Babies can develop enlarged kidneys -- a condition known as hydronephrosis -- while still in the womb, or after birth. This condition is the most frequently discovered problem revealed by ultrasound examinations of babies in the womb. The kidneys, a pair of bean-shaped organs, produce urine from excess water and waste products that they filter from the blood. The urine passes through the urinary tract to the outside of the body. Structural defects may occur during the development of the urinary tract, and these defects can block the flow of urine and cause an enlarged kidney in babies.

Bulge in the Ureter

The ureters -- the pair of tubes running between the kidneys and the bladder -- may develop an abnormality that results in blockage of the flow of urine. The lower end of the ureter may develop a bulge called an ureterocele that protrudes into the inner part of the bladder. This prolapsed part of the ureter can impede the flow of urine and cause it to build up in the ureter and kidney, and this may cause an enlarged kidney. A surgical intervention can repair the defective ureter.

Obstructions in the Ureter

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Babies may develop an enlarged kidney due to a congenital obstruction in the ureter known as a congenital ureteropelvic junction, or UPJ, obstruction 2. In babies with a UPJ obstruction, the ureter grows narrow at the place where it connects with the kidney. This reduces the flow of urine between the kidney and the bladder, and urine builds up inside the kidney, causing enlargement. UPJ obstruction may develop slowly and may result in partial blockage of the urine flow. In such cases, the kidney does not suffer any permanent effects, but in cases of more severe obstruction, the condition can result in kidney damage.

Obstructions in the Urethra

A congenital obstruction may occur at the bladder opening or along the urethra. The medical term for this condition is bladder outlet obstruction, or BOO, and it can cause enlarged kidneys in children of both sexes. One form of BOO, which occurs only in boys, involves abnormal urethral growths known as posterior urethral valve, or PUV. This condition may cause total obstruction of the flow of urine in some cases, resulting in severe kidney damage.

Reflux

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Urine flows through the ureters to the bladder, and the junction where this occurs functions like a valve to prevent urine from returning to the ureters. A defect in this area can cause malfunction of the valve, which allows urine to move backward, or reflux, into the ureters and the kidneys. This condition, known as vesicoureteral reflux, or VUR, can cause enlargement of one or both ureters and kidneys. In cases of severe reflux, kidney development may be impeded, and children with the condition may remain vulnerable to other infections as they grow.