For clinicians it is very difficult to evaluate the prognosis of a left congenital diaphragmatic hernia (CDH) at prenatal ultrasound examination. Surgical studies show that the presence of a large part of the liver in the chest is a criterion of poor prognosis. However, ultra-sonography encounters some difficulties in determining the precise position of the liver in the thoracic cavity. The aim of this anatomic study was to define the relationship between the position of the liver and the path of the ductus venosus and of the umbilical v., which are easily recognizable at prenatal sonography. Twenty dead fetuses were used for the study (12 with a left CDH and 8 without). All fetuses underwent radiographic assessment, anatomic dissection and cross-sectional study. The angle between the umbilical v. and the ductus venosus in different planes was measured. The more the liver was in the thorax, the greater was the angle between the ductus venosus and the sagittal plane, and the less the angle between the ductus venosus and the umbilical v. These angles can be easily measured by prenatal ultrasound examination of the fetus. Our findings suggest that it is now possible to offer the clinician a new and reliable way to determine the prognosis of a left CDH before birth.