INTRODUCTION — Fetal hydronephrosis (dilation of the renal pelvis with or without dilation of the renal calyces) is a common finding on antenatal ultrasound. In most cases, renal pelvic dilation is a transient physiologic state, however, congenital anomalies of the kidney and urinary tract (CAKUT) can present with fetal hydronephrosis due to urinary tract obstruction and vesicoureteral reflux (VUR). These conditions may be associated with impaired renal development and/or cause renal injury.
The definition, etiology, and management of fetal hydronephrosis are reviewed here. Postnatal evaluation of fetal hydronephrosis and specific urologic conditions that may present as fetal hydronephrosis are discussed separately. (See "Fetal hydronephrosis: Postnatal management" and "Congenital ureteropelvic junction obstruction" and "Primary megaureter in infants and children" and "Ectopic ureter" and "Clinical presentation and diagnosis of posterior urethral valves", section on 'Prenatal' and "Clinical presentation, diagnosis, and course of primary vesicoureteral reflux".)
DEFINITION AND GRADING
Overview — Several systems have been developed to diagnose and grade the severity of fetal hydronephrosis .
●Renal pelvic diameter (RPD)
●Society of Fetal Urology (SFU) criteria
●Urinary tract dilation (UTD) classification system
However, there is no consensus on the most appropriate grading criteria for the diagnosis of fetal hydronephrosis . In general, the likelihood of having a significant renal anomaly correlates with the severity of hydronephrosis. In our practice, we define fetal hydronephrosis as a renal pelvic diameter (RPD) >4 mm between 18 and 22 weeks gestation and in the third trimester, a RPD >10 mm or as Society of Fetal Urology (SFU) grade III or above. We believe the cutoff point of 10 mm provides a reasonable threshold to detect most fetuses at risk for congenital anomalies of the kidney and urinary tract (CAKUT) and avoids unnecessary testing in cases that are likely due to benign transient hydronephrosis  (algorithm 1). (See 'Etiology' below.)
Renal pelvic diameter (RPD) — The most generally accepted method to define and grade fetal hydronephrosis is ultrasound measurement of the maximum anteroposterior diameter of the fetal renal pelvis (APPD), also referred to as RPD, in the transverse plane . Fetal hydronephrosis is graded according to the RPD during the second and/or third trimester of pregnancy. RPD is a measure of collecting system dilation and does not reflect the extent of hydronephrosis and parenchymal changes, such as increased echogenicity, thinning, or caliectasis.
Several studies have established normative values for fetal renal size based upon gestational age (GA) [5-7]. However, there remains a lack of consensus on the threshold RPD that defines clinically significant fetal hydronephrosis, which has a high likelihood for renal pathology requiring postnatal follow-up:
●In general, RPD >10 mm in the second trimester is associated with an increased risk for CAKUT [4,6,8,9]. Fetuses with RPD >15 mm during the third trimester are at the greatest risk for CAKUT [4,6,8-10]. (See 'Congenital anomalies of the kidney and urinary tract (CAKUT)' below.)
●Mild renal pelvic dilation, also referred to as pyelectasia, is defined as an RPD ≥4 to 10 mm in the second trimester [4,6,8,11]. Although most cases of mild renal pelvic dilation will resolve and not have a clinical impact on neonatal renal development, there are reports of persistent cases that require postnatal intervention [9,12-14]. Most experts in the field use a value above 4 to 5 mm as the lowest cutoff for fetal hydronephrosis in the second trimester requiring follow-up prenatal ultrasound monitoring.
GA, maternal hydronephrosis, maternal hydration, and the degree of bladder distention may affect the RPD [7,15-17]. In addition, varying the minimal RPD threshold for normal can significantly alter the positive predictive value of RPD as a measure of postnatal pathology. Lower cutoffs will be more sensitive in detecting postnatal pathology; however, the trade-off is in higher false-positive rates and requires more subsequent ultrasound assessments .
Society of Fetal Urology (SFU) — The SFU developed criteria for the diagnosis and grading of fetal hydronephrosis based upon the degree and site of pelvic dilation, number of calyces seen, and the presence and severity of parenchymal atrophy (image 1) . The SFU grading system focuses on the degree of hydronephrosis in the kidney without directly assessing the state of the ureter and bladder:
●Grade 0 – Normal examination with no dilation of the renal pelvis
●Grade I – Mild dilation of the renal pelvis only (image 2)
●Grade II – Moderate dilation of the renal pelvis including a few calyces
●Grade III – Dilation of the renal pelvis with visualization of all the calyces, which are uniformly dilated, and normal renal parenchyma
●Grade IV – Similar appearance of the renal pelvis and calyces as grade III, plus thinning of the renal parenchyma (image 3)
Urinary tract dilation (UTD) classification system — The UTD classification developed by a panel of radiologists, nephrologists, and urologists is stratified based on GA, whether the detection is antenatal or postnatal, and the following six ultrasound findings  (figure 1). Severity of the ultrasound findings is donated by a numerical system (1 through 3, with 3 being the most severe grade) with grading based on the most severe finding.
●Anterior and posterior RPD
●Renal parenchymal thickness
●Renal parenchymal appearance
The UTD system requires more extensive modifiers to determine the level of urinary tract obstruction than the SFU system. For example, when communicating that the patient has SFU grade IV UPJ hydronephrosis or SFU grade IV UVJ-type hydronephrosis, there is a clear documentation of the severity and etiology of the urologic anomaly. In contrast, UTD grade 3 requires numerous additional modifiers to communicate the exact diagnosis and severity.
Several observational studies have studied the utility of UTD.
●In a study that compared SFU and UTD grading systems, the two methods had similar prognostic ability to predict resolution of prenatal hydronephrosis (defined as RPD <10 mm) .
●A study at four institutions reported fair to moderate interobserver agreement using the UTD grading system, but agreement was higher for the UTD compared with the SFU classification .
●A prospective study of 447 infants with isolated antenatal hydronephrosis reported that the UTD classification system accurately predicted the risk of chronic kidney disease long-term at a median follow-up of 9 years .
EPIDEMIOLOGY — The reported incidence of fetal hydronephrosis ranges from 0.6 to 4.5 percent of pregnancies. Differences in reported data may be due to different criteria used to define the disorder and the level of attention to the urinary system by the ultrasonographer [9,12,24-28]. Hydronephrosis occurs approximately twice as often in males as in females. It is bilateral in 20 to 40 percent of cases .
●In a meta-analysis that included 17 studies, fetal hydronephrosis was identified in 1678 fetuses of 104,572 women (1.6 percent) . However, criteria for the diagnosis of hydronephrosis differed among the included studies.
●In one prospective Belgian study of 5643 unselected women, mild fetal hydronephrosis, defined as renal pelvic diameter ≥4 mm, was detected by ultrasonography during the second trimester in 4.5 percent of pregnancies .
Transient hydronephrosis — Transient hydronephrosis by definition is prenatal dilation (as defined above) of the renal pelvis that resolves over time and is not clinically significant. Transient hydronephrosis is the most common cause of fetal hydronephrosis with a reported range of 41 to 88 percent of cases . It is thought to be related to a transient narrowing of the ureteropelvic junction early in development that resolves or remains stable without clinical implications as the fetus matures. Mild hydronephrosis with renal pelvic diameter (RPD) <6 mm in the second trimester is usually associated with transient hydronephrosis.
Congenital anomalies of the kidney and urinary tract (CAKUT) — Congenital anomalies of the kidney and urinary tract (CAKUT) are a common cause of fetal hydronephrosis that includes both upper/lower urinary tract obstructive (eg, ureteropelvic junction obstruction [UPJO]) and nonobstructive processes (eg, vesicoureteral reflux [VUR]). It is important to identify these conditions soon after birth as they are potential risk factors for urinary tract infection and chronic kidney disease. (See "Overview of congenital anomalies of the kidney and urinary tract (CAKUT)", section on 'Association with end-stage kidney disease (ESKD)'.)
In a meta-analysis of 1678 infants diagnosed with fetal hydronephrosis, postnatal evaluation identified congenital anomalies of the kidney and urinary tract (CAKUT) as the underlying cause of fetal hydronephrosis in one-third of the patients .
Causes of fetal hydronephrosis due to CAKUT include:
●UPJO (figure 2) was the most common diagnosis and increased in frequency with the severity of hydronephrosis. (See "Congenital ureteropelvic junction obstruction".)
●VUR was the second most common diagnosis. Moderate to severe reflux (grades III through V) is associated with a greater degree of renal pelvic dilation (renal pelvic diameter [RPD] >10 mm) both in utero and postnatally (figure 3) . (See "Clinical presentation, diagnosis, and course of primary vesicoureteral reflux".)
●Less common causes include:
•Megaureter (figure 4) (see "Primary megaureter in infants and children")
•Multicystic dysplastic kidney (see "Renal cystic diseases in children", section on 'Multicystic dysplastic kidney' and "Prenatal sonographic diagnosis of cystic renal disease", section on 'Multicystic dysplastic kidney')
•Ureterocele (image 4 and figure 5) (see "Ureterocele")
•Posterior urethral valves (PUV) (image 5) (see "Clinical presentation and diagnosis of posterior urethral valves")
•Ectopic ureter (see "Ectopic ureter")
•Prune-belly syndrome (see "Prune-belly syndrome")
•Duplex collecting system (image 6)
Genetic and malformation syndromes
Down syndrome — Mild hydronephrosis is a common finding in fetuses with Down syndrome. Studies that defined hydronephrosis as RPD ≥4 mm in the second trimester demonstrated hydronephrosis is greater in Down syndrome compared with normal control fetuses (18 versus 0 to 3 percent, respectively) [27,31,32]. If additional anomalies are noted, diagnostic testing for chromosomal abnormality (eg, Down syndrome) rather than a screening test is appropriate. Amniocentesis to obtain amniocytes for fetal karyotype or microarray will provide a definitive genetic diagnosis. (See "Down syndrome: Overview of prenatal screening", section on 'Diagnostic testing'.)
Other anomalies — Other nonrenal/urinary tract congenital abnormalities are often associated with fetal hydronephrosis . Hydronephrosis has been reported as part of a multiple malformation syndrome in more than 60 genetic and sporadic malformation syndromes [34,35]. If other fetal structural abnormalities are identified, the fetus is at risk of having a chromosomal abnormality . In this setting, a diagnostic test such as amniocentesis for fetal karyotype or microarray may provide a definitive genetic diagnosis.
PRENATAL KIDNEY AND UROLOGIC ULTRASOUND EXAMINATION — Detection of fetal hydronephrosis by ultrasound usually occurs in the second trimester with a renal pelvic diameter (RPD) cutoff of ≥4 mm when routine prenatal ultrasonography is performed. Mild hydronephrosis (RPD 4 to 10 mm or Society of Fetal Urology [SFU] grade I or II) can be associated with Down syndrome or other chromosome anomalies. More severe dilation increases the risk of congenital anomalies of the kidney and urinary tract (CAKUT). (See 'Congenital anomalies of the kidney and urinary tract (CAKUT)' above and 'Genetic and malformation syndromes' above.)
During the ultrasound examination, the appearance of the fetal renal system can vary in both normal fetuses without hydronephrosis and in those with hydronephrosis. Therefore, serial measurements should be taken during each examination .
If fetal hydronephrosis is detected, the following parameters need to be evaluated by ultrasonography as they may be helpful in determining the cause of hydronephrosis and also identify risk factors for CAKUT. Risk factors associated with CAKUT include increasing severity of hydronephrosis, bilateral hydronephrosis, evidence of post-bladder obstruction (dilated ureter, increase bladder wall thickness), renal parenchymal thinning or echogenicity, and decreased amount of amniotic fluid.
●Severity and persistence of hydronephrosis (image 2 and image 3) – The likelihood of CAKUT increases with the severity of RPD [3,9,37]. Repeat ultrasound performed during the third trimester is more informative in predicting neonatal CAKUT, which may require surgical intervention as illustrated by the following:
•In a meta-analysis of 1678 infants from 17 studies, the following incidences of renal/urinary tract anomalies (eg, ureteropelvic junction obstruction [UPJ], vesicoureteral reflux [VUR] or posterior urethral valves [PUV]) were noted based on fetal RPD measurements :
-Mild hydronephrosis (≤7 mm in the second trimester and/or ≤9 mm in the third trimester) – 12 percent
-Moderate hydronephrosis (7 to 10 mm in the second trimester and/or 9 to 15 mm in the third trimester) – 45 percent
-Severe hydronephrosis (>10 mm in the second trimester and/or >15 mm in the third trimester) (image 3 and image 7) – 88 percent
•In a subsequently published review of single-center studies, prenatal resolution was noted in one-quarter of the 1034 cases . At last follow-up (mean age 20.6 months), postnatal hydronephrosis persisted in 10, 25, and 72 percent of patients with mild, moderate, and severe fetal hydronephrosis, respectively. The incidence of surgical intervention based on the severity of fetal hydronephrosis:
-Mild hydronephrosis – 10 percent, 26 of 267 cases including VUR (n = 14), UPJO (n = 7), PUV (n = 2), and 1 case each of cloaca, neurogenic bladder, and ectopic ureterocele.
-Moderate hydronephrosis – 24 percent, 11 of 43 cases including VUR (n = 3), UPJO (n = 5), PUV (n = 1), and unknown in 2 cases.
-Severe hydronephrosis – 63 percent, 13 of 19 cases including VUR (n = 2), UPJO (n = 8), and 1 case each of ureterovesical obstruction, prune-belly syndrome, and unknown.
•In a prospective French single-center study of cases with severe prenatal hydronephrosis (defined as RPD >10 mm on first fetal examination), 33 of 70 (47 percent) patients had surgical intervention at a median age of five months 
●Unilateral versus bilateral involvement – Bilateral involvement increases the risk of a significant renal abnormality and the risk of impaired postnatal renal function, primarily due to post-bladder obstructive uropathy such as PUV .
●Ureter – Dilation of the ureter can be consistent with vesicoureteral reflux (VUR) or obstructive uropathy distal to the ureteropelvic junction (eg, ureterocele (image 4 and figure 5), megaureter (figure 4), or posterior urethral valves [PUV] (image 7)). (See "Congenital ureteropelvic junction obstruction" and "Primary megaureter in infants and children" and "Ectopic ureter" and "Clinical presentation, diagnosis, and course of primary vesicoureteral reflux".)
●Renal parenchyma – Thinning of the parenchyma and/or cortical cysts indicate injury or impaired development of the renal cortex. An echogenic renal cortex may indicate abnormal renal parenchymal development (dysplasia), which may be associated with VUR or obstructive uropathy. (See "Overview of congenital anomalies of the kidney and urinary tract (CAKUT)", section on 'Renal dysplasia and hypodysplasia'.)
●Bladder – Abnormalities of the bladder, such as increased thickness and trabeculation of the bladder wall, are findings associated with obstructive uropathy distal to the bladder (eg, PUV) . In addition, dilation of the proximal urethra (keyhole sign) may indicate PUV in male fetuses with a thickened bladder wall and hydronephrosis (image 7). Bladder enlargement can be defined as a sagittal diameter (measured in mm) of 2 plus the gestational age (GA) (weeks). So, for a fetus that is 24 weeks GA, bladder enlargement would be defined as a sagittal diameter ≥26 mm (ie, 2 plus 24) [40,41]. (See "Clinical presentation and diagnosis of posterior urethral valves".)
Ureteroceles may be visualized in the bladder and can cause bladder outlet obstruction if they insert ectopically within the urethra. (See "Ureterocele".)
●Amniotic fluid volume – Oligohydramnios is consistent with impaired renal function resulting in a decreased production of fetal urine (amniotic fluid). It is a consistent feature of severe renal disease, affecting either both kidneys or a solitary kidney. (See "Assessment of amniotic fluid volume" and "Oligohydramnios: Etiology, diagnosis, and management in singleton gestations", section on 'Clinical manifestations and diagnosis'.)
●Presence of urinoma or urinary ascites:
•Urinoma is a fluid mass formed by extravasated urine encapsulated in the perirenal fascia. Urinomas are secondary to urinary obstruction such as PUV or ureteropelvic junction obstruction. Although rare, the presence of a fetal urinoma is associated with a nonfunctional dysplastic ipsilateral kidney in 80 percent of cases .
•Urinary ascites can be secondary to spontaneous or iatrogenic rupture of the bladder and the renal calices due to lower obstruction and increased pressure or, rarely, due to neurogenic bladder.
Goal — The majority of cases of fetal hydronephrosis are not clinically significant, and therefore, excessive concern may lead to unnecessary testing of the fetus and newborn infant, and anxiety for parents/caregivers and health care providers. The goal of prenatal management is to detect those cases of fetal hydronephrosis that may adversely affect the health of the infant and require antenatal and postnatal evaluation and may benefit postnatally with a timely referral to a pediatric urologist and possible intervention to minimize adverse outcomes, while limiting testing in those cases that are due to a benign, transient condition.
Who should be evaluated — In our practice, evaluation is performed for all fetuses with an antenatal ultrasound finding of a renal pelvic diameter (RPD) >4 mm between 18 and 22 weeks gestation and in the third trimester, RPD >10 mm or a Society of Fetal Urology (SFU) grade III or above. We believe these cutoff points provide a reasonable threshold to detect most fetuses at risk for congenital anomalies of the kidney and urinary tract (CAKUT) and avoid unnecessary testing in the majority of cases with benign transient hydronephrosis  (algorithm 1). (See 'Definition and grading' above and 'Prenatal kidney and urologic ultrasound examination' above.)
Evaluation and management based on initial ultrasound findings — The management approach is dependent on the initial ultrasound findings including the presence and nature of associated genitourinary and extrarenal anomalies, severity of hydronephrosis, unilateral versus bilateral involvement, and the amniotic fluid volume. Management decision making is shared with a collaborative maternal-fetal team, consisting of a high-risk obstetrician, fetal ultrasonographer, and pediatric urologist, and the parents/caregivers of the fetus. We provide parents/caregivers with an explanation of all the initial ultrasound findings and their potential significance, the next steps of assessment, and if needed, prenatal and postnatal management options. In particular, if the fetal/neonatal prognosis is poor (ie, severe bilateral hydronephrosis, oligohydramnios, and evidence of thin or poorly developed renal parenchyma), legal termination, if possible, can be offered. (algorithm 1).
●Unilateral hydronephrosis – For fetuses with unilateral hydronephrosis defined as a renal pelvic diameter (RPD) >4 mm in the second trimester, a follow-up ultrasound scan in the third trimester (32 to 34 weeks of gestation) is performed. (See 'Congenital anomalies of the kidney and urinary tract (CAKUT)' above and "Fetal hydronephrosis: Postnatal management".)
•Those with resolution (RPD <10 mm in the third trimester) have a low risk of clinically significant pathology and do not need further antenatal or postnatal evaluation [43,44].
•Those with persistent hydronephrosis RPD >10 mm in the third trimester require postnatal evaluation. In one retrospective review, surgery was performed in about one-quarter of affected cases .
Of note, some experts in the field do not repeat a third trimester ultrasound for fetuses with an RPD between 4 and 7 mm in the second trimester but repeat a postnatal ultrasound at three to four weeks.
●Bilateral hydronephrosis or an affected solitary kidney – Fetuses with bilateral hydronephrosis or an affected solitary kidney >4 mm and normal amniotic fluid volume have serial ultrasound examinations every two to three weeks after diagnosis to evaluate for progression of dilation and amniotic fluid volume. Follow-up examinations are determined according to the serial ultrasound results.
●Other urologic findings – Ureteral dilation, a thickened bladder, and/or presence of a ureterocele are findings that may be associated with lower tract urinary obstruction . Although, measurement of biochemical markers in amniotic fluid, which is predominantly composed of fetal urine can be used to assess fetal renal function, these tests are not clinically accurate enough to fully predict postnatal renal function. These tests are as discussed separately. (See "Evaluation of congenital anomalies of the kidney and urinary tract (CAKUT)", section on 'Amniotic fluid' and 'Fetal surgery' below.)
However, because there are no other predictive tools, in our center we will perform bladder taps in some rare cases of post-bladder obstruction as part of the evaluation to predict renal prognosis for further intervention, including fetal intervention (eg, amnioinfusion and vesicoamniotic shunts). (See 'Fetal surgery' below.)
●Oligohydramnios – Management of oligohydramnios (ie, abnormally low amniotic fluid volume) is dependent on the GA of the fetus and is associated with a poor prognosis. The management of oligohydramnios is discussed separately. (See "Oligohydramnios: Etiology, diagnosis, and management in singleton gestations".)
●Assessment for other non-genitourinary anomalies and genetic syndromes – The finding of mild hydronephrosis should prompt a detailed assessment of fetal anatomy to identify other congenital anomalies. When pyelectasis is identified in an otherwise normal second trimester fetus and no previous maternal plasma or serum screening tests for trisomy 21 have been performed, the mother can be offered a cell-free DNA test to screen for trisomy 21. A normal result indicates a very low risk of an affected fetus and invasive diagnostic testing is typically avoided in these cases (see "Prenatal screening for common aneuploidies using cell-free DNA", section on 'Secondary screening'). A serum analyte test instead of a cell-free DNA test is a reasonable alternative for screening. (See "Down syndrome: Overview of prenatal screening", section on 'Second-trimester quadruple test'.)
Genetic counseling and testing are offered if additional fetal anomalies are detected, in women of advanced maternal age, and in women with abnormal maternal serum screening tests during the first or second trimester. (See "Sonographic findings associated with fetal aneuploidy", section on 'Trisomy 21 (Down syndrome)' and "Genetic testing".)
Fetal surgery — Although there have been several prospective and retrospective studies of antenatal surgery in fetuses with sonographic findings consistent with lower urinary tract obstruction, there is no good evidence that fetal intervention improves renal outcome or long-term patient survival [45-52]. These procedures increase the amount of amniotic fluid, thus potentially improving lung development and survival rate. However, there remains a high rate of fetal demise and chronic renal disease in the survivors, necessitating kidney replacement therapy in almost two-thirds of cases . If antenatal intervention is performed, it should only be performed in select centers with expertise after careful counseling of parents/caregivers on the risk and benefits of the procedure and available information on the neonatal survival and renal outcome. In very rare cases with severe bilateral hydronephrosis due to post-bladder obstruction from PUV, we will consider serial fetal bladder taps for amniotic fluid analysis and if the prognosis appears favorable, perform a vesicoamniotic shunt.
A randomized trial that aimed to determine if percutaneous vesicoamniotic shunting improves survival and renal outcome compared with conservative management could not be completed due to inability to recruit a sufficient number of cases, only enrolling 31 of the 150 planned pregnancies over a four-year period .
●In an intention-to-treat analysis of the 31 enrolled women with singleton pregnancies (16 allocated to vesicoamniotic shunting versus 15 to conservative management), there was a trend for improved survival with shunting at 28 days (relative risk [RR] 1.88, 95% CI 0.71-4.96), 1 year (RR 2.19, 95% CI 0.69-6.94), and 2 years of age (RR 2.19, 95% CI 0.69-6.94). Outcomes for the 16 pregnancies allocated to vesicoamniotic shunting were 12 live births, 1 intrauterine death, and 3 terminated pregnancies; for the 15 pregnancies managed conservatively, there were 12 live births, 1 intrauterine death, and 2 terminated pregnancies. A significant number of cases did not receive the allocated intervention; 3 of the 16 pregnancies allocated to shunting were managed conservatively, and conversely, 2 cases randomized to conservative management had shunting.
●Due to large crossover, an analysis based on treatment was performed, which showed a larger beneficial effect of shunting on survival at 28 days (RR 3.2, 95% CI 1.06-9.62), 1 year (RR 4.27, 95% CI 1.07-16.96), and 2 years of age (RR 4.27, 95% CI 1.07-16.96). Only two offspring who survived to two years of age had normal renal function; both were in the shunt group. Seven complications occurred in six fetuses who were in the shunt group and included spontaneous ruptured membrane, shunt blockage, and dislodgement, which resulted in four pregnancy losses.
●Postnatal diagnoses were made in all 12 perinatal survivors and in 3 neonates who died and included 9 cases of PUV (5 in the shunt group and 4 in the conservative management group), 5 cases of urethral atresia (4 in the shunt group and 1 in the conservative management group), and 1 case of urethral syrinx that caused obstruction (in the conservative management group). Diagnostic information was also available in three cases lost during pregnancy: two with urethral atresia and one with PUV.
The results from this study need to be interpreted carefully as four of the fetuses who underwent shunting did not survive, and the trial was limited by significantly too few cases and protocol deviation in a substantial number of the enrolled cases. Nevertheless, survival outcome in this study is consistent with the findings of observational studies that suggest the chance of postnatal normal renal function is low irrespective of whether or not vesicoamniotic shunting is performed.
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●Basics topics (see "Patient education: Prenatal hydronephrosis (The Basics)")
SUMMARY AND RECOMMENDATIONS
●Epidemiology – Fetal hydronephrosis (fetal renal pelvic dilation) is a common finding on antenatal ultrasonography occurring in 0.6 to 4.5 percent of pregnancies. It can be detected as early as the 12th week of gestation. (See 'Epidemiology' above.)
•Transient hydronephrosis is the most common cause of fetal hydronephrosis. Mild hydronephrosis with renal pelvic diameter (RPD) <6 mm in the second trimester is usually associated with transient hydronephrosis. (See 'Transient hydronephrosis' above.)
•Congenital anomalies of the kidney and urinary tract (CAKUT) are important causes of fetal hydronephrosis that should be diagnosed soon after birth as they are associated with chronic kidney disease and urinary tract infection. The two most common CAKUT are ureteropelvic junction obstruction (UPJ) and vesicoureteral reflux (VUR). (See 'Congenital anomalies of the kidney and urinary tract (CAKUT)' above.)
•In fetuses with Down syndrome and other genetic and malformation syndromes, mild hydronephrosis is a common finding. (See 'Genetic and malformation syndromes' above.)
●Definition and grading – Although there are many different scoring systems for fetal hydronephrosis, renal pelvic diameter (RPD) is the most commonly used scoring system based on prenatal ultrasound assessment. Mild hydronephrosis is defined as an RPD ≥4 to 10 mm and more severe hydronephrosis as an RPD >10 mm measured during the second trimester. Fetuses with RPD >15 mm during the third trimester are at the greatest risk for significant renal disease (ie, congenital anomalies of the kidney [CAKUT]). In our practice we use a cutoff of >10 mm in the third trimester to determine the need for postnatal evaluation and intervention (algorithm 1). (See 'Prenatal kidney and urologic ultrasound examination' above and 'Definition and grading' above.)
●Prenatal renal ultrasound – In fetuses with increased RPD, ultrasonographic imaging should assess the severity of hydronephrosis, bilateral or unilateral involvement, renal parenchyma, amount of amniotic fluid, and evidence of ureter or bladder abnormalities. Risk factors associated with CAKUT include increasing severity of hydronephrosis, bilateral hydronephrosis, evidence of post-bladder obstruction (dilated ureter, increase bladder wall thickness), renal parenchymal thinning or echogenicity, and decreased amount of amniotic fluid. (See 'Prenatal kidney and urologic ultrasound examination' above.)
•Goal – The goal of prenatal management is to detect those cases of fetal hydronephrosis that may adversely affect the health of the infant and require further evaluation, timely referral to a pediatric urologist if required, and possible intervention to minimize adverse outcomes, while limiting testing in those cases that are due to a benign, transient condition. (See 'Goal' above.)
•Who should be evaluated – In our practice, evaluation is performed for all fetuses with a prenatal ultrasound finding of a RPD >4 mm between 18 and 22 weeks gestation and in the third trimester, RPD >10 mm. (See 'Who should be evaluated' above.)
•Management approach – The management approach is dependent on the initial ultrasound findings including the presence and nature of associated genitourinary and extrarenal anomalies, severity of hydronephrosis, unilateral versus bilateral involvement, and the amniotic fluid volume. Findings associated with CAKUT include increased severity of hydronephrosis, bilateral hydronephrosis, evidence of post-bladder obstruction (dilated ureter, increase bladder wall thickness), renal parenchymal thinning or echogenicity, and oligohydramnios (decreased amount of amniotic fluid). (See 'Evaluation and management based on initial ultrasound findings' above.)
In general, fetuses with second trimester hydronephrosis (RPD >4 mm) should undergo repeat testing in the third trimester to assess progression and identify those who will benefit most from postnatal testing (algorithm 1). We typically perform a repeat examination 2 to 3 weeks later in fetuses with bilateral involvement (or an affected solitary kidney) and in those with unilateral involvement at 32 to 34 weeks gestation.
•Fetal surgery – Data are limited on whether percutaneous vesicoamniotic shunting compared with conservative observation in fetuses with lower urinary tract obstruction improves survival and renal outcome. As a result, we do not suggest that percutaneous vesicoamniotic shunting be routinely performed in fetuses with lower urinary tract obstruction versus observation and monitoring (Grade 2C). If antenatal intervention is performed, it should only be performed in select centers with expertise after careful counseling of parents/caregivers on the risk and benefits of the procedure and available information on the neonatal survival and renal outcome. (See 'Fetal surgery' above.)
ACKNOWLEDGMENT — The editorial staff at UpToDate would like to acknowledge Tulin Ozcan, MD, who contributed to an earlier version of this topic review.
2 : Variations in management of mild prenatal hydronephrosis among maternal-fetal medicine obstetricians, and pediatric urologists and radiologists.
3 : The outcome of antenatal ultrasound diagnosed anomalies of the kidney and urinary tract in a large Danish birth cohort.
7 : Mild pyelectasis: evaluating the relationship between gestational age and renal pelvic anterior-posterior diameter.
10 : Prognostic Value of Ultrasound Grading Systems in Prenatally Diagnosed Unilateral Urinary Tract Dilatation.
12 : Results of systematic screening for minor degrees of fetal renal pelvis dilatation in an unselected population.
16 : Renal pyelectasis in pregnancy: correlative evaluation of fetal and maternal collecting systems.
17 : Hydronephrosis index: a better physiological reference in antenatal ultrasound for assessment of fetal hydronephrosis.
18 : The Society for Fetal Urology consensus statement on the evaluation and management of antenatal hydronephrosis.
19 : Ultrasound grading of hydronephrosis: introduction to the system used by the Society for Fetal Urology.
20 : Multidisciplinary consensus on the classification of prenatal and postnatal urinary tract dilation (UTD classification system).
21 : Society for Fetal Urology Classification vs Urinary Tract Dilation Grading System for Prognostication in Prenatal Hydronephrosis: A Time to Resolution Analysis.
22 : Interobserver and Intra-Observer Reliability of the Urinary Tract Dilation Classification System in Neonates: A Multicenter Study.
23 : Evaluation of Urinary Tract Dilation Classification System for Prediction of Long-Term Outcomes in Isolated Antenatal Hydronephrosis: A Cohort Study.
25 : Antenatal ultrasonography to detect fetal renal abnormalities: a prospective screening programme.
27 : The association between fetal pyelectasis on second trimester ultrasound scan and aneuploidy among 25,586 low risk unselected women.
28 : Vesico-ureteral reflux in children with prenatally detected hydronephrosis: a systematic review.
35 : Congenital renal tract anomalies: outcome and follow-up of 402 cases detected antenatally between 1986 and 2001.
36 : Striking findings concerning the variability in the measurement of the fetal renal collecting system.
38 : Postnatal longitudinal evaluation of children diagnosed with prenatal hydronephrosis: insights in natural history and referral pattern.
39 : Predictive accuracy of prenatal ultrasound findings for lower urinary tract obstruction: A systematic review and Bayesian meta-analysis.
40 : Predictive accuracy of prenatal ultrasound findings for lower urinary tract obstruction: A systematic review and Bayesian meta-analysis.
41 : Fetal bladder sagittal length: a simple monitor to assess normal and enlarged fetal bladder size, and forecast clinical outcome.
43 : Prenatal anteroposterior pelvic diameter cutoffs for postnatal referral for isolated pyelectasis and hydronephrosis: more is not always better.
44 : Prenatal diagnosis and management of mild fetal pyelectasis: implications for neonatal outcome and follow-up.
46 : Effectiveness of vesico-amniotic shunt in fetuses with congenital lower urinary tract obstruction: An updated systematic review and meta-analysis.
48 : Fetal intervention in obstructive uropathy: prognostic indicators and efficacy of intervention.
50 : Systematic review of the effectiveness of antenatal intervention for the treatment of congenital lower urinary tract obstruction.
51 : Effectiveness of fetal cystoscopy as a diagnostic and therapeutic intervention for lower urinary tract obstruction: a systematic review.
52 : Fetal intervention for severe lower urinary tract obstruction: a multicenter case-control study comparing fetal cystoscopy with vesicoamniotic shunting.
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