Fetal head molding. Diagnosis by ultrasound and a review of the literature.

Head molding refers to changes in cranial bone relationships that occur in response to external compression force. In the normal term labor with vertex presentation, the suboccipito-bregmatic diameter shortens and the mentovertical diameter lengthens. This is accomplished partially through the unbending or straightening of the parietal bones rather than the frequently taught mechanism of overlapping sutures. The occipital and frontal bones may also contribute by an inward movement of their apex, using their basal portions as a hinge. A locking mechanism may occur in protracted labors as the free edges of the cranial bones are forced into one another, preventing further molding and providing more protection for the fetal brain. The preterm skull has weaker material properties and wider sutures. Thus, more molding at lower pressures is possible and the protective effect of "locking" may not be operational. A case of extreme antenatal preterm fetal head molding discovered at ultrasound is presented as an introduction to review the literature regarding molding.