The foetal skull is the largest and least compressible part of the foetus, making its passage through the birth canal a major mechanical challenge during childbirth. It is divided into two main regions: the face and the cranium. The cranium houses the brain and consists of several flat bones that are not yet fused together. This lack of fusion is a deliberate anatomical design that allows the head to change shape under pressure. Understanding this structure helps the nurse midwife anticipate how the foetal head will navigate the pelvis during labor.
Cranial sutures are fibrous joints connecting the flat bones of the foetal skull. These flexible membranes allow the bones to overlap during labor, reducing the overall head diameter.
Where cranial sutures intersect, spaces covered by tough membranes form. These spaces are called fontanelles. They serve as crucial clinical landmarks for determining foetal position and attitude during vaginal examination.
The presence of sutures and fontanelles is essential for safe vaginal delivery. Without them, the rigid cranium would cause prolonged obstructed labor or lead to severe intracranial trauma. They permit overlapping of the cranial bones, a process that temporarily decreases the dimensions of the foetal head to match the contours of the maternal pelvis. Furthermore, mapping these landmarks during a vaginal examination allows the midwife to track the progress of descent and rotation.
Moulding is the process by which the foetal cranial bones shift and overlap under the pressure of uterine contractions and maternal pushing. The parietal bones usually slide over the frontal and occipital bones, and one parietal bone may override the other. While moderate moulding is a normal, harmless physiological adaptation, excessive or rapid moulding can cause tearing of the bridging veins and intracranial hemorrhage. Documenting the degree of moulding provides insight into the disproportion between the foetal head and the maternal pelvis.
During vaginal examinations in labor, the midwife uses sterile-gloved fingers to feel the sutures and fontanelles. By identifying whether the anterior or posterior fontanelle is felt towards the front, side, or back of the pelvis, the exact position of the foetus is confirmed. Caput succedaneum or severe moulding can sometimes obscure these landmarks, making careful, gentle palpation necessary. Recognizing these structures ensures accurate partograph charting and timely intervention if labor fails to progress.