Fetal Hydronephrosis: Causes, Risk Factors, and What Parents Should Know
Hydronephrosis in fetuses is a relatively common condition detected during routine prenatal ultrasounds. It refers to the swelling of one or both kidneys due to the accumulation of urine caused by a blockage or reflux in the urinary tract. While the diagnosis can be concerning for expecting parents, it's important to understand that many cases are mild and resolve on their own after birth.
Understanding the Difference Between Unilateral and Bilateral Hydronephrosis
One of the key factors in determining the underlying cause of fetal hydronephrosis is whether the condition affects one kidney (unilateral) or both kidneys (bilateral). This distinction helps doctors assess potential complications and plan appropriate follow-up care after delivery.
Bilateral Hydronephrosis: Often Linked to Lower Urinary Tract Obstruction
When both kidneys are affected, the most common cause is an obstruction in the lower urinary tract. A frequent structural issue in such cases is posterior urethral valves (PUV), which occur only in male fetuses. These abnormal flaps of tissue in the urethra can partially block urine flow from the bladder, leading to back pressure that causes both kidneys to swell. Other possible causes include bladder outlet obstruction or developmental abnormalities affecting the entire lower urinary system.
Unilateral Hydronephrosis: Usually Due to Upper Urinary Tract Issues
In cases where only one kidney is involved, the problem typically originates higher up in the urinary system. The most common cause is a narrowing at the ureteropelvic junction (UPJ)—the point where the renal pelvis connects to the ureter. This blockage prevents urine from draining properly into the bladder, resulting in kidney dilation. Another possible cause is a condition known as megaureter, a congenital disorder in which one of the ureters is abnormally wide and functions poorly, leading to urine buildup.
Diagnosis and Monitoring During Pregnancy
Fetal hydronephrosis is usually identified during the second-trimester anatomy scan, typically around 18–20 weeks of gestation. Ultrasound imaging allows healthcare providers to measure the degree of kidney swelling and monitor changes over time. Mild cases may require only periodic monitoring, while more severe cases might prompt additional testing, such as fetal MRI or amniocentesis, to rule out genetic syndromes or other associated anomalies.
What Happens After Birth?
In many instances, fetal hydronephrosis improves or resolves completely before or shortly after birth. However, newborns diagnosed prenatally often undergo a postnatal ultrasound within the first few weeks of life to evaluate kidney function and confirm the diagnosis. Depending on the results, further imaging studies like a voiding cystourethrogram (VCUG) may be recommended to check for vesicoureteral reflux or other structural issues.
Treatment and Long-Term Outlook
The majority of babies with mild hydronephrosis do not require surgery and go on to develop normal kidney function. For those who do need intervention, treatment options vary based on the cause and severity. Surgical correction—such as pyeloplasty for UPJ obstruction—is highly effective and typically performed using minimally invasive techniques. With timely diagnosis and proper management, the long-term prognosis for most infants is excellent.
Expectant parents should work closely with their obstetrician and a pediatric urology team to understand the specifics of their baby's condition. Advances in prenatal imaging and neonatal care have significantly improved outcomes, making early detection a powerful tool for ensuring the best possible health for the child.