INTRODUCTION — Fetal hydronephrosis (dilatation of the renal pelvis collecting system) is a common, readily diagnosed finding on antenatal ultrasound examination occuring in 0.5 to 1 percent of pregnancies [1]. It can be detected as early as the 12th to 14th week of gestation [2].

Although renal pelvic dilatation is a transient, physiologic state in most cases, urinary tract obstruction and vesicoureteral reflux (VUR) can occasionally be causal. These conditions can prevent normal renal development and/or cause renal injury. However, the majority of cases of antenatal hydronephrosis are not clinically significant leading to unnecessary testing of the newborn baby and anxiety for patients and healthcare providers. The goal of prenatal management is to detect those cases of antenatal hydronephrosis that will impact the health of the infant and require antenatal and postnatal evaluation, timely referral to a pediatric urologist if required, and possible intervention to minimize adverse outcomes.

The definition, etiology, and prenatal issues of antenatal hydronephrosis are reviewed here. Postnatal evaluation of antenatal hydronephrosis and specific urologic conditions that may present as antenatal hydronephrosis are discussed separately. (See "Postnatal management of antenatal hydronephrosis" and "Ureteropelvic junction obstruction, congenital megaureter, ureterocele, and ectopic ureter".)

DEFINITION AND GRADING — Several systems have been developed to diagnosis and grade the severity of antenatal hydronephrosis [3]. In general, the likelihood of having a significant renal anomaly correlates with the severity of hydronephrosis. In addition, the underlying diagnosis often determines the extent of dilatation. As examples, ureteropelvic junction (UPJ) obstruction is a frequent cause of severe hydronephrosis. VUR is an exception to this rule and is often associated with milder dilatation.

Scoring systems differ based upon the fetal ultrasound criteria used, as follows:

• Society of Fetal Urology (SFU) criteria — SFU criteria for the diagnosis and grading of antenatal hydronephrosis are based upon the degree of pelvic dilation, presence of caliceal dilation, and the presence and severity of parenchymal thinning or atrophy (figure 1).
• Renal pelvic diameter — Measurement of the maximum anteroposterior diameter of the renal pelvis (APPD), also referred to as renal pelvic diameter (RPD), is the most generally accepted method to define antenatal hydronephrosis [4].

Several studies have established normative data for fetal renal size based upon gestational age [5-7]. However, despite these data, there remains a lack of consensus on the threshold RPD that defines clinically significant antenatal hydronephrosis [5-7]. (See 'Renal pelvic diameter (RPD)' below.)

• Changes in the degree of hydronephrosis — The relative change in hydronephrosis compared to subsequent prenatal ultrasound or the first postnatal ultrasound has been used in the clinical research setting [3]. However, testing its clinical applicability has been limited. As a result, relative changes in antenatal hydronephrosis are not generally used clinically.

Society of Fetal Urology (SFU) — The SFU developed criteria for the diagnosis and grading of antenatal hydronephrosis based upon the degree of pelvic dilatation, number of calyces seen, and the presence and severity of parenchymal atrophy (figure 1) [8].

• Grade 0 — Normal examination with no dilatation of the renal pelvis
• Grade 1 — Mild dilatation of the renal pelvis only (picture 1)
• Grade II — Moderate dilatation of the renal pelvis including a few calyces
• Grade III — Dilatation 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 (picture 2).

Renal pelvic diameter (RPD) — Although there is a lack of consensus on the threshold RPD that defines antenatal hydronephrosis, hydronephrosis is graded according to the RPD during the second and/or third trimester of pregnancy.

Mild renal pelvic dilatation, also referred to as pyelectasia, is defined by a RPD of ≥4 to 10 mm [4,6,9,10]. Although most cases of mild dilatation will resolve and not have a clinical impact on neonatal renal development, there are reports of persistent cases that require postnatal intervention [11-14].

In general, RPD >10 mm is associated with an increased risk of significant congenital anomalies of the kidney and urinary tract (CAKUT) [4,6,9,12]. Fetuses with RPD >15 mm during the third trimester are at the greatest risk for CAKUT [4,6,9,12].

EPIDEMIOLOGY — Hydronephrosis occurs approximately twice as often in males than in females. It is bilateral in 20 to 40 percent of cases [15]. The reported incidence of antenatal 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 [11,16-20].

• In a prospective study, antenatal hydronephrosis defined as RPD >5 mm was detected by detailed ultrasonography in the second trimester of pregnancy in 100 fetuses of 18,766 women (0.6 percent) cared for at antenatal clinics in Britain [16].
• In a prospective study, mild antenatal hydronephrosis defined as RPD ≥4 mm was detected by ultrasonography (with special attention to the urinary system) during the second trimester of pregnancy in 258 fetuses of 5643 unselected Belgium women (4.5 percent) [11].
• In a meta-analysis that included 17 studies, antenatal hydronephrosis was identified in 1678 fetuses of 104,572 women (1.6 percent) [12]. Criteria for the diagnosis of hydronephrosis differed among the included studies.

ETIOLOGY — Based upon postnatal evaluation, most cases of antenatal hydronephrosis are found to be transient (48 percent) or physiologic (15 percent) [4].

In a meta-analysis of 1678 infants diagnosed with antenatal hydronephrosis, postnatal evaluation identified a cause in 36 percent of patients, which were primarily renal/urinary tract abnormalities [12]. Ureteropelvic junction (UPJ) obstruction was the most common diagnosis and increased in frequency with the severity of hydronephrosis. In contrast, VUR, the second most common diagnosis, was not associated with the severity of hydronephrosis.

A second systematic review that included studies that evaluated infants with antenatal hydronephrosis for VUR by voiding cystourethogram reported 15 percent of patients had VUR and another 29 percent had CAKUT [20].

In a third review, causes of antenatal hydronephrosis and their relative frequency were determined as follows [4]:

• Transient — 48 percent
• Physiologic — 15 percent
• UPJ obstruction — 11 percent (see "Ureteropelvic junction obstruction, congenital megaureter, ureterocele, and ectopic ureter")
• VUR — 9 percent (see "Presentation, diagnosis, and clinical course of vesicoureteral reflux")
• Megaureter — 4 percent (see "Ureteropelvic junction obstruction, congenital megaureter, ureterocele, and ectopic ureter")

Multicystic dysplastic kidney — 2 percent (see "Renal cystic diseases in children" section on Multicystic dysplastic kidney.)

• Ureterocele — 2 percent (picture 3) (figure 2)
• Posterior urethral valves — 1 percent (see "Clinical presentation and diagnosis of posterior urethral valves".

Less common causes included ectopic ureter, prune belly, urachal cyst, duplex collecting system (picture 4), and urethral atresia.

Many of these disorders are discussed in detail separately. (See "Ureteropelvic junction obstruction, congenital megaureter, ureterocele, and ectopic ureter" and "Presentation, diagnosis, and clinical course of vesicoureteral reflux" and "Renal cystic diseases in children", section on Multicystic dysplastic kidney.)

PRENATAL ISSUES — Detection of antenatal hydronephrosis usually occurs in the second trimester with a RPD cutoff of ≥4 mm. Mild hydronephrosis (RPD of 4 to 10 mm or SFU grade 1 or 2) can be associated with Down syndrome and an increased risk of other chromosome anomalies. More severe dilatation increases the risk of renal/urinary tract disorders.

Down syndrome — Mild hydronephrosis is a common finding in fetuses with Down syndrome. Studies that defined hydronephrosis as RPD ≥4 mm demonstrated hydronephrosis is greater in Down syndrome compared to normal control fetuses (18 versus 0 to 3 percent, respectively) [19,21,22].

The finding of mild hydronephrosis requires a detailed assessment of fetal anatomy and review of clinical risk factors for Down syndrome. However, genetic amniocentesis is not warranted based solely on the isolated finding of hydronephrosis [21,23,24], because the incidence of Down syndrome remains low (<1 in 300 pregnancies) in fetuses with no maternal risk factors (eg, advanced age or abnormal maternal serum screen) or without any other sonographic features suggestive for Down syndrome [19,21,22]. (See "Sonographic findings associated with fetal aneuploidy", section on 'Trisomy 21 (Down syndrome)', and (see "Laboratory issues related to maternal serum screening for Down syndrome".

Congenital anomalies of the kidney and urinary tract (CAKUT)

Predictive factors — The likelihood that an infant postnatally will have a significant CAKUT increases with the severity of hydronephrosis [4,12,25].

This was illustrated in a meta-analysis of 1678 infants that demonstrated that the severity of antenatal hydronephrosis defined by RPD increased the risk of renal/urinary tract pathology, as follows [12].

• 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) (picture 2 and picture 5) — 88 percent

The risk of VUR was the same regardless of the degree of hydronephrosis. Although the risk of postnatal pathology was lower in patients with mild antenatal hydronephrosis, this analysis demonstrated that these patients still had a significant risk for postnatal pathology.

Similar results were seen in a subsequently published prospective study of 350 infants with RPD ≥7 mm in the third trimester from a single center [26]. If postnatal evaluation was limited to only patients with RPD greater ≥10 mm, 25 percent of those with pelviureteric junction and 50 percent with VUR would not have been identified.

Additional prenatal screening that gauges RPD changes may be useful in predicting postnatal outcome [27,28]. This was illustrated in a retrospective review of 280 infants who had two prenatal ultrasound examinations and a complete postnatal urological evaluation [27]. Pyelectasis was defined if RPD was between 5 and 10 mm on the first scan. The following is the total number of cases and number of cases requiring surgery based upon RPD changes on the second compared to the first prenatal ultrasound.

• Complete resolution with normal RPD on second ultrasound — 5 cases, of which 1 required surgery (2 percent)
• Reduced dilatation but not complete resolution — 97 cases, of which 3 required surgery (3 percent)
• No change — 86 case, of which 7 required surgery (8 percent)
• Increased dilatation (RPD > 10 mm on second scan) — 46 cases, of which 11 required surgery (24 percent)

Timing of prenatal ultrasonography is important in optimal detection of CAKUT. Ultrasonography performed before the 18 to 24th week of gestation can fail to detect significant disease [29,30]. In addition, studies performed in the third trimester have been suggested to be more helpful in predicting postnatal outcome than screening earlier in the pregnancy [29,31-33]. Thus, most authors agree that a repeat scan in the third trimester (28 to 34 weeks gestation) be performed to identify those fetuses potentially requiring postnatal intervention.

Despite numerous studies in neonates with antenatal hydronephrosis [7,11,12,28,34-36], there remains considerable debate on the accepted RPD threshold that predicts clinically significant disease in the neonate. Currently, no criteria have been identified that can identify all neonates with significant pathology and exclude those infants with transient or physiologic hydronephrosis. Thus, although a low threshold (eg, RPD >4 mm) in the second trimester will have a greater sensitivity of detecting infants with CAKUT, it would also include a large number of patients without renal disease who would undergo unnecessary testing. In our practice, these fetuses are reevaluated in the third trimester between 30 and 32 weeks to assess progression and select those who will benefit most from postnatal testing. At this third trimester examination, cutoff for further postnatal testing can be set between 7 and 10 mm with the understanding that a small number of obstructive disease and VUR cases may be missed as the cutoff is increased.

Patients should be given an explanation of all of the ultrasound findings and their potential significance (eg, transient insignificant finding, urinary tract obstruction, VUR, risk of renal insufficiency, association with Down syndrome). The absolute risk of any of these outcomes, however, depends upon several factors including the severity of the hydronephrosis, whether it is bilateral or unilateral, the amount of amniotic fluid volume, and the presence of other findings in the urinary system, associated anomalies, or maternal risk factors, and whether progression occurs. Prenatal and postnatal management options should also be explained. (See 'Management' below and "Postnatal management of antenatal hydronephrosis".)

Ultrasound examination — During the ultrasound examination, the appearance of the fetal renal system can vary in both normal fetuses and those with hydronephrosis. Therefore, this diagnosis should not be based upon a single measurement [37]. An increase of maternal hydration can also increase the RPD in both normal fetuses and those with hydronephrosis [38].

If fetal hydronephrosis is detected, the following parameters need to be evaluated by ultrasonography as they guide further need for evaluation and are helpful in determining the cause of hydronephrosis:

• Severity of hydronephrosis (picture 1 and picture 2). (See 'Renal pelvic diameter (RPD)' above.)

• Unilateral versus bilateral involvement. Bilateral involvement increases the risk of a significant abnormality and the risk of impaired postnatal renal function.
• Ureter — Dilatation of the ureter can be consistent with VUR or obstructive uropathy distal to the ureteropelvic junction [eg, ureterocoele (picture 3) (figure 2), megaureter, or posterior urethral valves (PUV)]. (See "Ureteropelvic junction obstruction, congenital megaureter, ureterocele, and ectopic ureter" and "Presentation, diagnosis, and clinical course of vesicoureteral reflux".)

• Renal parenchyma — Thinning of the parenchyma indicates injury or impaired development of the renal cortex. An echogenic renal cortex may indicate abnormal renal parenchymal development (dysplasia), which may be associated with vesicoureteral reflux.
• Bladder — Abnormalities of the bladder such as increased thickness and trabeculation of the bladder wall are consistent with obstructive uropathy distal to the bladder (eg, PUV). In addition, dilatation of the proximal urethra (key-hole sign) may indicate PUV in male fetuses patients with a thickened bladder wall and hydronephrosis (picture 5).
• Amniotic fluid — 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 both kidneys or a solitary kidney.

Other anomalies — Other non-renal/urinary tract congenital abnormalities are associated with antenatal hydronephrosis. In one case series of 104 fetuses with hydronephrosis, the three other major birth defects detected were ventricular septal defect in a patient with Down syndrome, microcephaly, and omphalocele [39]. Hydronephrosis also has been reported as part of a multiple malformation syndrome in more than 60 genetic and sporadic malformation syndromes [40]. In one case series, a chromosomal abnormality was detected in 7 of 81 fetuses that had karyotyping [41]. Of those, only two had isolated renal malformations.

Management — Decisions regarding evaluation and care are dependent on the presence and nature of associated renal and extrarenal anomalies, severity of hydronephrosis, unilateral versus bilateral involvement, gestational age, and the amniotic fluid volume.

• Fetal karyotype — In fetuses with mild hydronephrosis and normal amniotic fluid, a careful examination for genitourinary and extrarenal abnormalities should be performed. Clinical risk factors and ultrasound markers of Down syndrome should be evaluated. Assessment of the fetal karyotype should be offered if additional fetal anomalies are detected, in women of advanced maternal age, and women with abnormal maternal serum screening tests for Down syndrome. (See "Sonographic findings associated with fetal aneuploidy", section on 'Trisomy 21 (Down syndrome)',).
• Ultrasound follow-up — Fetuses with mild hydronephrosis should have a follow-up ultrasound scan in the third trimester (32 to 34 weeks of gestation). Those with resolution have a low risk of clinically significant pathology, and do not need further antenatal or postnatal evaluation.

By comparison, serial follow-up ultrasounds are indicated for fetuses with:

• - Moderate or severe hydronephrosis
• - Bilateral involvement
• - Progression and/or persistence of hydronephrosis
• - Oligohydramnios

The interval for follow-up examination and management are dictated by the initial findings and the amount of amniotic fluid.

• Although there have been case series of antenatal surgery in fetuses with severe hydronephrosis and oligohydramnios consistent with lower urinary tract obstruction [42-45], this intervention has not been shown to improve renal outcome. There remains a high rate of chronic renal disease in the survivors necessitating renal replacement therapy in almost two-thirds of the cases. These procedures may increase the amount of amniotic fluid, thus potentially improving lung development and survival rate. Antenatal surgery should only be performed in select centers with expertise. Currently, a clinical trial comparing conservative management without surgical intervention to vesicoamniotic shunting in singleton fetuses below 28 weeks gestation with isolated bladder outflow obstruction is recruiting sites and patients [46]. Results of this trial hopefully will answer whether surgical intervention improves clinical outcome.
• Severe oligohydramnios — Early delivery has been suggested for fetuses with severe oligohydramnios and documented lung maturation. Although there are no data documenting improved renal outcome with early delivery [41,45], early delivery may be indicated to reduce the risk of other adverse outcomes from oligohydramnios such as umbilical cord compression. (See "Oligohydramnios".)

• Normal amniotic fluid volume

• - Fetuses with unilateral involvement and normal amniotic fluid volume should have a repeat prenatal ultrasound examination at 32 to 34 weeks of gestation to evaluate and guide postnatal evaluation.
• - Fetuses with bilateral hydronephrosis and normal amniotic fluid volume have repeat ultrasound examination two to three weeks after diagnosis to evaluate for progression and amniotic fluid volume. Follow-up examinations are determined according to the serial ultrasound results.

The postnatal management of antenatal hydronephrosis is discussed separately. (See "Postnatal management of antenatal hydronephrosis".)

SUMMARY AND RECOMMENDATIONS

• Fetal hydronephrosis (fetal renal pelvic dilatation) is a common, readily diagnosed 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.)

• Although antenatal hydronephrosis is most often transient or clinically insignificant, urinary tract obstruction or vesicoureteral reflux (VUR) are important causes that should be diagnosed soon after birth because they can result in renal impairment or cause further renal damage. (See 'Etiology' above.)

• Although there is lack of consensus, several grading systems have been developed to describe the severity of antenatal hydronephrosis. We use the renal pelvic diameter (RPD) to grade severity with mild hydronephrosis defined as a RPD of ≥4 to 10 mm and more severe hydronephrosis as RPD >10 mm. Fetuses with RPD >15 mm during the third trimester are at the greatest risk for significant renal disease. VUR is also the second most common cause for antenatal hydronephrosis and is not associated with the severity of hydronephrosis. (See 'Definition and grading' above.)

• Because fetuses with hydronephrosis are at increased risk for Down syndrome and congenital anomalies including renal and urinary tract abnormalities, a comprehensive prenatal evaluation is required for these conditions. (See 'Prenatal issues' above.)

• The risk of renal and urinary tract abnormality increases with the severity of hydronephrosis, persistence of hydronephrosis into the third trimester, bilateral involvement, and the presence of oligohydramnios. (See 'Predictive factors' above.)
• Repeat antenatal ultrasound examinations are performed to help guide management decisions. The timing is dependent on findings on the initial examination. Fetuses with second trimester hydronephrosis should undergo repeat testing in the third trimester to assess progression and select those who will benefit most from postnatal testing. In general, we perform a repeat examination two to three weeks later in fetuses with bilateral involvement (or an affected solitary kidney) and at 32 to 34 weeks gestation in those with unilateral involvement. (See 'Management' above.)