~&aet-Fetal head molding is the change in shape of the fetal head due to forax of labor. The biomechanics ofthis process are poorly understood. To understand it better, &ssical e@eering structural analysis is being a&cd to analyze the process. A fundamental part of this analysis is to describe the mechanical properties of the eaPstituent materials, a knowledge which has been lacking for fetal cranial bone. As a first step toward defining the machaniad properties, 86 specimens of fetal cranial bone ob&d from spe&mens ranging in estimated gestational age from 25 to 40 weeks were tested in three-point bending. In addition, 12 specimens from a 6-yrdkl ash&urn were tested for comparative pqoses. The data indicate that the daatic modulus is highly sensitive to gestational age and fiber orientation. Elastic modulus values for specimem with par&l fiber orientation are in tbe range of 1.65 x 10s MPa for preterm bone to 386 x 10' MPa for term bone. The results arc discussed with respect to the limitations of the test method. Finally, clinical inferences are made. INIXODUCTION Molding is the change in shape of the fetal head due to the forces of labor. The process is a normal @rt of every labor and as such it allows the fetal head to more easily fit the shape of the maternal birth passage. However, excessive molding with subsequent cerebral trauma has been linked to conditions ranging from subtle psycho-neurol@cal disabilities to lhental ro-tar&ion, ccr&al palsy and even death (Willerman, 1970; Glenting, 1970; Fianu, 1976). Deq.&e the undisputed importance of head molding in both normal and abnormal labor, little research has heen done to understand better the biomechanics of the proars. Those studies which have been undertake have basn widely sp8ccd in time, using a number of very d&rent methods (

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Ahawact-Fetal head molding is the change in bead shape due to the forces of labor. Previous investigations of the molding process have ban largely qualitative in nature. In order to provide a first step toward a quantitative description of the &formations of the fetal head, a fmitc eiement analysis of the par&al bone of the fetal skull is pcr&tncd. The structural geometry of the bone is derived from orthogonal radiographs of a full term anatomical specimen. Load and material parameters are taken from previous investigations The bone is modeled using 63 thin shell elements from the SAP IV element library. Scaled models of prctcrm par&al bone are compared to term bone models. The results indicate that the models undergo deformations which are qualitatively similar to those seen in normal labor. The pretcrm parictal bone model exhibits deformations 2-4 times greater than the term bone model for the same load distribution. The significance of these results and their relationship to the molding process are discussed. INlRODUCnON Due to the forces of labor, the fetal head changes shape as it passes through the birth canal-a procas termed "mokling". Exceaaive molding itas been implicptai in conditions ran&

Molding at parturition, äs perceived subjectively, ranges from no reshaping to marked distortion of the infant's head. While clinicians easily recognize ,the highly molded head, there are no criteria for quantitating the degree of molding. The purpose of the present study is to describe quantitatively neonatal skull molding and the restitution of the newborn's skull from the molded state during the early neonatal period.

Forty-five specimens of fetal cranial bone ranging in estimated gestational age from 25540 weeks were tested in three-point bending. Five specimens of a six-year-old calvarium were tested for comparative purposes. The elastic modulus of each specimen was derived from the bending stiffness and geometrical property data. Following mechanical testing, each specimen was oven-dried and ashed to a constant weight. Ash content was determined as the ratio ofash weight to dry specimen weight. The results indicate that the ash content of fetal cranial bone increases significantly (from 50?,0 to 687,) with increasing gestational age. (The elastic modulus of fetal cranial bone significantly increases with increasing ash content.) However, the ash content of the term fetal cranial bone or the six-year-old cranial bone is not significantly different from that found in the adult. These results are discussed in relation to a structural hypothesis for increasing cranial bone stiffness with increasing maturity *