The finding of fluid in a baby’s kidneys during a prenatal ultrasound is a common occurrence known as fetal hydronephrosis. This condition represents a dilation of the renal pelvis, the basin in the center of the kidney where urine collects before draining. It is one of the most frequently detected anomalies on prenatal scans, appearing in an estimated 1% to 3% of pregnancies. Although the diagnosis can be concerning, the majority of cases are mild, temporary, and resolve without causing lasting issues.
How the Fluid Accumulation Occurs
The core reason for fluid accumulation is a disruption in the normal, continuous flow of urine from the kidney to the bladder. The urinary tract system, consisting of the kidneys, ureters, bladder, and urethra, is designed for one-way drainage. When this drainage is slowed or blocked, the accumulating urine causes pressure and dilation of the renal pelvis.
The most frequent scenario is transient hydronephrosis, where a specific cause for the dilation cannot be identified. Experts believe this often stems from a temporary immaturity or narrowing in the developing urinary tract that corrects itself as the fetus grows. Approximately half of all prenatally diagnosed cases resolve spontaneously, sometimes even before the baby is born.
A more concerning cause involves a physical obstruction, much like a kink in a garden hose, impeding the flow of urine. The most common site for this is the Ureteropelvic Junction (UPJ), the point where the kidney connects to the ureter. This UPJ obstruction is the leading cause of significant fetal hydronephrosis and prevents the kidney from fully emptying its collected urine.
Obstruction can also occur lower down at the Ureterovesical Junction (UVJ), where the ureter enters the bladder, or at the bladder outlet itself. In male fetuses, a rare but serious obstruction is caused by Posterior Urethral Valves (PUV), which are abnormal folds of tissue in the urethra. This blockage severely restricts the outflow of urine from both kidneys, leading to a significant backup of fluid.
Another mechanism involves Vesicoureteral Reflux (VUR), where a valve malfunction allows urine to flow backward from the bladder up into the ureter and the kidney. Normally, the junction between the ureter and the bladder prevents backflow during bladder contraction. If this mechanism fails, pressure from the bladder pushes urine back toward the kidney, leading to dilation.
Identifying the Severity of the Condition
Assessing the risk associated with fetal hydronephrosis relies on precise ultrasound measurement of the kidney’s dilation. Physicians rely on the Anteroposterior Pelvic Diameter (APD), which measures the widest distance across the renal pelvis in millimeters. This measurement is important because the degree of dilation directly correlates with the likelihood of a persistent issue after birth.
The APD thresholds used for grading change as the pregnancy progresses due to the baby’s growth. Hydronephrosis is considered mild if the APD is less than 5 millimeters in the second trimester and less than 7 millimeters in the third trimester. Such mild findings have a very high chance of resolving without any need for intervention.
A measurement of 10 millimeters or greater in the third trimester is often used as a threshold to classify the condition as severe or persistent. Higher APD measurements, along with the presence of other features, suggest a greater probability of a true obstruction or significant reflux.
Beyond the APD measurement, sonographers look for other distinguishing signs to determine the underlying cause and severity. They check whether the dilation is unilateral or bilateral, as bilateral cases carry a higher risk. The appearance of the ureter is also noted; if it is also dilated, it points to a problem lower down, such as VUR or UVJ obstruction.
The overall volume of amniotic fluid is an important indicator, since fetal urine is the primary component in the second half of pregnancy. If severe bilateral hydronephrosis impairs the kidneys’ ability to produce urine, the amniotic fluid level may be low (oligohydramnios). This low level is a sign of potentially compromised renal function and requires careful postnatal follow-up.
Monitoring and Treatment Strategies
Once fetal hydronephrosis is diagnosed and graded, the management plan centers on careful monitoring throughout the rest of the pregnancy. For mild cases, the medical team will typically schedule repeat ultrasounds to track the APD and ensure the fluid accumulation does not worsen. The goal is to monitor for progression or resolution.
Following birth, a postnatal ultrasound is necessary because the baby’s hormonal status changes, which can temporarily reduce the dilation, making an immediate scan less accurate. This follow-up scan, usually performed within the first week of life, determines if the hydronephrosis is still present and guides further evaluation. If the dilation persists or is moderate to severe, additional specialized tests are often required.
Diagnostic Testing
If a blockage or reflux is suspected, specialized tests are performed. One such test is a Voiding Cystourethrogram (VCUG), which involves placing a catheter into the bladder and filling it with a liquid that is visible on X-ray. This procedure is used specifically to check for Vesicoureteral Reflux by observing if the liquid flows backward from the bladder up toward the kidneys.
If a blockage is suspected, a Diuretic Renography, often using a MAG-3 scan, may be performed. The MAG-3 scan uses a radioactive tracer injected into the bloodstream to evaluate how well each kidney is functioning and how quickly it drains the urine. This test helps differentiate between a truly obstructive blockage and a non-obstructive dilation that can be safely observed, which is important for determining the need for surgery.
Observation and Surgery
Most children with mild or moderate hydronephrosis only require observation with serial ultrasounds. They may also receive antibiotic prophylaxis to prevent urinary tract infections, especially if reflux is present.
Surgical intervention is reserved for severe cases where persistent obstruction is confirmed, or if the kidney function begins to decline. The most common surgical procedure for UPJ obstruction is a pyeloplasty, which involves removing the narrow or blocked segment and reconnecting the ureter to the renal pelvis to create a wider channel. This procedure is typically performed after the baby is born and is generally highly successful in preserving long-term kidney health.