As was mentioned above, the newborn skull is designed to provide maximum accommodation to the forces of labor, minimum trauma to the infant's brain, and complete restoration to free mobility of all its parts once the stress of labor is over.

Just before birth, the infant in utero is positioned for delivery by presenting the smallest diameter of his head to the largest diameter of the mother's pelvis; this is the position of full fetal flexion. As contractions continue, the infant is conducted by the inclination of the maternal pelvic floor into the midline for delivery around the pubic symphysis by a process of extension of the head.

This descent in full flexion, progressing to birth by extension of the head, is of profound significance to the initiation of pulmonary respiration. The respiratory activity associated with the vigorous vocal activity of the newborn serves to expand the cranial mechanism and restore the bones and membranes to their anatomic relationships (permitting their free physiologic motion). Healthy sequential development of the central nervous system within can then continue.

These ideal circumstances, however, seldom occur in our modern, civilized world. Owing to such factors as poor nutrition of the mother, structural inadequacies before and during pregnancy, drug abuse, inadequate preparations for labor, and, sometimes, the mechanical or artificial acceleration of labor by an impatient obstetrician, only a relatively few infants are born without undue skein or cranial trauma.

Instead, structural inadequacies of the maternal pelvis may cause the fetus to assume a degree of extension (and lateral cervical flexion) greater than the ideal; the result will be a presentation of a portion of the head greater than the minimum occipitobregmatic diameter. This can range from a moderate extension to posterior occiput, to transverse arrest, to brow presentation, or even to a complete extension in which the face itself presents-a position in which vaginal delivery is impossible. In such a circumstance, cesarean section will be necessary if the baby is to survive.

But the compressive forces will have already traumatized the head as the uterine contractions force it progressively towards the birth canal. Prominence of the base of an anterior maternal sacrum may obstruct descent of the head on one side, and such asynclitism can distort the cranial mechanism. The presence of large twins, both striving to present the head at the same time, may cause cranial stress to one or both even before active labor begins. These are only a few of the mechanical insults that may occur before birth.

So much for the passage of the infant into the birth canal. Now let us consider the structure of the infant skull itself at the time of birth.