Introduction

Foetal pyelectasis (FP) is a congenital abnormality where the foetus develops one or two kidneys with a dilated or enlarged pelvis.¹ In short, the renal pelvis is a funnel-shaped structure where urine accumulates and is collected after the blood has been filtered. The pelvis then converges into a tube that connects to the bladder, known as the ureter. It is often detected using ultrasound in the second trimester of pregnancy, and its significance is determined by the size of the enlargement. Although urine production is generally unaffected in mild and moderate cases, long-term effects include eventual decline of kidney function if left unmonitored, which requires early intervention after birth.² As a result, early detection is important to prevent these long-term complications and maintain the child’s normal kidney function.

Many maternal risk factors, including lifestyle, diet and certain conditions, have been linked to abnormalities in foetal renal development, including FP.³ Therefore, recognising and minimising these risk factors is imperative for normal foetal development and the decreased incidence of FP.

Overview of foetal renal development

Role of the renal pelvis in urine formation

The main role of the kidney is to filter blood to remove waste products and toxins, and regulate water volume and blood pressure.⁴ The functional unit of the kidney is known as a nephron, in which each kidney has approximately one million nephrons located in the renal cortex and renal medulla (see diagram below).⁵ The nephron is responsible for filtering blood and selectively reabsorbing nutrients and water back into the blood and concentrating urine. After the urine has been filtered, it flows through a tubule called a collecting duct, which converges and empties into the renal pelvis. The renal pelvis acts as a funnel that connects the nephrons to the ureter, allowing the urine to flow to the bladder.⁶

https://www.hopkinsmedicine.org/health/wellness-and-prevention/anatomy-of-the-urinary-system

Key stages of renal development in utero

Kidney development begins in the fourth week of gestation and continues until around week 36, when nephron development is complete, by which point foetal urine contributes to a large percentage of amniotic fluid, which is critical for lung development and cushioning the foetus.^8,9

Formation of kidneys and renal pelvis occurs during the first and second trimesters. An embryonic tissue known as the ureteric bud branches at approximately six weeks, giving rise to the ureters, renal pelvis, calyces, and collecting ducts. Any interference in this development can increase the risk of FP.

Risks and abnormal development 

The development of FP is currently poorly understood but may be ultimately due to obstruction of urine flow or vesicoureteral reflux (VUR), in which urine flows back up into the kidney from the bladder.¹⁰ In both instances, this may result in the buildup of urine in the renal pelvis, causing it to expand.

Whilst most cases of FP are self-limiting, in rarer, severe cases where the condition develops into hydronephrosis (an advanced form of FP, where the renal pelvis is severely dilated), the restricted urine flow to the bladder can reduce the volume of amniotic fluid. This presents more severely when both kidneys are affected. Oligohydramnios is the term used when the amniotic fluid volume is lower than expected for the gestational age and can increase the risk of a number of complications, including impaired lung development (pulmonary hypoplasia) and early birth induction. 

In the case of the affected kidney itself, if obstruction occurs, the kidney is more prone to injury, mainly due to restricted blood flow, which can give rise to kidney cell death. As a result, in such extreme and rare cases, early detection and intervention are crucial to prevent further complications. 

Maternal risk factors for foetal pyelectasis

Maternal diabetes

Maternal and gestational diabetes (when the mother develops diabetes during pregnancy) can increase the risk of several foetal developmental issues if blood sugar levels are not properly controlled.¹¹ This includes normal kidney development of the foetus, particularly branching of the ureteric bud. Whilst there is no study correlating maternal and gestational diabetes to FP specifically, it is important to highlight that elevated blood sugar levels are linked to the reduction in the number of nephrons developed, damage to the kidney and hydronephrosis.¹²

Currently, one of the safest medications used to control blood sugar levels during pregnancy is insulin, a hormone that the body naturally produces to lower blood sugar levels, which is absent or reduced in diabetics.¹³ Insulin is administered via injection into subcutaneous fat, usually in the abdomen or thigh, and needs to be monitored using a blood glucose machine. It is considered safer because it does not travel through the placenta to the foetus, and there is no evidence of any impact on normal foetal development.

Advanced maternal age

Mothers aged over 35 are considered to have advanced maternal age and are categorised as such due to associated risk factors. One of these risk factors is the development of chromosomal abnormalities such as Down’s Syndrome, which has been associated with FP.¹⁴

Advanced maternal age is a risk factor for gestational diabetes and hypertensive disorders; therefore, blood glucose levels should be monitored and an appropriate management plan implemented to reduce the risk of FP development.¹⁵

Vitamin A deficiency

Vitamin A is essential for embryonic growth and organ development. Similar to high blood sugar levels, low vitamin A has been linked to impaired branching of the ureteric bud, which in turn increases the risk of FP development.¹⁶ Causes of vitamin A deficiency include: 

• Excessive alcohol intake 
• Use of cocaine 
• Insufficient vitamin A intake from diet
• Impaired absorption in the gut 
• Liver disorders 

High blood pressure and pre-eclampsia 

When pregnant mothers have high blood pressure or develop it during pregnancy (known as pre-eclampsia), blood flow from the mother to the foetus, via the placenta, becomes restricted.¹⁷ Depending on the stage of pregnancy in which this restriction occurs, the foetus can have impaired growth and development due to reduced oxygen and nutrients to aid these processes. Kidney development may also be affected and give rise to FP. Antihypertensive medications such as labetalol are considered safe during pregnancy and are commonly used to maintain blood pressure within target ranges. 

Medications affecting ureteric bud branching

• Angiotensin ll receptor blockers (ARBs)¹⁶
• Angiotensin-converting enzyme inhibitor (ACE) 
• Corticosteroids (when used in high or prolonged doses)
• Cyclophosphamide 
• Mycophenolate mofetil 

These medications should be avoided and replaced with safer alternatives under medical supervision.

Diagnosis and monitoring of foetal pyelectasis

Prenatal ultrasound screening

FP is usually detected and diagnosed at 20 weeks (second trimester of pregnancy) using ultrasound scans.¹⁸ The renal pelvis is measured anteroposteriorly (from front to back) and is categorised by size to determine severity:

• Mild: between 5 and 7 mm
• Moderate: between 7 and 10 mm
• Severe: over 10mm 

The majority of those with mild FP spontaneously resolve after birth or during late stages of pregnancy; however, those with moderate to severe FP may require surgery to prevent deterioration of kidney function.¹⁴ Hydronephrosis and blockage of urine need to be ruled out, which is usually accompanied by other signs and symptoms such as reduced volume of the amniotic fluid, which presents as a smaller than expected pregnant abdomen.¹⁹

https://pmc.ncbi.nlm.nih.gov/articles/PMC9955995/#B20-children-10-00407

Follow-up monitoring

At around weeks 28-32 of pregnancy, a follow-up ultrasound is performed to monitor any changes in the size of the renal pelvis.²⁰ Because most cases resolve spontaneously, monitoring continues after birth to ensure normal kidney function and successful resolution. If blockage or reflux is suspected or the renal pelvis has maintained a large size or increased in size, then surgical intervention will be considered after birth, following further monitoring.

Association with down’s syndrome

There has been an association made between the risk of Down’s Syndrome and the detection of FP in the second trimester and it has thus been used as an informal marker.¹⁴ In cases where the foetus only presented with FP, 1 in 390 pregnancies resulted in the child developing Down’s syndrome; therefore, this rare outcome should not warrant a need for genetic testing in such cases.²¹ However, genetic testing may be offered to mothers with additional risk factors, such as mothers over the age of 35 and those with low serum alpha-fetoprotein (AFP). 

Monitoring maternal health during pregnancy

It is important to manage the maternal risk factors for FP in order to minimise the risk of the foetus developing it and the associated complications. If a mother is diabetic or develops gestational diabetes, blood sugar needs to be controlled through an appropriate diet and medication that can be safely taken during pregnancy. 

Blood pressure needs to be monitored and managed throughout pregnancy to prevent impairment in development. Medications such as labetalol can be taken safely by pregnant women to reduce blood pressure that is persistently high.²²

Mothers over the age of 35 should be monitored more frequently due to an increased risk of multiple risk factors of FP.²³ Early detection allows for early intervention, thus reducing the risk of FP and the development of severe subtypes. Ensuring adequate intake of essential vitamins, especially vitamin A, either through diet or supplements.

Summary 

• FP is the enlargement of the renal pelvis in the kidney of a foetus, likely due to disruption of its development
• The exact development of FP is poorly understood and further study is needed to understand the relationship between the risk factors and FP development 
• Most cases of FP self-resolve, but more severe forms can develop into hydronephrosis and oligohydramnios 
• Maternal risk factors of FP include diabetes, high blood pressure, low Vitamin A, certain medications and advanced maternal age 
• Managing these risk factors and sufficient monitoring are important to assess the extent of FP and prevent its development