Fetal Skull Anatomy PDF

**FETAL SKULL** **INTRODUCTION** The fetal skull is the most difficult part of the baby to pass through the mother's pelvic canal, due to the hard bony nature of the skull. Understanding the anatomy of the fetal skull and its diameter will help to recognise how a labour is progressing, and whether...

**FETAL SKULL** **INTRODUCTION** The fetal skull is the most difficult part of the baby to pass through the mother's pelvic canal, due to the hard bony nature of the skull. Understanding the anatomy of the fetal skull and its diameter will help to recognise how a labour is progressing, and whether the baby's head is 'presenting' correctly as it comes down the birth canal. This will give a better understanding of whether a normal vaginal delivery is likely, or if the mother needs referral because the descent of the baby's head is not making sufficient progress. The skull bones encase and protect the brain, which is very delicate and subjected to pressure when the fetal head passes down the birth canal. Correct presentation of the smallest diameter of the fetal skull to the largest diameter of the mother's bony pelvis is essential if delivery is to proceed normally. But if the presenting diameter of the fetal skull is larger than the maternal pelvic diameter, it needs very close attention for the baby to go through a normal vaginal delivery. It consists of vault, face and base. 1. The vault is composed of: - 2 frontal bones separated by the frontal suture, - 2 parietal bones separated by the sagittal suture, - the occipital bone separated by the lambdoidal suture from the parietal bones, while the coronal suture separates the frontal from the parietal bones. Each of the 2 parietal bones is separated from the temporal bone on each side by the temporal suture. 2. The face is the area from the junction of the chin and neck to the root of the nose and supra-orbital ridges. It consist of 16 bones which are firmly united. 3. The brow is the area from the nose and supra-orbital ridges to the anterior fontanelle and coronal suture. **Bone of the vault consist of the following;** - The *frontal bone*, which forms the forehead or sinciput. In the fetus, the frontal bone is in two halves, which fuse (join) into a single bone after the age of eight years. Its ossification Centre on each is the frontal eminence. - The two *parietal bones*, which lie on either side of the skull and occupy most of the skull. Its ossification centers on each side of the parietal bone is called the parietal eminence. - The *occipital bone*, which forms the back of the skull and part of its base. It joins with the cervical vertebrae (neck bones in the spinal column, or backbone). It contains the foramen magnum; this helps to protect the spinal cord as it leaves the skull. The ossification centre can be easily defined and called the occipital protuberance. - The two *temporal bones,* one on each side of the head, closest to the ear. The vertex is the area of the vault bounded ; - anteriorly by the anterior fontanelle and the coronal suture, - posteriorly by the posterior fontanelle and lambdoidal suture, - laterally by 2 lines passing by the parietal eminencies. Note; Understanding the landmarks and measurements of the fetal skull will help to recognise normal and abnormal presentations of the fetus during antenatal examinations, labour and delivery. **Sutures** Sutures are cranial joints between the bones of the skull, they are membranes between the skull bones where ossification has not been completed. In the fetus they can 'give' a little under the pressure on the baby's head as it passes down the birth canal. - The *lambdoid suture* forms the junction between the occipital and the frontal bone. - The *sagittal suture* joins the two parietal bones together. - The *coronal suture* joins the frontal bone to the two parietal bones. - The *frontal suture* joins the two frontal bones together. **Fontanelles** A fontanelle is the space created by the joining of two or more sutures. It is covered by thick membranes and the skin on the baby's head, protecting the brain underneath the fontanel from contact with the outside. Identification of the two large fontanels on the top of the fetal skull helps you to locate the angle at which the baby's head is presenting during labour and delivery. 1.The **anterior fontanel** (also known as the bregma) - Is large and a diamond-shaped space towards the front of the baby's head. - Found at the junction of the sagittal, coronal and frontal sutures. - Approximately 2.5 - 4cm in length, 1.5 -2cm wide, - The floor is completely ossified or closes by 18months after birth. - The surrounding bones are not overlapping during moulding. - It is very soft and you can feel the fetal heart beat by placing your fingers gently on the fontanel. The skin over the fontanel can be seen 'pulsing' in a newborn. 2\. The **posterior fontanel** (or lambda). - It is small and has a triangular shape, and is found towards the back of the fetal skull +-----------------------------------------------------------------------+ | - Its floor is bony, surrounded by 3 bones (2 parietal and | | occipital bones). | +=======================================================================+ | | +-----------------------------------------------------------------------+ | | +-----------------------------------------------------------------------+ - It is formed by the junction of the lambdoid and sagittal sutures. - The surrounding bones are overlapping during moulding **Regions and landmarks in the fetal skull** 1.The **vertex** is the area midway between the anterior fontanel, the two parietal bones and the posterior fontanel. - A **vertex presentation** occurs when this part of the fetal skull is leading the way. - This is the normal and the safest presentation for a vaginal delivery. 2.**Bregma** - The anterior fontanelle. 3.The **brow** is the area of skull which extends from the anterior fontanel to the upper border of the eye. A brow presentation is a significant risk for the mother and the baby. 4.The **face** extends from the upper ridge of the eye to the nose and chin (lower jaw). A face presentation is also a significant risk for the mother and baby. 5.The **occiput** is the area between the base of the skull and the posterior fontanel. It is unusual and very risky for the occiput to be the presenting part. 6\. **Glabella** -- the bridge of the nose 7\. **Mentum** -- the chin **Diameters of the fetal skull** Longitudinal diameters - Suboccipito-bregmatic (SOB) = 9.5cm - from below the occipital protuberance to the centre of the anterior fontanelle (bregma). - It is the engagement diameter in occipito-anterior with complete flexion. - Suboccipito-frontal (SOF) = 10 cm - from below the occipital protuberance to the centre of the sinciput or frontal suture. - It is the engagement diameter in occipito anterior with incomplete flexion. - It is the diameter that distends the vulva in occipito anterior if the head is allowed to extend after crowning. - Occipito-frontal (OF) = 11.5 cm - form the occipital protuberance to the root of the nose or glabella. - The head is erect. - It is the engagement diameter in occipito-posterior position. - It is the diameter that distends the vulva in face to pubis delivery. - It is the diameter that distends the vulva if the head extends before crowing in occipito anterior. - Submento-bregmatic (SMB) = 9.5 cm - from the junction of the chin and neck to the centre of the bregma. - It is the engagement diameter in face presentation when the head is completely extended. - Submento-vertical (SMV) =11.5 cm - Measures from the junction of the chin and neck to the vertical point which is a point on the sagittal suture midway between anterior and posterior fontanelles (vetex). - It is the engagement diameter in the incompletely extended face. - It is the diameter that distends the vulva during face delivery. - Mento-vertical (MV) =13.5 cm- 14cm - from the tip of the chin to the highest point of the vertex, slightly toward the posterior than the anterior. - It is the engagement diameter in brow presentation. As it is longer than the largest diameter of the pelvic brim, the head cannot enter the pelvis. - Vaginal delivery is usually impossible. Transverse diameters - Biparietal =9.5 cm - between the 2 parietal eminencies. - Subparietal supraparietal = 9cm - from below one parietal eminence to above the opposite eminence. - It is the engagement diameter in case of asynclitism. - Bitemporal = 8.2 cm - between the anterior ends of the temporal sutures. - Between the two the extreme points of the coronal suture. - Bimastoid = 7.5cm - between the tips of the 2 mastoid processes. -- --