Fetal skull obg explain in detail
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fetal skull anatomy obstetrics diameters sutures fontanelles OBG detailed
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Fetal Skull - Detailed OBG Notes
The fetal skull is of immense obstetric importance because it is the largest and least compressible part of the fetus. Its ability to adapt to the maternal pelvis determines the ease or difficulty of labor.
1. Divisions of the Fetal Skull
The fetal skull has three main parts:
| Part | Description |
|---|---|
| Vault (Cranium) | Upper dome-shaped part above a line drawn through the base of the nose, supra-orbital ridges, and external auditory meatus |
| Face | Below the vault, from supra-orbital ridges to the chin |
| Base | Bones at the floor of the skull; firmly fused - NOT compressible |
The vault bones are important obstetricaly - they are thin, poorly ossified, and connected by membranes (sutures), making the vault compressible and moldable.
2. Bones of the Vault
The vault is formed by 5 bones (remember: 2 pairs + 1):
- 2 Frontal bones (fuse into 1 after birth)
- 2 Parietal bones
- 1 Occipital bone (squamous part)
- (Parts of the temporal bones also contribute)
3. Sutures of the Fetal Skull
Sutures are membranous joints between skull bones. They allow molding during labor.
| Suture | Location | Bones Joined |
|---|---|---|
| Sagittal suture | Runs anteroposteriorly in the midline | Joins the 2 parietal bones |
| Coronal suture | Transversely across the skull | Joins frontal bones to parietal bones |
| Frontal (Metopic) suture | Midline anteriorly | Joins the 2 frontal bones |
| Lambdoid suture | Posterior, V-shaped | Joins occipital bone to parietal bones |
| Temporal sutures (2 pairs) | Lateral | Join temporal bones to parietals |
Obstetric importance of sutures:
- Help identify fetal head position (e.g., sagittal suture in AP or transverse diameter of pelvis)
- Allow overlapping (molding) of bones during labor
- At the time of labor, vault sutures are unossified membranes; base sutures are firmly united
4. Fontanelles
Fontanelles are membranous areas at the junctions of sutures. There are 6 total, but only 2 are clinically important.
Anterior Fontanelle (Bregma) - DIAMOND-SHAPED
- Formed by junction of 4 sutures: frontal (anteriorly), sagittal (posteriorly), and 2 coronal sutures (on either side)
- Surrounded by 4 bones: 2 frontal + 2 parietal
- Shape: Diamond/lozenge-shaped
- Dimensions: ~3 cm AP × 3 cm transverse
- Floor: Membranous (soft, pulsatile - the "soft spot")
- Ossification: Closes at ~18 months after birth (pathological if open beyond 24 months)
- Obstetric significance:
- Identifies the sinciput (front of head)
- Helps determine degree of flexion/extension of head
- Used to identify vertex presentation
- Enlarged in hydrocephalus
Posterior Fontanelle (Lambda) - TRIANGULAR
- Formed by junction of 3 sutures: sagittal (anteriorly) + 2 lambdoid sutures (on either side)
- Surrounded by 3 bones: 2 parietal + 1 occipital
- Shape: Triangular
- Dimensions: ~1.2 × 1.2 cm
- Floor: Bony at term (strictly speaking, not a true fontanelle at term)
- Ossification: Closes at term (or shortly before/after)
- Obstetric significance:
- Identifies the occiput - used to diagnose position of fetal head in labor (OA, OP, OT)
- Key landmark in vaginal examination during labor
Comparison Table: Anterior vs Posterior Fontanelle
| Feature | Anterior (Bregma) | Posterior (Lambda) |
|---|---|---|
| Shape | Diamond/Lozenge | Triangular |
| Size | Large (~3×3 cm) | Small (~1.2×1.2 cm) |
| Sutures meeting | 4 | 3 |
| Bones surrounding | 4 (2 frontal, 2 parietal) | 3 (2 parietal, 1 occipital) |
| Floor | Membranous | Bony at term |
| Closes | 18 months post-birth | At term |
| Overlapping during molding | No | Yes |
| Identifies | Sinciput/degree of flexion | Occiput/position of head |
Sagittal Fontanelle
- Inconsistent in presence
- Located on the sagittal suture at junction of anterior 2/3 and posterior 1/3
- No obstetric importance
5. Regions of the Fetal Skull
| Region | Boundaries |
|---|---|
| Vertex | Bounded anteriorly by the anterior fontanelle and coronal suture, posteriorly by the posterior fontanelle and lambdoid suture, laterally by the temporal ridges. This is the MOST favorable presenting part. |
| Sinciput (Brow) | From the root of the nose and supra-orbital ridges to the anterior fontanelle and coronal suture |
| Occiput | From the posterior fontanelle and lambdoid suture to the nape of the neck |
| Face | From the supra-orbital ridges to the chin (mentum) |
6. Diameters of the Fetal Skull
The engaging diameter depends on the degree of flexion or extension of the fetal head.
A. Anteroposterior (Longitudinal) Diameters
| Diameter | Measurement Point | Length | Clinical Significance |
|---|---|---|---|
| Suboccipitobregmatic (SOB) | From below occipital protuberance to center of anterior fontanelle (bregma) | 9.5 cm | Engaging diameter in complete flexion (fully flexed vertex/OA) - SMALLEST, most favorable |
| Suboccipitofrontal (SOF) | From below occipital protuberance to the anterior end of bregma (frontal suture) | 10 cm | Engaging diameter in incomplete flexion; also distends vulva in OA after crowning |
| Occipitofrontal (OF) | From occipital protuberance to glabella (root of nose) | 11.5 cm | Engaging diameter in occipitoposterior (OP) position (deflexed vertex); distends vulva in face-to-pubis delivery |
| Mentovertical (MV) | From tip of chin to highest point on vertex | 13.5 cm | Presenting diameter in brow presentation - LARGEST, usually too big for vaginal delivery |
| Submentobregmatic (SMB) | From junction of chin and neck to center of bregma | 9.5 cm | Engaging diameter in face presentation with complete extension - favorable for vaginal delivery |
| Submentovertical (SMV) | From junction of chin and neck to a point midway between anterior and posterior fontanelles | 11.5 cm | Engaging diameter in incompletely extended face; distends vulva during face delivery |
B. Transverse Diameters
| Diameter | Measurement Point | Length | Significance |
|---|---|---|---|
| Biparietal (BPD) | Between the 2 parietal eminences | 9.5 cm | Widest transverse diameter; used in USG for fetal age estimation |
| Bitemporal | Between the anterior ends of the coronal suture (temporal areas) | 8 cm | Narrowest transverse diameter |
| Bimastoid | Between the tips of the 2 mastoid processes | 7.5 cm | Smallest transverse diameter |
| Subparietal-supraparietal | From below one parietal eminence to above opposite | 9 cm | Diameter in asynclitism |
7. Attitude of the Fetal Head and Presenting Diameters
The attitude refers to the relationship of the fetal head to the fetal spine (degree of flexion/extension):
| Attitude | Presentation | AP Diameter | Vaginal Delivery |
|---|---|---|---|
| Complete flexion | Vertex (OA) | SOB = 9.5 cm | Easiest - most favorable |
| Partial deflexion | Vertex (OP) | OF = 11.5 cm | Difficult but possible |
| Moderate extension | Brow | MV = 13.5 cm | Usually impossible vaginally |
| Complete extension | Face | SMB = 9.5 cm | Possible if mentoanterior |
8. Molding
Molding is the change in shape of the fetal skull during passage through the birth canal due to overlapping of skull bones at the sutures.
- Possible because: Vault sutures and fontanelles are membranous at birth
- Which bones overlap: Parietal bones slide under the frontal and occipital bones
- In the posterior fontanelle, the bones DO overlap during molding
- In the anterior fontanelle, bones do NOT overlap during molding
- Caput succedaneum is the serosanguineous swelling of the scalp seen over the presenting part (crosses suture lines - distinguishes from cephalohematoma)
- Excessive molding is a sign of disproportion (CPD)
9. Clinical Importance Summary
- Vaginal examination in labor - identifying sutures and fontanelles tells you:
- Presenting part (vertex, face, brow)
- Position (OA, OP, OT)
- Degree of flexion
- Ultrasound measurement - BPD (9.5 cm) is the standard for gestational age estimation
- Prediction of labor outcome - smaller presenting diameter = easier delivery
- Diagnosis of malpresentations - brow/face presentations identified by palpating sutures
- Molding assessment - excessive molding + caput warns of CPD
Quick Recall - Diameters Mnemonic:
- SOB (Suboccipitobregmatic) = 9.5 cm - best! (Vertex, full flexion)
- SOF (Suboccipitofrontal) = 10 cm (partial deflexion)
- OF (Occipitofrontal) = 11.5 cm (OP position)
- MV (Mentovertical) = 13.5 cm - worst! (Brow - can't deliver vaginally usually)
- SMB (Submentobregmatic) = 9.5 cm (Face, full extension - mento-anterior)
- BPD (Biparietal) = 9.5 cm - standard transverse
Sources: Foetal Skull - El-Mowafi, GFMER | Midwives Revision Uganda - Fetal Skull | University of Anbar - Maternal Pelvis and Fetal Skull PDF | Creasy & Resnik's Maternal-Fetal Medicine
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