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2011-03-17-10 Duplex kidney © Cerekja
Duplex kidney

Albana Cerekja
, MD, PhD*, Juan Piazze MD, PhD**.

*   Ultrasound Division, ASL Roma B, Rome, Italy.
** Ultrasound Division, Ceprano Hospital, Ceprano, Italy.



Duplex kidney is one of the most common major congenital abnormalities of the urinary tract. It's incidence is 1% of all live births with higher frequency in females than in males [1]. Duplicated collecting systems (also known as duplex collecting systems) can be defined as renal units containing 2 pyelocaliceal systems that are associated with a single ureter or with double ureters. The two ureters empty separately into the bladder or fuse to form a single ureteral orifice [2]. The majority of duplex kidneys is unilateral. When double ureters drain a double kidney, the two ureteral orifices in the bladder are characteristically inverted in relation to the collecting systems they drain. The orifice of the lower pole ureter occupies the more cranial and lateral position and upper pole ureter has a caudal and medial position. These findings are part of the spectrum described as a Weigert-Meyer rule [3,4,5] "The upper pole obstructs and the lower pole refluxes". The caudal orifice of the upper pole ureter is often stenotic and results in marked dilatation of the ectopic ureter and the upper renal collecting system, hydronephrosis and ureterocele [6].

Ultrasound findings

Ultrasonographic criteria for prenatal diagnosis of duplex kidney are represented by the following [7]:

  • Length of the kidney in the sagittal view > 95° percentile (see the table below)
  • Cystic-like structure surrounded by a rim of renal parenchyma in the upper pole of the kidney (second pelvis)
  • Kidney with two separate non communicating renal pelvises
  • Dilated ureter usually draining the upper pole. Normally, the fetal ureters are too narrow to image (1-2 mm) and they are considered dilated if their diameter is ≥3 mm.
  • Echogenic cystic structure in the bladder (ureterocele)

Normal fetal renal dimensions (mm) correlated with gestational age

Gest.Age Thickness* Width* Length* Volume*
16 2/6/10 6/10/13 7/13/18 0/0.4/2.6
18 4/8/12 6/10/14 12/17/22 0/0.7/2.9
20 6/10/13 7/11/15 15/21/26 0/1.1/3.3
24 9/13/17 10/14/18 22/28/33 0.3/2.5/4.7
28 12/16/20 13/17/21 28/33/38 2.5/4.7/6.9
32 15/19/23 17/20/24 33/38/43 5.4/7.5/9.7
36 17/21/25 19/23/27 36/41/47 8.1/10.2/12.4


* 5th/50th/95th percentile
Adapted from Romero R. Pilu G. Jeanty P. Prenatal Diagnosis of Congenital Anomalies. East Norwalk, CT. Appleton & Lange, 1989.

Although the antenatal diagnosis of fetal hydronephrosis is simple, the diagnosis of fetal renal duplication is infrequent. This is probably due to unfamiliarity of sonographers with this entity and because the kidneys don't look markedly abnormal, especially in case the collecting system is not dilated.

Case report

A 33-year-old patient G1 P0 was scanned for the first time at 22 weeks. It was a female fetus and both kidneys looked normal with no pyelectasis. A small left choroid plexus cyst (5 mm) was evidenced. Amniocentesis was not performed.

At 31 weeks an elongated anechoic avascular structure was seen in the right pelvic area near the spine. Cranially, it communicated with the upper third of the kidney and distally it was connected to the bladder although the level of insertion into the bladder could not be shown. This structure was thought to be a hydroureter and measured 4.8 mm in diameter. No hydronephrosis was evidenced. The length of the right kidney was 44 mm, which was above the 95th percentile for this gestational age. The contralateral kidney looked normal, although it was technically difficult to scan because of the position of the baby.

A follow-up exam in 2 weeks didn't show any hydroureter. Fetus had changed position so the left kidney was better visualized and two small non communicating pelvises were visualized in both kidneys. The length of the left kidney and right kidney was 47 mm and 49 mm respectively, which is above the 95th percentile.

Same findings were obtained at 35 weeks and the suspicion of a bilateral duplex kidney was raised.

A healthy female neonate of 2900 grams was born spontaneously at 39 weeks. A renal sonogram performed at 1 month of age showed a right duplex kidney. The right duplex system measured 54 mm (superior to the upper limits for the age), there was a slight pyelectasis (the upper pelvis measured 7 mm) and  the renal parenchyma appeared normal. There was also a slight dilatation of the proximal tract of the upper pole of the ureter (5 mm). The left kidney looked normal and measured 48 mm. The bladder wall had a normal thickness and there was no ureterocele.
Parents were counseled and explained the need for the growth check and regular blood and urinary laboratory work to rule out any urinary tract infections.

Images 1,2: 22 weeks, Image 1 shows both kidney in the sagittal view. Image 2 shows a transverse view of the renal pelvis.

Images 3,4
: 31 weeks, image 3 shows a dilated renal pelvis and ureter, no urine flow visible in the ureter. Image 4 shows a transverse view of the both kidneys.

Images 5,6: 33 weeks, image 5 shows a sagittal view of the left kidney with duplicated collecting system, note 2 renal pelvises. Image 6 shows a transverse view of the both renal pelvises of the left kidney.

Images 7,8: Coronal view of the left kidney, measured is the length of the kidney, note the duplicated renal pelvis.


Images 9,10: Image 9 shows a sagittal view of the right kidney with duplicated collecting system. Image 10 shows a normal looking urinary bladder.

Images 11,12: 35 weeks, Images show a coronal view of the left kidney with duplicated pelvis.

Images 13,14: Image 13 shows the length of the left kidney. Image 14 shows a transverse view of both left kidney pelvises.

Images 15,16: Transverse view of the both kidneys. Image 16 shows transverse view of the upper renal pelvises (left) and lower renal pelvises (right).

Videos 1,2: Videos showing dilated collecting system and ureter.

Video 3: 31 weeks, video showing the urinary bladder with ureterocele.


1. Privett JTJ, Jeans WD, Roylance J. The incidence and importance of renal duplication. Clin Radiol 1976;27(4):521-30.
2. Khan AN, Chandramohan M, MacDonald S. Duplicated Collecting System Imaging. eMedicine Specialties . Radiology, Genitourinary, updated Feb 22, 2008.
3. Weigert C. Ueber einige Bildunsfehler der Ureteren. Virchows Arch 1877,70:490-2.
4. Meyer R. Normal and abnormal development of the ureter in the human embryo–a mechanistic consideration. Anat Rec 1946;96:355-8.
5. Manohar S. Double renal collecting system.
6. Mestcatello VJ, Smith EH, Carrera CF, Berger M, Teele RL. Ultrasonographic evaluation of the obstructed kidney. Am J Roentgenol 1977;129:113-16.
7. Abuhamad AZ, Horton Jr CE, Ewans AT. Renal duplication anomalies: clues for prenatal diagnosis. Ultrasound Obstet Gynecol 1996;7:174-77.

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