Vertex Birth Molding

GENESIS

Vertex birth molding describes the mechanical changes in fetal head shape due to external compression on the cranium from bony adjustments within the cranial vault that occur as the neonate in vertex presentation passes through the birth canal. Additional soft tissue swelling can significantly alter the shape of the neonatal head, and pressure against the fetal cranium can delay normal ossification in the vertex region, resulting in benign vertex craniotabes. Humans have an unusual pelvis, a large fetal head, and a complicated mechanism of labor. One major feature of human evolution is marked delay in neural development, such that our brains continue to grow slowly over a much longer period than do the brains of other primates. The modern human brain is only 25% of its adult size at birth and continues to grow at a rapid rate throughout the first year of life, when it reaches 50% of its adult size. Human brains are 95% of adult size by 10 years of age. In comparison, a chimpanzee brain is already 40% of its adult size by birth and reaches 80% of the adult volume by the end of the first year. A 1-year-old Homo erectus brain was closer to that of apes in its growth pattern and measured 72% to 84% of adult size at that age, which implies differences in the development of cognitive abilities in Homo erectus compared with modern humans. The early delivery of the human head appears to leave the cranium much more vulnerable to mechanic forces than in other species. The birth canal is a deep curved tube through which a mature fetal head can only pass by rotating as it descends. A number of factors influence the individual fetal cranial response to the normal forces of labor around the time of delivery, such as fetal head position and size, gestational age, maternal pelvic shape and dimensions, and the quality of uterine contractions.

During normal vertex molding, anteroposterior compression causes the frontal and occipital bones to slide under the parietal bones along the entire length of the coronal and lambdoid sutures. This elongates the occipitofrontal diameter to its greatest possible extent so as to diminish the vertical diameter of the fetal head to its smallest dimensions ( Fig. 35.1 ). The fetal pathologist John Ballantyne noted that during vertex molding, the occipital bone rotated in an anteroposterior direction on an “occipital hinge,” with a range of motion that is much greater in a 7-month fetus than in a term infant. He also noted that an infant’s head recovered to its unmolded state within 6 days after delivery. Holland studied the impact of mechanical stress on the fetal head and noted that the dura mater underlying the cranial bones acted as a protective mechanism to reduce the stress transmitted to the cranial contents. Decreases in dural growth-stretch tension across a suture can trigger synostosis, whereas excessive dural pressure can decrease ossification, leading to craniotabes and increased cranial flexibility. Holland described the fetal skull as a pliable shell composed of loosely jointed plates attached to a rigid cranial base, suggesting that the frontal and parietal bones bend under pressure in relation to movement at the occipital hinge.

Moloy examined the hinge action of occipital and frontal bones radiographically in stillborn infants and noted that the cranial base was capable of bending slightly to allow elevation of the occipital plates, and that biparietal pressure decreased the transverse diameter enough to prevent the frontal and occipital bones from overriding the parietal bones when longitudinal pressure was applied. Borell and Fernstrom used radiographs to assess molding as the fetal head passed through the birth canal, and they confirmed that vertex molding was characterized by an elevation of the vertex, an increase in the biparietal diameter, and an inward displacement of the occipital and frontal bones, which caused an overall reduction in the occipitofrontal diameter (see Fig. 35.1 ). They noted an association between the amount of molding and the length of labor, and they attributed normal vertex molding to pressures from the soft tissues rather than the bony pelvis. They noted that a contracted pelvis resulted in more severe fetal head molding than seen in normal deliveries, and that excessive muscular contraction in the lower uterine segment resulted in excessive vertex molding, with increased elevation of the fetal vertex ( Figs. 35.2–35.4 ).

In 1980, McPherson and Kriewall used engineering structural analysis techniques to investigate the biomechanics of fetal head molding. They observed that fetal cranial bone is capable of deforming under load distributions typical of normal labor, with preterm parietal bone capable of undergoing two to four times the amount of deformation than term parietal bone for the same load distribution. Although this may not result in more obvious extensive cranial molding, the transmission of pressure may be a contributing factor to the increased incidence of birth trauma and intracranial bleeds in preterm infants. In a photographic and anthropometric study of vertex molding in 319 term infants delivered vaginally, several factors influenced the degree of molding. Infants born to primiparous women, after oxytocin-stimulated labors, and via vacuum extraction showed significantly more molding. The duration of the first stage of labor did not influence the degree of molding, but a prolonged second stage in primiparous mothers was associated with more extensive molding. Infants born in the occipitoposterior and in breech presentation showed significantly less molding than those born in occipitoanterior presentations. Some degree of molding does occur within the uterus prior to labor, and repetitive Braxton-Hicks contractions throughout pregnancy were also a factor that influenced the head shape of infants before the onset of labor. Extreme fetal head elongation due to vertex molding from fetal cephalic fixation with persistent uterine contractions has been noted via prenatal ultrasonography as early as 30 weeks of gestation. More recently sutures and fontanels can be modeled, characterized, and studied in silico to simulate the second stage of labor in the vertex presentation and assess a time-dependent response and effects of a prolonged second stage of labor on head molding.