The kidneys, a pair of organs located in the lower back, are composed of millions of tiny tubes called nephrons. As blood passes through the nephrons, vital substances are retained in the circulation while nitrogen-containing wastes, which form the urine, drip out into a central pooling area, the renal pelvis. From the renal pelvis, urine travels to the bladder, from which it is excreted.

Of all the organs in the body, the kidneys are the most vulnerable to congenital malformations. Fortunately, many of the more common congenital kidney problems cause no symptoms. They may, in fact, go completely unnoticed unless they are detected on X rays taken for some other reason.

HOW DO KIDNEY MALFORMATIONS DEVELOP?
Normally, the first tiny kidney tubules appear during the fourth week of prenatal development. By the eighth week, a complete (if small) rudimentary pair of kidneys is present, although not in the location where they should be at birth. Through the remainder of the prenatal period, the kidneys rise and rotate to take their eventual place in the lower back. Many kidney malformations develop at this latter stage, when one of the kidneys moves only part of the way into its normal position. More rarely, problems occur in the first eight weeks. In these cases, one kidney may be incompletely formed or improperly attached to the blood supply or ureters (the tubes that carry urine to the bladder).

WHEN SHOULD I SUSPECT THAT MY CHILD HAS A KIDNEY MALFORMATION?
Most kidney malformations do not produce symptoms. Before birth, a kidney malformation in the fetus may cause an excessive or deficient amount of amniotic fluid (the liquid in which the fetus floats) in the uterus. If the obstetrician detects this problem, a fetal ultrasound may be ordered to check kidney structures.

Repeated urinary tract infections and urinary stones are also possible signs of kidney malformations. In addition, kidney malformations may occur in certain congenital syndromes (groups of symptoms and features that occur together) such as fetal alcohol syndrome.

IS MEDICAL ATTENTION NECESSARY?
Since the kidneys are able to function normally in the majority of kidney malformations, special medical care may not be necessary. If a child has only one functioning kidney, however, special care should be taken to avoid urinary tract infections, since damage to the remaining kidney can lead to kidney failure.

TYPES OF KIDNEY MALFORMATIONS

• Ectopic kidney.
This malformation occurs when one kidney fails to rise to the midsection, remaining below the waist instead. Ectopic kidneys function normally despite their location.

• Kidney malrotation.
A kidney that does not rotate properly during fetal development is malrotated. A malrotated kidney faces the wrong way, but it works.

• Renal agenesis.
This malformation occurs when a kidney fails to develop. However, as long as one healthy kidney remains, the absence of the other kidney poses no serious threat.

• Hypoplastic kidney.
A hypoplastic kidney is small and underdeveloped. Since such a kidney may contain precancerous cells, its presence is a greater cause for concern.

• Supernumerary kidneys.
In this malformation, two normal kidneys drain into a duplicated or triplicated renal pelvis.

• Horseshoe kidney.
In this malformation, the two kidneys are fused into one large, horseshoe-shaped organ. A horseshoe kidney lies low in the back because it is too large to rise.

Excerpt from THE DISNEY ENCYCLOPEDIA OF BABY & CHILD CARE, © 1995 DSH Communications, Inc. This material is based on current medical research and, to the best of the editors' knowledge and understanding, is accurate and valid. However the reader should not use information contained in this material to alter a medically prescribed regimen or as a form of self-treatment, without seeking the advice of a licensed physician.