Microdissection of the oval fossa from its right and left atrial aspects showed that the rim of the fossa was formed from an infolding of the atrial wall, which began between the base of the superior caval vein and the insertion of the right pulmonary vein to the left atrium. Because the rim varied in shape from circular to oval, we used morphometric methods to obtain patterns of normality of the diameters of the oval fossa, oval foramen, and the area of right and left atrial aspects of the septum. These values were found to correlate with log fetal and neonatal weight. Due to technical advances in prenatal diagnosis, these structures can now be examined in fetal life. Therefore, our morphometric data, when appropriately interpreted, have immediate clinical and surgical applications in the treatment of fetal and neonatal cardiovascular disorders.