Introduction 

The kidneys remove waste from the blood and produce urine, which passes through tubes called ureters to the bladder.¹⁷ The renal pelvis is the area where urine collects before moving into the ureters.¹ Pyelectasis is a condition in which the renal pelvis of the kidney becomes enlarged ². It is often detected via prenatal ultrasound, and its clinical significance can vary. Fetal renal pelvis dilation is observed in about 1% to 5% of pregnancies and is usually a temporary, harmless variation within normal development. This condition is more frequently observed in male fetuses, with some studies reporting a male predominance. Although most cases resolve on their own, pyelectasis may sometimes reflect underlying urinary tract problems or, less commonly, be linked to chromosomal disorders such as Down syndrome.³ 

Understanding the different causes of pyelectasis can help parents feel more in control and informed about monitoring and treatment options. 

Congenital causes 

Pyelectasis is not inherited, but certain genetic disorders or conditions can increase the likelihood of its development. Risk factors include a family history of urinary tract disorders, genetically inherited anatomical abnormalities, and maternal medical conditions, including gestational diabetes.⁵ 

These are congenital conditions related to the formation of the urinary system in the fetus:

• Ureteropelvic Junction Obstruction (UPJO): This occurs when there is an obstruction where the kidney joins the ureter, preventing normal urine flow. It makes urine flow backwards into the kidney, enlarging the renal pelvis as there is an increased volume of urine overwhelming the renal pelvis. UPJO is one of the most common underlying causes for pyelectasis and typically affects only one kidney⁴
• Posterior Urethral Valves (PUV): This is found in male fetuses and involves abnormal flaps of tissue in the urethra that block urine flow. The blockage causes urine to flow backwards in the opposite direction, into the bladder and kidneys, resulting in pyelectasis. PUV can be severe and may require treatment after birth¹¹
• Vesicoureteral Reflux (VUR): VUR happens when urine flows backwards from the bladder into the ureters and kidneys. This backward flow can lead to renal pelvis dilation due to the overwhelming amount of urine backflow. VUR can be mild, which often resolves on its own, while severe cases may require treatment to prevent kidney damage¹⁸

Genetic links 

As mentioned above, a family history of urinary tract disorders and genetically inherited anatomical abnormalities can increase the risk of pyelectasis. 

Pyelectasis is also associated with certain chromosomal abnormalities, such as Down syndrome (Trisomy 21).⁸ Studies have shown that fetal pyelectasis is more common in chromosomally abnormal pregnancies and can serve as a marker for such conditions, though it is not definitely predictive.⁹

In some families, pyelectasis is observed across multiple generations, suggesting a genetic predisposition. Although no specific gene has been identified, familial clustering of pyelectasis and related renal abnormalities can sometimes be observed.³

Obstructive causes 

These involve physical barriers that obstruct the normal urine flow from the kidneys. There are two main categories:

• Intrinsic Obstructions: These occur within the urinary tract itself, such as UPJO. They may result from abnormal development or tightening of the urinary tract¹¹
• Extrinsic Obstructions: These happen when external structures compress the urinary tract, causing an obstruction. For example, a blood vessel may press against the ureter, obstructing urine flow and leading to pyelectasis¹⁹

Physiological causes

• Pregnancy-Related Changes: During pregnancy, hormonal fluctuations and increased blood flow to the kidneys can lead to temporary widening of the renal pelvis.¹⁰ This effect, known as physiological pyelectasis, typically resolves spontaneously after birth without medical intervention¹²

Transient causes

• Mild Hydronephrosis: Fetal pyelectasis is often associated with mild hydronephrosis, which often resolves on its own during pregnancy. Hydronephrosis is a condition in which one or both kidneys become enlarged due to the accumulation of urine within them.⁷ This can result from factors such as a temporary kink in the ureter, bladder overdistension, or changes in fluid dynamics during pregnancy
• Normal variations during development: Pyelectasis is usually a variation of fetal development, especially when the enlargement is mild.¹⁵ The condition will usually resolve as the fetus grows without any treatment
• Transitory Blockages: Occasionally, transitory situations like the positioning of the fetus or bladder distension lead to temporary backup of urine, resulting in transient pyelectasis.¹⁵ These cases usually resolve on their own

Frequently asked questions 

What is the difference between hydronephrosis and pyelectasis?

• Hydronephrosis often indicates a more serious underlying issue and may require medical intervention to prevent kidney damage. However, pyelectasis specifically refers to the mild enlargement of the renal pelvis without involving the entire kidney⁷

Is pyelectasis a serious condition?

• In most instances, pyelectasis is mild, particularly in fetuses or newborns. It usually resolves on its own without any medical interventions.¹³ However, if pyelectasis is secondary to obstruction or another structural issue, further studies and treatment may be required

Can pyelectasis be a sign of an underlying serious condition?

• Yes, in some cases, pyelectasis can be a sign of more serious conditions like hydronephrosis or chromosomal abnormalities. If detected during pregnancy or infancy, additional testing may be recommended to rule out other causes
• If pyelectasis progresses to hydronephrosis, a pediatric urologist or pediatric nephrologist may be involved in postnatal care.

How is pyelectasis diagnosed?

• Pleural effusion is generally diagnosed by prenatal ultrasound studies. After birth, it can be diagnosed with imaging tests like ultrasound, or, in more severe cases, a voiding cystourethrogram (VCUG) or renal scan¹²
• If pyelectasis is detected in isolation and aneuploidy screening has not been performed, prenatal cell-free DNA screening is recommended. If results are reassuring, no further diagnostic testing is necessary. However, if other anomalies are present, genetic counselling and further diagnostic options, such as amniocentesis, may be offered¹²

Is there a connection between pyelectasis and preterm birth?

• Pyelectasis is generally considered a benign condition, and its association with preterm birth is not well established.

Can pyelectasis recur?

• Research shows that mild fetal pyelectasis does not increase the risk of urinary tract problems during childhood. Additionally, studies suggest that children who had mild pyelectasis in utero do not face higher rates of urinary tract issues compared to those without the condition
• In rare cases, if pyelectasis is linked to other urinary issues, some follow-up may be required. For example, posterior urethral valves can cause blockages in the urinary tract that, if untreated, may damage the kidneys. However, if pyelectasis is isolated and not linked to other conditions, long-term monitoring is typically unnecessary¹⁴

Why are males more likely to have pyelectasis than females?

• The reasons for the higher prevalence of pyelectasis in males are not completely understood. However, differences in fetal kidney development between males and females may play a role. Studies show that male renal anomalies like pyelectasis are more common amongst male fetuses, possibly due to anatomical differences and hormonal influences

Are there any symptoms of pyelectasis?

• During pregnancy, pyelectasis is typically identified through routine prenatal ultrasounds and does not present noticeable symptoms in most cases
• However, if it persists after birth, it can lead to urinary tract infections, abdominal pain, or blood in the urine

Summary 

Pyelectasis is a condition in which the renal pelvis (the area of the kidney where urine is collected) is mildly dilated. It is frequently detected on prenatal ultrasounds. Generally, pyelectasis resolves on its own, without intervention. However, its underlying cause should be investigated, as it may reflect either a harmless, transient condition or a more serious issue that requires medical attention.

Common causes of pyelectasis are congenital abnormalities such as ureteropelvic junction obstruction (UPJO), posterior urethral valves (PUV), and vesicoureteral reflux (VUR), which are typically structural or anatomical. Genetic issues, such as urinary tract abnormalities or genetic test findings like Down syndrome, can also increase the likelihood of pyelectasis. While some cases are due to an obstruction, others result from benign, transient factors during pregnancy, mild hydronephrosis, or short-term obstruction that will likely resolve after delivery. Understanding the underlying cause can help caregivers and parents to feel reassured and make informed decisions about monitoring and treatment.