Prepyloric atresia or pyloric atresia is the congenital membranous obstruction of the gastric outlet, with vomiting of gastric contents only. Prepyloric means situated or occurring anterior to the pylorus.The pylorus is the region of the stomach that connects to the duodenum (the beginning of the small intestines), which is divided into two parts:

• the pyloric antrum, which connects to the body of the stomach.
• the pyloric canal, which connects to the duodenum.