Over 50% of neonatal abdominal masses arise from the kidney. Hydronephrosis and multicystic kidney are the most common etiologies, followed by polycystic kidney, renal vein thrombosis, and solid tumors. Workup should include careful physical examination and abdominal palpation, ultrasound, computed tomography (CT) scan, and intravenous (IV) urography. Note the immature neonatal kidney requires high doses of contrast, up to 4 mL/kg, because of its poor concentrating ability (see Chapter 15).

Hematuria in the neonate, particularly gross hematuria, is an emergency. Consider renal vein or artery thrombosis, renal cortical necrosis, obstructive uropathy, cystic renal disease, and tumors.

The kidney is the second most common cause of systemic hypertension in the neonate. Renal artery thrombosis is the most important etiology to not only rule out but also consider hydronephrosis secondary to obstruction or reflux, adrenogenital syndrome, and, rarely, pheochromocytoma, Cushing’s disease, and hyperaldosteronism.

Delay of micturition for more than 24 hours after birth, especially if associated with a distended palpable bladder, is cause for immediate concern. Ninety percent of newborns will have urinated about 10 mL within the first 24 hours of life. Consider lower urinary tract obstruction such as posterior urethral valves.
Note that magnesium sulfate given to the mother for toxemia during delivery can cause transient neonatal urinary retention.

A scrotal mass is rare in the neonate but must be given immediate attention. Consider torsion, epididymitis, incarcerated inguinal hernia, torsion of the appendix testis or epididymis, and testicular tumor. Management is generally surgical exploration (see Chapter 10).

Ambiguous genitalia are medical and social emergencies and require immediate evaluation (see Chapter 16).

Urine is the most common reason for ascites in the neonate, and obstructive uropathy is the most common underlying cause. Posterior urethral valves and ureteropelvic junction obstruction are the most frequent etiologies followed by urethral stricture or atresia, ectopic ureterocele, and neurogenic bladder. Voiding cystourethrography and CT scan or IV urography are essential to diagnosis.