Hydro­nephrosis: kidney dilation in children

What is hydronephrosis?

Urine made in the kidney first collects in the calyces, then in the renal pelvis, and then travels down the ureter to the bladder. Hydronephrosis is dilation of this collecting system, that is, the renal pelvis and calyces. It is often called “water on the kidney.”

Hydronephrosis is not the name of a disease. It is a finding seen on ultrasound. Behind it there may be a completely harmless condition that resolves on its own, or a narrowing or reflux that needs surgical treatment. So the main question is not whether dilation is present, but why it developed and whether it is harming the kidney.

It is one of the most common findings in pediatric urology: dilation of the kidney collecting system is seen on fetal ultrasound in about 1–3% of pregnancies.

Causes of hydronephrosis

• Transient (physiological) dilation. This is the most common situation. There is no blockage or reflux, and the dilation decreases and resolves over time.
• UPJ obstruction. A narrowing where the renal pelvis joins the ureter. It is the most common cause of hydronephrosis that needs surgical treatment.
• Vesicoureteral reflux. Urine flowing backward from the bladder up to the kidney.
• Megaureter. Dilation of the ureter itself, often with a narrowing at its lower end.
• Duplex kidney and ureterocele.
• Posterior urethral valves. A congenital membrane in the urethra in boys. It usually shows up as dilation on both sides and a thick-walled bladder, and needs early treatment.
• Multicystic dysplastic kidney. Sometimes mistaken for hydronephrosis on ultrasound.

Grades: what do the numbers say?

The measurement most often taken on ultrasound is the anteroposterior diameter of the renal pelvis (in millimeters). But this number alone is not enough to make a decision: dilation of the calyces, the thickness and appearance of the kidney tissue, and the condition of the ureter and bladder are all assessed together.

After birth (after 48 hours), the UTD classification looks like this:

Group	Renal pelvis anteroposterior diameter (APD)	Other findings
Normal	APD < 10 mm	Calyces, kidney tissue, ureter and bladder normal
P1 (low risk)	10 ≤ APD < 15 mm and/or central calyceal dilation	Kidney tissue, ureter and bladder normal
P2 (intermediate risk)	APD ≥ 15 mm	Or dilation of the peripheral calyces, or dilation of the ureter; kidney tissue and bladder normal
P3 (high risk)	Any size	Thinning or abnormal appearance of the kidney tissue, or a bladder abnormality

Dilation of the central calyces does not by itself mean severe hydronephrosis: if the APD is below 15 mm (even below 10 mm) and there are no other findings, it is P1. During pregnancy the thresholds are different: at weeks 16–27, 4 ≤ APD < 7 mm, and after week 28, 7 ≤ APD < 10 mm, with no other changes, is low risk (A1); a larger measurement (APD ≥ 7 mm at weeks 16–27, APD ≥ 10 mm after week 28) or additional findings means increased risk (A2–3).

This is a simplified table based on the international UTD system. The full classification weighs all ultrasound findings together, and the final decision is made by the doctor. Older reports may also list SFU grades (1–4): 1–2 usually means mild and 3–4 severe dilation. You can check your own result for orientation with the hydronephrosis tool.

What to do if it was found during pregnancy

To spot urinary tract anomalies in time, two screening ultrasounds are recommended in pregnancy: at weeks 11–14 and at weeks 17–22.

For mild dilation found during pregnancy, in most cases no extra steps are needed: the measurements are followed on the next scheduled ultrasounds. The method of delivery usually does not change.

The following situations need closer monitoring, and it helps to consult a pediatric urologist before birth:

• severe dilation in both kidneys,
• a large, thick-walled bladder, especially in a male fetus,
• reduced amniotic fluid,
• a solitary kidney or changes in the kidney tissue.

In these situations the plan is made by a team: a maternal-fetal medicine specialist, a clinical geneticist, a pediatric urologist and nephrologist, a neonatologist and, when needed, a psychologist. Treatment before birth (for example, a shunt draining the baby’s bladder into the amniotic fluid) is considered only in selected cases: when urine outflow from the bladder is blocked (for example, by posterior urethral valves), amniotic fluid is reduced in the second trimester, and serious genetic anomalies have been ruled out. Hydronephrosis of one kidney does not need treatment before birth.

After birth: which tests and when?

A kidney and bladder ultrasound is the first step. In a newborn, the body holds relatively little fluid in the first 1–2 days, so the dilation can look smaller than it really is. For this reason the ultrasound is usually done after 48 hours, often in the first weeks of life. The doctor sets the exact timing based on the findings in pregnancy. In severe cases, however, meaning dilation on both sides, a solitary kidney, low amniotic fluid in pregnancy or suspected posterior urethral valves, there should be no waiting: the scan is done right after birth.

Further tests are chosen based on the ultrasound result:

• VCUG (voiding cystourethrogram). To check for reflux and look at the urethra if the ureter is dilated, the dilation is on both sides, the bladder looks abnormal, or there has been a febrile infection.
• MAG3/DTPA renogram. In severe dilation, to see whether urine drains from the kidney and how well each kidney works. It is done under standard conditions (good hydration, a diuretic), usually after 4–6 weeks of age.

For most children with mild dilation, periodic ultrasound is all that is needed.

Treatment: monitoring or surgery?

Hydronephrosis itself is not treated; its cause is. In most children, active monitoring is the right path: the dilation decreases or stays stable in the first 1–2 years, and the kidney develops normally.

Surgery is usually discussed in these situations:

• split renal function on MAG3/DTPA is below 40% or drops by more than 10 percentage points during monitoring (for example, from 48% to 36%),
• urine drains poorly from the pelvis after the diuretic,
• the dilation increases on consecutive ultrasounds or is high grade (SFU III–IV),
• there are recurrent febrile infections, flank pain or stones.

The decision is based not on a single number but on how the dilation and kidney function change over time.

For UPJ obstruction, the operation is pyeloplasty. Its success rate is about 90–95%. Treatment of reflux, megaureter and valves is explained separately on the relevant pages.

How is follow-up done?

The follow-up schedule is set individually based on the degree of dilation. A typical example: after the first postnatal ultrasound, the next one at 1–2 months, then every 3–6 months, and less often as the condition stabilizes. Follow-up stops when mild dilation has fully resolved. UTD P2 and P3, however, carry long-term risk, so these children are checked periodically throughout childhood. Children who have had surgery are followed for longer, usually for several years.

During follow-up, the parent’s most important job is to take fever seriously: if a baby with hydronephrosis has a fever with no clear cause, a urine test must be done.