What is a duplex kidney?
Normally each kidney has one collecting system (renal pelvis) and one ureter. In some people the kidney is divided into two parts, and each part has its own collecting system. This is called a duplex collecting system (duplex kidney). It is the most common kidney anomaly and occurs in about 1 in every 100 people.
- Incomplete duplication: the two ureters join along the way and open into the bladder through one opening. It usually causes no problems.
- Complete duplication: the two ureters open into the bladder separately. Problems mostly occur with this type.
What problems can it cause?
In complete duplication, each of the two ureters has its own risk:
- The ureter of the lower part often opens into the bladder through a short tunnel, and reflux develops.
- The ureter of the upper part opens into the bladder lower than normal. A ureterocele may form at its end, or it may open outside the bladder (ectopic ureter).
Ectopic ureter
In girls, an ectopic ureter can open below the bladder sphincter, into the urethra or the vagina. In this case, urine from that part is not held and dribbles out constantly. The typical picture is this: the girl pees normally, but her underwear stays wet, even if only slightly, throughout the day. This is different from ordinary daytime wetting and can be fully cured with surgery.
In boys, an ectopic ureter opens above the sphincter, so it does not cause wetting, but it can lead to infections.
Diagnosis and treatment
A duplex kidney is usually seen on ultrasound. If there is no dilation and there are no symptoms, no further tests or treatment are needed.
If a newborn has a dilated ureter and kidney (hydroureteronephrosis) because of an ectopic ureter or ureterocele, a small daily dose of antibiotic is started to prevent infection. In these cases, tests show how well each part of the kidney works and whether there is reflux or blockage (VCUG, DMSA scan, and if needed a MAG3 renogram or MR urography).
Treatment is tailored to each child: the child’s condition and age, the function of each part, reflux and blockage, and the preferences of the family and the surgeon are all taken into account. In general it depends on the type of problem: for reflux, antibiotic prophylaxis or endoscopic injection; for ureterocele, an endoscopic incision; and for an ectopic ureter, reattaching the ureter to the bladder or joining it to the ureter of the lower part. If the upper part of the kidney does not work at all and causes problems, that part can be removed.
When to seek urgent care
If any of the following is present, do not wait for a routine appointment: call the doctor or go to the nearest emergency department.
- Fever, especially in a baby
- Sudden flank pain and vomiting
Note for pediatricians and family doctors
In a girl, continuous dribbling incontinence throughout the day alongside normal voiding, with underwear that never stays dry even after toilet training, is typical of an ectopic ureter and must be distinguished from ordinary daytime wetting. These children should be referred to a pediatric urologist; the diagnosis is often made with MR urography. Neonates with hydroureteronephrosis due to an ectopic ureter or ureterocele are started on continuous antibiotic prophylaxis; imaging assesses moiety function, reflux and obstruction, and treatment is individualised.
What happens at the visit?
- Ultrasound assesses both parts, any dilation and the bladder.
- If there is dilation or there are symptoms, the function of each part, reflux and blockage are assessed: a VCUG (voiding cystourethrogram), a DMSA or MAG3 scan, and MR urography if an ectopic ureter is suspected.
- Monitoring, endoscopic or surgical treatment is explained according to the problem.
Frequently asked questions
The ultrasound report says duplex kidney, but our child is healthy. What should we do?
If there is no dilation and no symptoms, usually no treatment is needed. If there is a febrile infection, it is enough to get a urine test and let the doctor know.
Related topics
Sources
References to the EAU Paediatric Urology guidelines were checked against the 2026 edition (October 2026).
This information is for educational purposes and does not replace medical advice. Decisions about your child’s treatment are made with the doctor after an examination.
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