Congenital ascites is the abnormal accumulation of fluid in the peritoneal cavity during the neonatal period. There are multiple causes of congenital ascites, which can be distinguished by the composition of the ascitic fluid. Perforations of the biliary tract due to congenital abnormalities may lead to bilious ascites. Some causes of biliary ascites include congenital weakness of bile duct wall and rupture of choledochal cysts. Congenital abnormalities of the genitourinary tract can lead to urinary ascites, the most common abnormality being a posterior urethral valve leading to obstruction and subsequent perforation. Finally, chylous ascites can develop due to abnormalities of the lymphatic tract.

Ascites present as abdominal distension with dullness to percussion. Ascites can sometimes be identified prenatally during ultrasonography. Treatment for congenital ascites varies by cause, but often requires surgical intervention. Some forms of chylous ascites, however, can resolve with bowel rest.