The discovery of an abdominal mass on physical examination in the newborn period causes concern and the need for a rapid diagnosis. The incidence of an abdominal mass is 1/1000 live births. 

With the introduction of fetal ultrasound, many abdominal masses are diagnosed in utero. Many dilatations of the urinary tract diagnosed in utero resolve spontaneously. In utero procedures to correct most anomalies are still experimental . The majority of masses are of benign origin and greater than 50% are of renal origin. The majority will be diagnosed with a good history, physical examination, and ultrasound evaluation.

History

1. Was there a prenatal ultrasound performed?
2. Are there any GI symptoms such as vomiting and poor feeding?
3. How much amniotic fluid was present?
4. Any family history of masses or renal disease?

1. Location of the mass- flank, mid-abdomen, or suprapubic
2. Is the mass solid, cystic, smooth, or tender?
3. Is there hepatosplenomegaly?
4. Other physical findings unrelated to the mass- facies, rectal, lung exam, other anomalies.

1. Renal (55%)
1. Multicystic dysplastic kidney- usually a flank mass and irregular surface. If bilateral, usually some intrauterine obstruction. On ultrasound hypoechogenic. Usually removed surgically prior to development of hypertension.
2. Hydronephrosis- neonates are usually asymptomatic and present with a flank mass. Most are secondary to an obstruction at the ureteropelvic junction. May also be associated with reflux. Surgically repair and may need nephrostomy to decompress first.
3. Polycystic Disease
1. Infantile is inherited as autosomal recessive and associated with hepatic cysts and pulmonary hypoplasia. Poor prognosis.
2. Adult Polycystic Disease- autosomal dominant inheritance and rarely seen in childhood
4. Posterior Urethral Valves -may have enlarged kidneys or bladder
5. Renal Vein Thrombosis- history of dehydration and hemoconcentration. May have hematuria and Proteinuria.
6. Mesoblastic Nephroma-a benign hamartoma that may have associated hematuria. Remove and pathology will differentiate from rare Wilm's tumor.
2. Adrenal Masses and other retroperitoneal(10%)
1. Hemorrhage- associated with birth trauma. 
2. Neuroblastoma- may have calcifications on plain film of the abdomen
3. Enlarged liver - (5%) there may be cysts, tumors, and hemangiomas. Choledochal cyst of the gall bladder often presents with jaundice
4. Duplications of the Gastrointestinal tract-and other GI lesions (15%) Duplications most commonly in the ileocecal area and most do not communicate with the intestines. 
5. Pelvic and Genital Tract (15%)
1. Ovarian cysts
2. Hydrometrocolpos
3. Distended bladder 
4. Teratomas
5. Anterior meningomyeloceles

1. History and Physical examination
2. Abdominal radiograph- will show gas pattern, displacement of organs may identify location of the mass. May also show calcifications associated with neuroblastoma, meconium peritonitis, and hepatoblastoma. 
3. Ultrasound- Will differentiate solid from cystic and locate which organ the mass is located in. 
4. CT and MRI- will provide more anatomic detail. 
5. Include the obstetrican, surgeon, and urologist in the process of evaluating abdominal masses.

1. Pearl MH et al. The Approach to Common Abdominal Diagnoses in Infants and Children: Part II. Pediatric Clinics of North America. 1998; 45(6):1287-1326.
2. Schwartz, Marshall, and Shaul, Donald. Absominal Masses in the Newborn. Pediatrics in Review. December 1989.