Congenital megacolon (Hirschsprung disease) results from the failure of neural crest cells to migrate into the walls of the colon. Incomplete separation of the cloaca would result in anal agenesis either with or without the presence of a fistula. The failure of recanalization of the colon results in rectal atresia, wherein both the anal canal and rectum exist but are not connected due to incomplete canalization or no recanalization. Defective rotation of the hindgut can cause volvulus or twisting of its contents. Oligohydramnios is a deficiency of amniotic fluid, which can cause pulmonary hypoplasia but would not cause Hirschsprung disease.