The superior mesenteric vein appears as a fusion between irregularly-shaped slits of the midgut mesentery tissue at 5-6 weeks. In contrast, there might be no report when and how the inferior mesenteric vein (IMV) develops. We aimed to find the human initial IMV.We examined 1) sagittal histological sections of 7 human fetuses with 45-75 mm crown rump length or CRL (10-12 weeks); 2) horizontal sections of 15 fetuses with 70-155 mm CRL (12-18 weeks) and 3) horizontal sections of 12 late-term fetuses with 225-328 mm CRL (28-41 weeks).In the mesentery of the descending colon-rectum, the initial IMV lumen opened at 10-12 weeks of gestation, but the vein was difficult to trace upward to the anterior surface of the left adrenal. At 13-14 weeks, irrespective of whether it accompanied a colic artery, the IMV ran medially along the adrenal and it sometimes became thick near the pancreatic head. Earlier than the IMV, the middle colic vein appeared at the left aspect of the pancreatic head. Until late-term after establishment of the dorsal mesogastrium fusion with the mesocolon transversum, the IMV provided a peritoneal fold at the duodenojejunal junction.A venous drainage via the IMV was much delayed possibly because, in early and midterm fetuses, an ongoing fusion of the midgut mesentery and a changing topographical relation among the abdominal viscera interfered with the venous flow. Instead, well-developed lymphatics seemed to be responsible for the drainage of the left-sided colon.