Introduction

It is essential for a midwife to understand the parameters and characteristics of the fetal skull because of its significance during the mechanism of labour. Two key functions of the fetal skull are the protection of the brain, which is subjected to pressure as it descends through the birth canal during labour, and an ability to change shape, adapting to the process of labour in response to uterine contractions and the size and shape of the pelvis. By assessing the landmarks of the fetal skull, such as sutures and fontanelles, a midwife is able to diagnose the position and attitude of the fetal head in the pelvis and determine the most likely mechanism of labour and mode of delivery.

Development of the fetal skull

As the fetus develops in utero, the mesenchyme layer surrounding the brain starts to ossify, forming the various bones of the fetal skull. This process is called intramembranous ossification and begins between 4 to 8 weeks of gestation. The initial development of the skull occurs from this intramembranous structure, derived from neural crest cells and mesoderm. The intramembranous structure is divided into two major components, the neurocranium, which forms the protective case of the skull, and the viscerocranium, forming the bones of the face.

The neurocranium can be subdivided into the chondrocranium and the dermatocranium. The chondrocranium (cartilaginous part) is formed by the fusion of cartilages, and following ossification becomes the occipital, temporal, sphenoid and ethmoid bones. The dermatocranium (membranous part) is thought to arise from the external dermal scales developed to protect the brain. This lies under the superficial layers of the skin, covering and protecting the dorsal section of the brain, giving rise to the parietal and frontal bones.

The earliest visible signs of development can be seen on ultrasonography at about 4 weeks’ gestation with calcification of the membranes and the development of the occiput. This becomes easier to determine from approximately 8 weeks, when intramembranous ossification is more prominent. At 12 weeks, the outline of the individual bones become evident (Moore & Persaud 2007, Sadler 2009) (see Fig. 30.1). Ossification of the bones continues throughout pregnancy with individual bones ossifying from their centre. At term, the bones of the skull are thin and pliable, enabling some movement of bones to take place during labour. The two frontal bones have usually united by term.

Figure 30.1 Ultrasound images illustrating development and ossification of the fetal skull at 9, 11 and 14 weeks’ gestation.

(Reproduced by kind permission of the Ultrasound Department at the Luton and Dunstable Hospital NHS Trust.)

The external structures of the newborn skull (Fig. 30.2)

Following birth, the midwife examines the external structures of the newborn head in order to identify any unusual characteristics or abnormalities in the skull structure. A baseline measurement of the newborn skull is sometimes taken during this procedure and documented within the child’s neonatal records.

Figure 30.2 External structures and circumferences of the newborn skull.

The skull (Fig. 30.3)

The fetal skull is a complex structure consisting of 29 irregular flat bones with 22 of these paired symmetrically: 8 bones form the cranium, 14 the face, and 7 the base. Knowledge of the fetal skull in the antenatal period enables a midwife to assess the size of the fetal head in relation to the size of the pelvis, assess engagement of the fetal skull in the pelvis (Dietz & Lanzarone 2005). It also helps inform reviews of ultrasonography and pelvimetry reports.

Figure 30.3 Characteristics of the fetal skull.

Sinuses

A sinus is a naturally occurring cavity in the body. Sinuses enable blood to circulate throughout the skull and into the brain membranes. The sinuses associated with the frontal, ethmoidal, sphenoidal and maxillary bones change shape during puberty and are thought to be associated with voice tone.

Reflective activity 30.1

Use the diagrams of the fetal skull in Figure 30.19 to revise the information outlined within this chapter. Photocopy the diagrams and revise the structures until you are confident that you know all of the components of the fetal skull and their application to midwifery practice.