Annular pancreas is an uncommon congenital anomaly.The presentation in adults differs from that in children.
A lot of adults patients with annular pancreas are asymptomatic. When symptomatic, the presentation is usually in the third to sixth decade with abdominal pain, postprandial fullness, vomiting, upper gastrointestinal bleeding from peptic ulceration, acute or chronic pancreatitis, or in rare instances, biliary obstruction. 
The pancreas develops from a single dorsal and two ventral buds since in the fifth week of gestation. The two ventral buds rapidly fuse. By the seventh gestational week, expansion of the duodenum causes the ventral bud to rotate and pass behind the duodenum from right to left and fuse with the dorsal bud.

Annular pancreas results from failure of the ventral bud to rotate with the duodenum, resulting in envelopment of the duodenum. This can be complete or incomplete.