The Passenger (Fetal Anatomy)
The Fetal Skull
The skull bones, sutures, and fontanelles.
- The fetal skull is made up of:
- The vault.
- The face.
- The base.
- Sutures: The lines formed where the individual bony plates meet each other.
Fetal Skull - Fontanelles
The fontanelles are the junctions of the various sutures.
- The anterior fontanelle (bregma) is:
- at the junction of the s*agittal, frontal, and coronal *sutures.
- Diamond in shaped. On vaginal examination four suture lines can be felt.
- The posterior fontanelle:
- at the junction of the sagittal and lambdoidal sutures.
- Is smaller & triangular shaped.
- The fact that sutures are not fixed is important for labour because it allows the bones to move together and even to overlap (moulding).
Fetal Skull Diameters
- Suboccipito-bregmatic diameter (occipito-anterior position):
- The diameter is from the suboccipital region to the centre of the bregma.
- Diameter = 9.5 cm. Head well flexed.
- Submento-bregmatic diameter:
- From the centre of the bregma to the angle of the mandible.
- Measuring 9.5 cm. This is the presenting diameter when the neck is hyperextended.
Effect of Fetal Attitude on Presenting Diameter
| Attitude | Well flexed | Less well flexed (partially extended) or deflexed | Extended ‘brow presentation’ | Hyperextended ‘face presentation’ |
|---|---|---|---|---|
| Diameter | Suboccipito-bregmatic | Occipito-frontal | Occipito-mental | Submento-bregmatic |
| Measurement | 9.5 cm | 11.5 cm | 13.0 cm | 9.5 cm |
The Powers (Uterus)
- The result of contractions is the development of the:
- Thicker, actively contracting ‘upper segment’.
- At the same time, the lower segment of the uterus becomes thinner and more stretched.
- This results in cervical (effacement) and then dilation.
- And the fetus descends in response to this directional force.
Fetal Lie
- Is the relation of the long axis of the fetus to the long axis of the mother. Lie types:
- Longitudinal (the only normal lie).
- Oblique lie.
- Transverse lie.
Fetal Presentation
- That part of the fetus entering the pelvis first. Types of presentation:
- Vertex (96.8%)
- Breech (2.5%)
- Face (0.2%)
- Shoulder (0.4%)
- Compound
- Brow (0.1%)
Fetal Denominator
- Is an arbitrary part of the presentation. Types of denominator:
- Occiput in vertex presentation.
- Sacrum in breech presentation.
- Mentum in face presentation.
Vaginal delivery is not possible unless the baby is very small since the presenting diameter is the mento-vertical diameter (14 cm) and the largest pelvic diameter is 12.5 cm.
Fetal Position
- Orientation of the presenting part in relation to the maternal public symphysis.
Types of position:
- Occipito-anterior (RT & LT).
- Occipitolateral (RT & LT).
- Occipito-posterior (RT & LT).
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Fetal Attitude
- Is the relationship between fetus and himself (posture of the fetus).
Types of attitude:
- Flexion.
- Deflexion.
- Extension.
Synclitism
- The parallelism between the plane of the pelvis and that of the fetal head.
- The head position is considered to be synclitic when:
- the biparietal diameter is parallel to the pelvic plane.
- and the sagittal suture is midway between the anterior (symphysis pubis) and posterior (sacral promontory) planes of the pelvis.
Asynclitism
- When the head plane is not parallel to the pelvic plane.
- Or, fetal head position is tilted to one side.
- Synclitism
- Anterior Asynclitism: Anterior parietal bone is over the inlet.
- Asynclitic birth
