Abstract In utero surgical correction of fetal diaphragmatic hernia is a new therapeutic alternative for selected prenatally diagnosed patients. With increasing experience, the authors have found that fetuses with herniation of the liver through the diaphragmatic defect have a high incidence of perioperative death. The hypothesis was that reduction of the liver during fetal diaphragmatic hernia repair caused distortion of fetal vascular anatomy resulting in fetal demise. To study this, the authors performed angiograms through the umbilical vein in fetal and neonatal cadavers, with and without diaphragmatic hernias, and simulated in utero repair. Large fetal congenital diaphragmatic hernias can be associated with dramatic changes in the vascular anatomy of the liver. The liver does not simply rotate up into the chest through a diaphragmatic defect; instead, the liver more likely develops in the chest, with its vascular anatomy situated accordingly. Attempts to reduce a herniated liver can result in significant distortion of the fetal vasculature, leading to fetal death. Techniques to prevent vascular compromise during hepatic manipulation may improve the outcome for fetuses during in utero repair of diaphragmatic hernias.