The fetal head, from an obstetrical viewpoint, and in particular its size, is important because an essential feature of labor is the adaptation between the fetal head and the maternal bony

pelvis The pelvis (plural pelves or pelvises) is the lower part of the trunk, between the abdomen and the thighs (sometimes also called pelvic region), together with its embedded skeleton (sometimes also called bony pelvis, or pelvic skeleton). The ...

. Only a comparatively small part of the head at term is represented by the face. The rest of the head is composed of the firm skull, which is made up of two frontal, two parietal, and two

temporal bone The temporal bones are situated at the sides and base of the skull, and lateral to the temporal lobes of the cerebral cortex. The temporal bones are overlaid by the sides of the head known as the temples, and house the structures of the ears. Th ...

s, along with the upper portion of the occipital bone and the wings of the sphenoid. These bones are separated by membranous spaces, or sutures. The most important sutures are the frontal, between the two frontal bones; the

sagittal The sagittal plane (; also known as the longitudinal plane) is an anatomical plane that divides the body into right and left sections. It is perpendicular to the transverse and coronal planes. The plane may be in the center of the body and divid ...

, between the two parietal bones; the two coronal, between the frontal and parietal bones; and the two

lambdoid The lambdoid suture (or lambdoidal suture) is a dense, fibrous connective tissue joint on the posterior aspect of the skull that connects the parietal bones with the occipital bone. It is continuous with the occipitomastoid suture. Structure Th ...

, between the posterior margins of the

parietal bone The parietal bones () are two bones in the Human skull, skull which, when joined at a fibrous joint, form the sides and roof of the Human skull, cranium. In humans, each bone is roughly quadrilateral in form, and has two surfaces, four borders, an ...

s and upper margin of the

occipital bone The occipital bone () is a neurocranium, cranial dermal bone and the main bone of the occiput (back and lower part of the skull). It is trapezoidal in shape and curved on itself like a shallow dish. The occipital bone overlies the occipital lobe ...

. Where several sutures meet, an irregular space forms, which is enclosed by a membrane and designated as a fontanel. The greater, or anterior fontanel, is a lozenge-shaped space that is situated at the junction of the

and the coronal sutures. The lesser, or posterior fontanel, is represented by a small triangular area at the intersection of the

and

sutures. The localization of these fontanels gives important information concerning the presentation and position of the fetus. The temporal, or casserian fontanels, have no diagnostic It is customary to measure certain critical diameters and circumferences of the newborn head. The diameters most frequently used, and the average lengths thereof, are: #The occipitofrontal (11.5 cm), which follows a line extending from a point just above the root of the nose to the most prominent portion of the occipital bone #The biparietal (9.5 cm), the greatest transverse diameter of the head, which extends from one parietal boss to the other. #The bitemporal (8.0 cm), the greatest distance between the two temporal sutures. #The occipitomental (12.5 cm), from the chin to the most prominent portion of the

occiput The occipital bone () is a cranial dermal bone and the main bone of the occiput (back and lower part of the skull). It is trapezoidal in shape and curved on itself like a shallow dish. The occipital bone overlies the occipital lobes of the cereb ...

#The suboccipitobregmatic (9.5 cm), which follows a line drawn from the middle of the large fontanel to the undersurface of the occipital bone just where it joins the neck The greatest circumference of the head, which corresponds to the plane of the occipitofrontal diameter, averages , a size too large to fit through the pelvis without flexion. The smallest circumference, corresponding to the plane of the suboccipitobregmatic diameter, is . The bones of the cranium are normally connected only by a thin layer of fibrous tissue that allows considerable shifting or sliding of each bone to accommodate the size and shape of the maternal pelvis. This intrapartum process is termed molding. The head position and degree of skull ossification result in a spectrum of cranial plasticity from minimal to great and in some cases, undoubtedly contribute to fetopelvic disproportion, a leading indication for cesarean delivery.

References

{{Refend Obstetrics Midwifery