Determining kidney size for children is important in certain circumstances because enlarged kidneys can be evidence of obstruction along the ureters or urethral. Left untreated, some obstructions may lead to kidney failure. Kidney size is measured by ultrasound and is a routine part of a prenatal exam. Ultrasounds may be indicated in infants and children after urinary tract infections or the finding of an abdominal mass.

Assessing Kidneys

Your kidneys are located along the lower ribs and muscles of your back, just above the level of your belly button. The right kidney sits a little lower than the left because it is under your liver, the largest internal organ. Kidney size and other characteristics can be easily determined by ultrasound, as is done routinely during prenatal ultrasounds. After birth, routine blood and urine labs can add useful information about the kidneys' function.

When to Assess Kidneys

There are a couple of instances in which pediatricians might order an ultrasound of the kidneys. By far the most common is a urinary tract infection in girls up to age 6 or in boys of any age. Many pediatricians will forgive one urinary tract infection in older girls if they don't have a fever with it. The most urgent reason to get a kidney ultrasound is feeling a mass in your child's abdomen. Abdominal masses in children are almost always related to their urinary tract.

Enlarged Kidneys

In infants and children, an enlarged kidney can be evidence of a blockage, either along the ureter from a kidney to the bladder, or along the urethra from the bladder to the outside world. A ureter obstruction affects the kidney on that side, while a urethral obstruction affects both kidneys. Obstructions are uncommon and most frequently caused by a ureter on one side not connecting to the bladder properly. If there's an obstruction, the urine that backs up will enlarge the kidney and cause abnormal pressure within it, eventually causing the kidney to fail. Finding an obstruction early can avoid such bad consequences, and treatment may require medication or surgery.

Normal Kidney Size

The kidneys of newborns are about 4.5 cm long, almost 2 inches, and weigh just less than an ounce. Kidneys of adults are about 12 cm long, nearly 5 inches, and weigh about 5 ozs. The growth of kidneys correlates more with a child's growth in height rather than age, and the normal length of the kidneys can be estimated using a simple mathematical formula based on your child's height. The kidneys grow rapidly in the first year of life, from 4.5 cm to 6.5 cm, and then gradually into adulthood, only about 0.3 cm, an eighth of an inch, per year on average.

Kidney Health

Kidneys are complex organs, not only filtering your bloodstream of wastes but also helping to regulate your blood pressure. They are such efficient filters that only one-fourth of one kidney is needed to properly filter the bloodstream, but they also need to last a lifetime. So keep your child well hydrated at all times, particularly during illnesses, and keep up with your child's regular health check-up appointments.