To assess the development of the duodenal window in fetuses, we examined semiserial histological sections of 59 human fetuses with a crown-rump length of 27–156 mm (∼4–18 weeks of gestation). In 44 of the 54 fetuses with horizontal sections, the duodenal window was formed by interdigitation of the anterior and posterior muscle slips from the proper duodenal circular muscle coat. The anterior slips approached the common bile duct from the anterior side and wound around the bile duct from the right aspect, whereas the posterior slips approached the main pancreatic duct from the posterior side, reaching the left or outer aspect of the duct without winding. These slips may become longitudinal muscles in the ampulla after birth. Six specimens showed variations in this typical pattern, in that the posterior muscle slips as well as the duodenal longitudinal muscle coat wound around the bile duct. In the remaining four specimens, we observed an abnormal union of the bile and pancreatic ducts, with the duodenal circular muscles suddenly ending along the window or slightly inserted into the right side of the common duct after joining. In all later-stage fetuses, the common sphincter surrounded both the bile and pancreatic ducts in the ampulla. Consequently, at and along the duodenal window, the proper duodenal circular muscle seemed to contribute to fetal sphincter formation. The window was not a simple hiatus but a functional interface between the sphincter and the duodenal wall. Clin. Anat. 26:598–609, 2013. © 2012 Wiley Periodicals, Inc.