Annular pancreas is one of the most frequent congenital pancreatic anomalies formed by a thin band of normal pancreatic tissue that completely or partially encircles the second portion of the duodenum. During embryonic development, the ventral pancreatic primordium moves to the right and rotates posteriorly until it comes to lie to the left of the duodenum, subsequently fusing to the dorsal primordium. Annular pancreas is believed to be due to a defect in the normal rotation of the ventral primordium. In more than two thirds of affected infants there are associated malformations, such as Down syndrome, intestinal malrotation, duodenal and esophageal atresia, tracheoesophageal fistula, imperforate anus, and a variety of cardiac defects.

Although the genetic and molecular basis for this anomaly remains obscure, Indian hedgehog or sonic hedgehog inactivation in mice has been shown to cause overgrowth of ventral pancreatic tissue, a phenotype strikingly similar to annular pancreas.