Discovering fluid in a baby’s kidneys during pregnancy can be a concerning finding for expectant parents. This condition, known as fetal hydronephrosis, is identified when an ultrasound reveals swelling in one or both of the baby’s kidneys. It is a relatively common occurrence, often detected during routine prenatal examinations. This article explores the nature of fetal hydronephrosis, its common causes, how its severity is assessed, and the typical monitoring and care provided both before and after birth.
Understanding Fetal Hydronephrosis
Fetal hydronephrosis refers to the abnormal dilation of the fetal renal collecting system, which includes the renal pelvis and sometimes the calyces. This swelling occurs when urine accumulates in the kidney due to impaired drainage. It is one of the most frequently identified abnormalities during prenatal ultrasound examinations, with an estimated prevalence ranging from 0.6% to 5.4% of pregnancies, and it is more common in male fetuses.
The condition is typically identified during a routine prenatal ultrasound. The diagnosis is based on observing a dilatation of the collecting system of the kidney. The presence of fluid indicates that urine may not be flowing properly from the kidney, though the exact reason for this impaired flow varies.
Common Underlying Causes
Fluid accumulation in a baby’s kidneys can stem from several factors, ranging from temporary conditions to more significant structural issues. The most frequent cause is physiological or transient hydronephrosis, which often resolves on its own either before or shortly after birth without requiring intervention. This type does not involve an identifiable structural problem.
Ureteropelvic junction (UPJ) obstruction is another common cause. This blockage where the kidney meets the ureter prevents urine from draining properly into the bladder, leading to a buildup of urine and kidney swelling. UPJ obstruction is estimated to occur in about 1 in 1,000 infants.
Vesicoureteral reflux (VUR) can also cause fluid in the kidneys. This condition is characterized by the backward flow of urine from the bladder into the ureters and potentially back into the kidneys. Many VUR cases improve without surgical intervention. Posterior urethral valves (PUV) are a rarer but more serious cause, affecting male babies when a membrane obstructs urine flow from the bladder, leading to kidney dilation. Less common causes include duplicated collecting systems, where a kidney has two ureters, and conditions like prune belly syndrome.
Assessing Severity and Potential Outcomes
Fetal hydronephrosis is graded based on the amount of fluid observed during ultrasound examinations. This grading categorizes cases as mild, moderate, or severe, determined by the anteroposterior diameter of the renal pelvis. For instance, in the second trimester, mild hydronephrosis might involve a renal pelvis diameter of 4-7 mm, while severe cases could exceed 10 mm.
The degree of dilation indicates the likelihood of an underlying issue and the potential need for intervention. Mild cases often resolve spontaneously either before birth or in the early neonatal period. In contrast, more severe cases may indicate a greater chance of an underlying obstruction or other issues that could affect kidney function if left unaddressed. While rare, severe, persistent hydronephrosis, if not managed, can lead to complications such as impaired kidney function or an increased risk of urinary tract infections.
Monitoring and Post-Birth Care
Upon detection of fetal hydronephrosis, continued monitoring throughout the pregnancy uses ultrasounds to track any changes. The frequency of these follow-up ultrasounds depends on the initial severity; for example, mild cases might have an additional ultrasound after 32 weeks’ gestation, while severe cases may warrant repeat examinations every 4-6 weeks. This ongoing surveillance helps determine if the hydronephrosis is resolving, remaining stable, or worsening.
After birth, a postnatal evaluation is recommended to confirm the diagnosis and identify the underlying cause. This evaluation includes further ultrasounds to assess the baby’s kidneys and bladder, and sometimes a voiding cystourethrogram (VCUG), an X-ray that shows how well the bladder is working and checks for reflux. Other tests, such as a diuretic renal scan, might be performed in more severe cases to diagnose an obstruction, often delayed until the baby is at least one month old for greater accuracy.
Based on postnatal findings, interventions vary. For vesicoureteral reflux, antibiotics might be prescribed to prevent urinary tract infections, with ongoing monitoring through periodic ultrasounds and voiding cystograms. If a significant obstruction like a UPJ obstruction is identified, surgical correction such as a pyeloplasty may be considered to restore proper urine flow. The overall outlook for most babies diagnosed with fetal hydronephrosis is positive, with many cases resolving without extensive treatment.