When evaluating the fetal kidneys on ultrasound, a duplicated collecting system can sometimes be surprisingly difficult to confirm.
One particularly interesting situation is when the number of renal arteries does not match the number of visible renal pelves.
For example, during a fetal ultrasound examination, we may encounter findings such as:
- Right kidney: two renal arteries, but only one renal pelvis
- Left kidney: one renal artery, but two renal pelves
At first glance, this can be confusing.
Does two renal arteries mean a duplex kidney?
Can a kidney with only one renal artery still have a duplicated collecting system?
The answer to both questions is important.
Renal Artery Number Does Not Define a Duplex Kidney
A duplex kidney, or duplicated collecting system, refers primarily to duplication of the urinary collecting system, not duplication of the renal arterial supply.
Accessory or multiple renal arteries can occur as an anatomical vascular variation.
Therefore:
Two renal arteries + one renal pelvis does not automatically mean a duplex kidney.
If the renal parenchyma appears otherwise normal and only a single collecting system is identified, the additional vessel may simply represent an accessory renal artery.
This is why color Doppler findings should always be interpreted together with the grayscale anatomy.
What If Two Renal Pelves Are Seen With Only One Renal Artery?
This is the more interesting situation.
If two distinct renal pelves or upper- and lower-pole collecting systems are visualized within one kidney, a duplex collecting system should be considered even when only one renal artery is identified.
The diagnosis of a duplicated collecting system is based much more on the collecting-system anatomy than on the number of renal arteries.
In other words:
One renal artery does not exclude a duplex kidney.
This is an important practical point during prenatal ultrasound.
Ultrasound Findings That Support a Duplex Collecting System
When duplication is suspected, I look carefully for several features.
1. Two separate renal pelves
The upper and lower portions of the kidney may demonstrate separate collecting systems.
Scanning the kidney slowly in coronal and sagittal planes can make this easier to appreciate.
2. An elongated or enlarged kidney
A duplex kidney may appear longer than the contralateral kidney because two renal moieties are present.
Kidney size alone, however, is not diagnostic.
3. Separation of the upper and lower renal moieties
Sometimes the renal architecture itself gives the impression of two components rather than one uniform collecting system.
4. Dilatation of one collecting system
One moiety may be more dilated than the other.
In a complete duplication, complications may preferentially involve either the upper- or lower-pole system.
5. Dilated ureter or ureterocele
Visualization of a dilated ureter, particularly when associated with upper-pole dilatation, increases suspicion for a duplicated system.
A ureterocele within the fetal bladder can also provide an important clue.
However, the ureters may be difficult or impossible to visualize when they are not dilated.
Can Ultrasound Determine Complete vs. Incomplete Duplication?
Not always.
A complete duplicated collecting system has two ureters entering the bladder separately.
In an incomplete duplication, the two collecting systems eventually join before reaching the bladder.
Prenatally, distinguishing these can be difficult when the ureters are not dilated.
For this reason, if two collecting systems are suspected but the entire ureteral anatomy cannot be demonstrated, it may be more appropriate to describe the finding as:
“Suspected duplex collecting system.”
This avoids overcalling complete duplication based on limited prenatal visualization.
A Useful Practical Lesson
Consider this ultrasound pattern:
Right kidney
- Two renal arteries
- One renal pelvis
Left kidney
- One renal artery
- Two renal pelves
The right-sided finding may represent an accessory renal artery without collecting-system duplication.
The left-sided finding, despite visualization of only one renal artery, raises much stronger suspicion for a duplex collecting system because two renal pelves are present.
This illustrates an important principle:
Renal vascular anatomy and collecting-system anatomy do not necessarily duplicate together.
When evaluating a suspected fetal duplex kidney, the collecting system should therefore remain the primary focus.
What I Recheck During the Examination
When I suspect duplication, I usually go back through the kidney slowly rather than relying on a single transverse image.
I reassess:
- Both kidneys in transverse and longitudinal planes
- Upper- and lower-pole collecting systems
- Renal length and morphology
- Renal pelvis dilatation
- Visible ureters
- Fetal bladder
- Possible ureterocele
- Amniotic fluid volume
- Renal vascular anatomy with color Doppler
Color Doppler is useful, but it is supportive information rather than the defining feature of a duplex collecting system.
Take-Home Message
When assessing a possible fetal duplex kidney:
Do not count renal arteries and assume they represent the number of collecting systems.
Two renal arteries can supply a kidney with a single collecting system, while a kidney with a duplicated collecting system may not demonstrate two separate renal arteries on prenatal ultrasound.
The most important question is:
Are there truly two separate collecting systems?
That distinction can prevent both overdiagnosis and missed suspicion of a fetal duplex kidney.
About the Author
I am a sonographer with 20+ years of clinical ultrasound experience, including obstetric, breast, and thyroid ultrasound. This article reflects practical observations from hands-on ultrasound imaging and is intended for educational purposes rather than as a substitute for individual medical diagnosis.
