The foetal skull is divided into two main parts: the base, which is firmly ossified to protect vital centres, and the vault, which consists of flat bones that remain compressible to ease birth. Several key landmarks guide the midwife during abdominal and vaginal examinations.
Sutures are the membranous spaces that separate the ununited flat bones of the foetal skull vault, allowing the cranial bones to overlap during labour. The main sutures include the frontal suture between the two frontal bones, the sagittal suture between the parietal bones, the coronal suture between the frontal and parietal bones, and the lambdoid suture between the parietal and occipital bones.
Fontanelles occur where multiple sutures intersect. The anterior fontanelle (bregma) is diamond-shaped, measures about three by two centimetres, and typically closes by eighteen months of age. The posterior fontanelle is small and triangular, closing much earlier around six to eight weeks postnatal. Identifying these fontanelles on vaginal examination tells the midwife the exact position and attitude of the foetal head.
The anteroposterior diameters of the foetal skull vary depending on the degree of flexion or extension of the head. When the head is well-flexed, a smaller diameter presents to the pelvis, making birth much easier.
| Diameter | Extent | Measurement | Presenting Attitude |
|---|---|---|---|
| Suboccipito-bregmatic | From beneath the occipital bone to the center of the bregma | 9.5 cm | Complete flexion |
| Suboccipito-frontalis | From beneath the occipital bone to the bridge of the nose | 10.0 cm | Moderate flexion |
| Occipito-frontalis | From the root of the nose to the most prominent part of the occiput | 11.5 cm | De-flexed (median) |
| Mentovertical | From the chin to the highest point of the vertex | 14.0 cm | Partial extension (brow) |
| Submento-vertical | From the junction of the chin and neck to the vertex | 11.5 cm | Face presentation |
Transverse diameters measure the skull side-to-side and determine whether the widest part of the head can pass through the corresponding transverse diameters of the maternal pelvis.
When the head is neither fully flexed nor fully extended, oblique diameters come into play. The position in which the foetus is held in utero is known as its attitude, which directly dictates which skull diameter presents to the pelvic brim.
Complete flexion brings the suboccipito-bregmatic diameter into the pelvis, allowing smooth engagement. Moderate flexion results in the suboccipito-frontalis diameter. When the head extends into a brow presentation, the massive mentovertical diameter of fourteen centimetres tries to fit through the pelvis, causing mechanical obstruction. Midwives must assess head attitude early to anticipate abnormal labour patterns and prevent prolonged obstructed labour.
Moulding is the overlapping of the cranial bones at the sutures during labour, driven by uterine contractions and maternal pushing against the resistance of the bony pelvis. This natural adaptation reduces the overall volume of the foetal head without significantly reducing its perimeter, helping the head squeeze through tight spaces.
Excessive or rapid moulding can cause intracranial haemorrhage or tearing of the tentorium cerebelli due to extreme stress on the dural membranes. Midwives grade moulding from zero to plus three to monitor progress and safety. A zero indicates no overlapping with easily felt sutures, while a plus three indicates severe, fixed overlapping where the bones override each other significantly, signaling potential cephalopelvic disproportion.