowning of the head)
- Systemic changes in the maternal organism during labor
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