Ultrasound Diagnosis of Malformations of the Fetal Kidneys and Urinary System

Abstract

Malformations of the fetal kidneys and urinary system are common and easily visualized and diagnosed on ultrasound. This article presents the typical sonographic findings of these abnormalities during the various stages of pregnancy. Because malformations of the urogenital tract often have an association with genetic diseases/ciliopathies, these are also discussed. To complete the article, we provide a brief overview of the normal anatomy of the kidneys and urinary system. The normal anatomy and malformations of the genitalia will not be discussed in this article due to their complexity.

Malformations of the fetal kidneys and urinary system are common and easily visualized and diagnosed on ultrasound. This article presents the typical sonographic findings of these abnormalities during the various stages of pregnancy. Because malformations of the urogenital tract often have an association with genetic diseases/ciliopathies, these are also discussed. To complete the article, we provide a brief overview of the normal anatomy of the kidneys and urinary system during the various stages of pregnancy.

Normal Anatomy of the Fetal Kidneys and Urinary System on Ultrasound

The description of the normal anatomy of the fetal urogenital tract follows the points during pregnancy at which an ultrasound examination is usually performed: in the first trimester (gestational week 11–13 + 5 days), the screening examination (GW 22), and in the third trimester (GW 28–32). In principle, both the fetal kidneys and the filled urinary bladder can be visualized at each of these points in time [1] [2] [3] [4] [5] [6] [7] [8].

Normal Anatomy of the Kidneys and Urinary System in the First Trimester

During this ultrasound examination it is usually possible to visualize the fluid-filled urinary bladder. This scan is performed either on a longitudinal plane (sagittal or coronal) or, more usually, by taking a transverse view between the two umbilical arteries.

Using a high-resolution ultrasound device and from GW 12 onwards, the kidneys can be visualized as relatively echogenic structures lateral to the spine with a hypoechoic renal pelvis (transverse view, sagittal longitudinal view, and coronal longitudinal view) ([Fig. 1]). During the first trimester, the amniotic fluid depends primarily on the mother and is therefore not an indicator of fetal renal function [4] [5] [6] ([Fig. 1]).

Fig. 1 Illustration of a normal urogenital tract in the first trimester (GW 13 + 2): a Frontal view of both kidneys. b Transverse view of both kidneys. It is easier to make out the area near the transducer. c Illustration of the filled urinary bladder in transverse view. d Illustration of the filled urinary bladder in transverse view between the umbilical arteries shown in color. e Longitudinal view of the filled urinary bladder.

Normal Anatomy of the Kidneys and Urinary System in the Second Trimester

At this point, the fetal kidneys with the renal pelvis and pelvicalyceal system can be visualized in all three planes (transverse, longitudinal sagittal). At this time the renal arteries can also be visualized using color Doppler ultrasound. However, this does not conclusively prove that kidneys are present, as in the absence of kidneys the image usually focuses on the adrenal arteries.

Under normal conditions, the ureter cannot be visualized.

The urinary bladder can be visualized both in the longitudinal plane (sagittal, coronal) or, even better, in the transverse plane between the two umbilical arteries. A normal urethra can sometimes be visualized using a very high-resolution probe, but this is not standard ([Fig. 2]).

Fig. 2 Illustration of the normal urogenital tract in the 2nd trimester (GW 22 + 2): a Frontal view of the two kidneys lateral to the aorta. b Parasagittal longitudinal view showing the kidney on the psoas muscle. c Transverse view in the dorsoanterior position, showing both kidneys lateral to the spine. d Transverse view (back is right), also showing both kidneys. e Illustration of the urinary bladder in transverse view in B-mode. f Illustration of the filled urinary bladder between the umbilical arteries.

The amniotic fluid gives an indication of kidney function from about the 16^th to the 18^th gestational week, and certainly after GW 20. As a rule, the urinary bladder fills during an ultrasound examination [1] [8] [9].

Normal Anatomy of the Kidneys and Urinary System in the Third Trimester

The basic presentation of the kidneys in the third trimester is similar to that of the second trimester. Because more fluid is produced, the renal pelvis and pelvicalyceal system often appear more fluid-filled. They often appear circular and hypoechoic; it is important not to confuse them with renal cysts ([Fig. 3]).

Fig. 3 Illustration of normal kidneys in the 3 rd trimester (GW 30 + 4): a Longitudinal view. The normal renal pelvic-calyceal system can be easily visualized. b Transverse view. Due to acoustic shadowing from the spine, it is usually only possible to see the kidney that is close to the transducer (as shown in the image). c Frontal view of both kidneys.

As a rule, a normal ureter cannot be visualized. The urinary bladder is best visualized in the transverse view between the umbilical arteries. The amniotic fluid is predominantly an indicator of the fetal renal function ([Fig. 3]).

In both the second and third trimester, fetal urination can sometimes be detected in the amniotic fluid [2] [3].

Note

The urinary bladder is best visualized in the transverse view between the umbilical arteries at any gestational age. Detection of a filled urinary bladder always means that at least one kidney is present.

Malformations of the Fetal Kidneys and Urinary System

In order to present them in a systematic way, it makes sense to classify the malformations according to the respective gestational age.

Malformations in the First Trimester

In principle, it is possible to diagnose bilateral or unilateral renal agenesis or pelvic kidney ([Fig. 4]) during the first trimester; however, this is technically very challenging. The presence of a filled urinary bladder always implies the presence of a functioning kidney.

Fig. 4 Fetus with initial megacystis: a Megacystis at GW 13 + 0. Hypertrophy of the bladder wall is visible. b Same fetus at GW 17. A normal quantity of amniotic fluid is present. c Same fetus at GW 22. Unchanged hypertrophy of the bladder wall and normal amniotic fluid volume. d+e The hypertrophy of the bladder wall can be seen in particular in the color Doppler, as the umbilical arteries are located far from the lumen in this image. The finding remained stable, and a healthy child was born.

The malformation of the urinary system that is typically detected during the first trimester is megacystis (lower urinary tract obstruction – LUTO); this is explained in detail below.

Megacystis/Lower Urinary Tract Obstruction (LUTO)

Due to a permanent or temporary drainage obstruction, urine is retained in the urinary bladder, making it appear significantly enlarged. This usually leads to more or less massive displacement of abdominal organs and significant bulging of the abdominal wall. Usually, both kidneys appear significantly enlarged and echogenic. The renal pelvis is also significantly enlarged, and both ureters appear as hypoechoic sections. A distinction is made between megacystis with a diameter of 7–15 mm, which is often associated with chromosomal abnormalities, and severe cases of megacystis with a diameter > 15 mm, which is usually an expression of drainage impairment. The drainage impairment may be permanent (this occurs in 40 % of LUTO cases, often in female fetuses); this is referred to as urethral atresia. Typically, the portion of the urethra that is close to the urinary bladder is distended (keyhole phenomenon) ([Fig. 5], [6]).

Fig. 5 Initial diagnosis of megacystis at GW 17 + 1: a Megacystis in the transverse view with enlargement of the urethra. b Megacystis in the longitudinal view with bulging of the abdominal wall. c 3 D visualization of megacystis; in this technique (minimum mode), fluids are displayed in black. d Frontal view showing both hyperechoic and dysplastically altered kidneys. e Transverse view showing megacystis and both hyperechoic and dysplastic kidneys. There is still a normal amount of amniotic fluid, which argues against urethral aplasia.

Fig. 6 Illustration of two fetuses with pelvic kidney in the first trimester: a Transvaginal ultrasound at GW 12 + 5. Sagittal longitudinal view, the kidney is very deep. b Frontal view of the same fetus, the kidneys can be seen in the pelvis. c Transabdominal ultrasound in a fetus at GW 14 + 5, the orthotopic kidney can be seen, and a second kidney can be seen which is located much deeper, in the pelvis above the urinary bladder.

As a result of pronounced drainage impairment involving megalourethra, megacystis, and megaureter, there is urinary retention and dysplastic remodeling of the kidneys. Pronounced megacystis can also lead to diaphragmatic elevation and pulmonary hypoplasia. Renal failure that develops in untreated cases can also cause pulmonary hypoplasia due to developing anhydramnios and due to the “exhalation” of pulmonary fluid into the amniotic fluid. Pulmonary hypoplasia can arise from very different causes, and its severity is virtually impossible to predict prenatally.

Permanent drainage impairments (often with a diameter > 15 mm) do not regress; in such cases, the full range of prenatal medical options (conservative, intrauterine therapy, termination of pregnancy due to a medical indication) should be discussed with the parents.

There is also the possibility of temporary drainage impairment (often with a diameter 7–15 mm) (60 % of LUTO cases, often found in male fetuses), e. g., due to a valve (urethral valves – often posterior valves, more rarely anterior valves). Spontaneous remission can occur in this situation, so if megacystis is detected, a follow-up examination should be performed promptly (after 1 week). With temporary changes, the prognosis is significantly better; there is even a possibility that the child will be born healthy.

However, LUTO can also be an expression of a more complex disorder of the urogenital tract (cloacal dystrophy, anal atresia, neural tube defect), even when there is spontaneous remission; it is therefore important to conscientiously perform a detailed ultrasound examination.

In all cases, a diagnostic puncture is recommended, as the more moderate cases of megacystis (< 7 mm) in particular may be associated with chromosomal abnormalities (e. g., trisomy 13).

Female fetuses with megacystis also have a 3 to 4-fold higher risk of megacystis-microcolon-intestinal hypoperistalsis syndrome, which is diagnosed by molecular genetics [2] [3] ([Fig. 5], [6]).

Note

The malformation of the urinary system typically detected in the first trimester is megacystis/LUTO. This can be a dynamic condition, which is why prompt follow-up is recommended.

Malformations of the Kidneys and Urinary System in the Second and Third Trimester

The second and third trimesters can be summarized together. Due to the increasing size of the fetus and the increasing production of urine during the course of pregnancy, malformations often become more prominent with increasing gestational age.

Bilateral Renal Agenesis

From about GW 16–18, this malformation results in a significant reduction in the quantity of amniotic fluid. After GW 20, there is clear anhydramnios.

At no point is it possible to detect a filled urinary bladder (look for the umbilical arteries), and no kidneys can be visualized at the typical paravertebral site. The kidneys are normally located on the psoas muscle. Because the kidneys are absent, this space is taken up by the adrenal glands which then appear large (hypoechoic); the intestine also moves into this space. Visualization of the renal arteries is not helpful here, as the adrenal arteries can usually be visualized.

Sometimes the umbilical cord has the appearance of small puddles of amniotic fluid, so the use of color Doppler to detect the vessels is helpful here.

As the fetus discharges more and more of its pulmonary fluid into the amniotic fluid, pulmonary hypoplasia develops. Small lungs typically result in a large heart, often with overly developed muscle (myocardial hypertrophy) and pericardial effusion [2] ([Fig. 7]).

Fig. 7 Initial diagnosis of a fetus with bilateral renal agenesis at GW 21 + 5. Note the poor examination conditions due to anhydramnios: a Frontal view with empty renal beds on both sides. b The same view as a) shows the large adrenal gland near the transducer, which is sliding in a caudal direction. c) Frontal view with the aorta and lack of evidence of renal arteries. d Pronounced pulmonary hypoplasia with a normal sized heart.

Unilateral Renal Agenesis

This is usually an incidental finding in the context of the examination. The urinary bladder is filled, and the contralateral kidney is usually visible and normal. Only when searching systematically for the second kidney does it become apparent that this is missing at the typical location. Here, again, the space is taken up by the adrenal gland and intestine.

An important differential diagnosis to consider is pelvic kidney, in which the kidney is typically located in the small pelvis of the fetus, ventral to the aortic bifurcation.

Unilateral renal agenesis has a favorable prognosis, although a possible association with malformations of the internal genitalia in female fetuses must also be considered. These often appear very late, so in all cases we recommend a follow-up at GW 28–30 [2] ([Fig. 8], [9]).

Fig. 8 Illustration of various fetuses with unilateral renal agenesis. In each case, an orthotopic normal kidney and an empty renal bed are found on the contralateral side: a GW 29 + 1, frontal view with normal kidney, aorta, and empty renal bed on the side distant from the transducer. b The same fetus with evidence of a normal renal artery supplying the kidney. c GW 23 + 5, sagittal longitudinal view of the normal kidney. d The same fetus in a sagittal longitudinal view of the contralateral side and empty renal bed. Note the large adrenal gland. e GW 22 + 4, sagittal longitudinal view of the normal kidney. f Same fetus on the contralateral side. The kidney’s space is occupied by the intestines and adrenal gland, which makes the diagnosis challenging.

Fig. 9 Frontal view of a fetus with pelvic kidney at GW 21 + 4 B-mode (a) and with color Doppler (b). The pelvic kidney with normal renal structure can be seen near the pelvic vessels.

Pelvic Kidney

Pelvic kidney can be unilateral or bilateral (which is rare). Again, during the ultrasound examination you typically visualize the normally filled urinary bladder, then look for the kidneys and find that one or both of them are not in the usual location. In this case, you must look for the kidney in the small pelvis of the fetus. Typically, it is located towards the center in front of the aortic bifurcation. The vascular supply mainly comes from the iliac artery. The structure may be completely unremarkable. However, various signs of urinary retention may occur, usually due to an atypical joining of the ureter and the urinary bladder [2] [18]. Malformations of pelvic kidneys can also occur ([Fig. 9], [10], [11]).

Fig. 10 Frontal view of a fetus with pelvic kidney at GW 33 + 5. The normal kidney in an orthotopic location can be seen in (a) and the pelvic kidney with normal structure cranial to the urinary bladder can be seen in (b).

Fig. 11 A fetus at GW 28 + 5 with a normal orthotopic kidney (a) and a multicystic dysplastic pelvic kidney (b).

The prognosis for pelvic kidney is in principle good.

Note

If unilateral renal agenesis is suspected, always think of pelvic kidney. Follow-up of these findings at around GW 28 is recommended.

Horseshoe Kidney

Renal fusion, or horseshoe kidney, is a special form of renal abnormality in which the two lower poles of both kidneys fuse at the center. This fusion is located ventral to the aorta; the position of the kidney often appears slightly deeper than expected. A typical feature is vascular supply of the medial junction from the aorta, which can be visualized in a transverse view. Generally the horseshoe kidney functions normally, which means that both urinary bladder filling and the quantity of amniotic fluid may appear unremarkable. Because horseshoe kidney is usually associated with other malformations, especially chromosomal abnormalities (trisomy 13/18), genetic diagnostic testing is always recommended [2] ([Fig. 12]).

Fig. 12 GW 28 + 5. Illustration of a horseshoe kidney with fusion of both kidneys ventral to the aorta in a fetus with a diaphragm hernia on the left side.

Cystic Kidney Disease

Cystic kidney disease leads to structural changes in one or both kidneys. These changes can be multicystic or polycystic; in both cases, the entire organ is always affected. It is important to distinguish this condition from rare cases of isolated renal cysts with normal renal structure [2] [3].

Multicystic Dysplastic Kidney (MCDK)

With an incidence of 1:1400 live births, this is a common renal malformation. It is usually diagnosed in the second trimester. Ultrasound shows multiple cysts that do not communicate with each other, and no normal renal structure is detectable. The affected kidney is enlarged and dysfunctional.

Bilateral occurrence (20 % of cases) is characterized by anhydramnios due to renal failure. Unilateral MCDK with contralateral renal agenesis (10 % of cases) is also characterized by anhydramnios; in both cases, the prognosis is poor.

Unilateral MCDK with a normal contralateral kidney has a favorable prognosis. In the course of the disease, it is important to watch out for the possible development of a urinary transport disorder in the healthy contralateral kidney. If necessary, induced early birth or intrauterine shunting should be considered, although such cases are fairly rare. As a rule, the kidney affected by MCDK becomes significantly smaller after birth, so surgical removal is only performed in rare cases.

Genetic diagnostics are recommended, preferably using a high-resolution method (genotyping array, trio exome sequencing), as genetic changes are found in up to 15 % of cases [19] [20] [21] [22] [24] ([Fig. 13], [14]).

Fig. 13 Various fetuses with a multicystic dysplastic kidney: a Transverse view at GW 25 + 5. b Same fetus in a longitudinal view. The size of the change can be seen. c Transverse view of a fetus with a multicystic dysplastic kidney (on the side distant from the transducer, with a normal kidney close to the transducer, GW 22 + 6. d The same fetus in a frontal view. e Transverse view of a fetus with multicystic dysplastic kidney at GW 22 + 6. The affected kidney crosses the midline. The healthy kidney near the transducer is visible. f Sagittal longitudinal view of the affected kidney of the same fetus.

Fig. 14 Various fetuses with MCDK and complications: a Unilateral MCDK and contralateral urinary retention at GW 28 + 5 (b). c Bilateral MCDK at GW 21. Anhydramnios can be seen. d Unilateral MCDK and contralateral renal agenesis and anhydramnios GW 22 + 2. e MCDK as horseshoe kidney with additional brain malformations GW 28 + 2.

Polycystic Renal Dysplasia

A distinction is made between autosomal recessive polycystic kidney disease (ARPKD) and autosomal dominant polycystic kidney disease (ADPKD) [2] [3]. Both appear similar on ultrasound.

Autosomal Recessive Polycystic Kidney Disease (ARPKD)

The autosomal recessive form (ARPKD) is considered to be a ciliopathy; ultimately, it always affects several organs (kidney, liver, bile ducts and pancreas). A mutation of the PKHD1 gene may be a fundamental cause, which is why genetic diagnostic testing that includes the parents is always recommended. If this mutation is detected in the parents, there is a 25 % risk of it recurring in their offspring (autosomal recessive). The incidence is reported to be 1:40 000 live births.

On ultrasound, the kidneys appear massively enlarged and hyperechoic; they are unable to form a typical renal structure with a distinct renal cortex and renal medulla. It is not possible to distinguish individual cysts. This disease always affects both kidneys. To date, there have been no reports of cysts being detected in the other organ systems during the prenatal stage.

Because the disease is always bilateral, it is assumed in all cases that the affected child will be clinically ill. If anhydramnios is detected prenatally, prenatal renal failure can be assumed. This often results in typical myocardial hypertrophy of the heart. The prognosis is poor.

If a normal amount of amniotic fluid is still detectable prenatally, after birth the disease characteristically takes a chronic course involving pulmonary hypertension, liver fibrosis, and biliary dysgenesis, with a need for dialysis and transplantation. In such cases, the progression of the disease over time cannot be predicted during the prenatal stage. Consequently, providing prognostic advice to the parents is very challenging [2] [3] [19] [22] [25] [26] [27] [28] ([Fig. 15], [16]).

Fig. 15 Polycystic kidney disease at GW 23 + 2. a Transverse view, the enlarged echogenic kidneys can be seen on both sides. b Frontal view, there is virtually no normal renal structure visible. Renal function is still present, the amount of amniotic fluid is normal.

Fig. 16 PCKD GW 22 + 3 in transverse view. The bilateral hyperechoic kidneys are easily visible.

Because this form manifests clinically during childhood, it is also called the infantile form.

Autosomal Dominant Polycystic Kidney Disease (ADPKD)

Autosomal dominant polycystic kidney disease (ADPKD) is significantly more common, with an incidence of 1:1000. Again, this is a ciliopathy which may affect the liver, bile ducts, and pancreas in addition to both kidneys. The majority of cases are due to a single mutation in the PKD1 gene (85 %), or more rarely in the PKD2 gene (15 %).

The postnatal course is characterized by delayed onset of renal failure; accordingly, this is called the adult form. Because of the dominant inheritance, it is recommended that the parents undergo ultrasound examination and genetic testing.

Ultrasound diagnosis is usually made in the second trimester based on hyperechoic kidney enlargement without normal renal structure. It may be possible to visualize the renal pelvis and pelvicalyceal system. Usually there is a normal amount of amniotic fluid present. Prenatal renal failure is fairly rare [1] [2] [3] [19] [25] [26] [27] ([Fig. 17]).

Fig. 17 Illustration of fetuses with PCKD, all with a normal quantity of amniotic fluid: a Longitudinal view, GW 22. b Transverse view, GW 23 + 2. c Longitudinal view, GW 28 + 2. d Frontal view, GW 28.

Polycystic Renal Dysplasia with Other Malformations

Bilateral polycystic renal dysplasia can also occur as a ciliopathy (multi-organ disease) in the context of other malformations (see examples in [Table 1]).

Table 1 Syndromes (ciliopathies) that present bilateral hyperechoic cystic renal dysplasia (selection).

Malformation/syndrome	Malformations	Gene mutation
Meckel-Gruber	Hyperechoic polycystic renal degeneration, CNS malformations (e. g., encephalocele), polydactylia	MKS1, MKS2, or MKS3 gene
Bardet-Biedl/McKusick-Kaufman	Hyperechoic polycystic kidneys, polydactylia, genital malformation	BBS1–14 genes
Joubert	Hyperechoic polycystic kidneys, brain malformations (molar tooth sign)	37 genes
ARPKD	Large hyperechoic medullary sponge kidney	PKHD1 gene
ADPKD	Hyperechoic kidneys, discreetly enlarged	PKD1, PKD2

[2] [3] [22], ([Fig. 18], [19]).

Fig. 18 PCKD in the context of other malformations: a GW 13, trisomy 18 with AVSD and increased intracranial translucency and growth restriction. b GW 15 + 5, trisomy 13 with alobar holoprosencephaly and cleft lip, jaw, and palate cleft. c Frontal view both kidneys appearing hyperechoic in this fetus.

Fig. 19 PCKD in a fetus with Zellweger syndrome in transverse view (a), frontal view (b), and sagittal view (c). The bilateral hyperechoic dysplastic kidneys can be seen. The quantity of amniotic fluid is normal.

Note

In cases of bilateral hyperechoic kidney, high-resolution genetic diagnostic testing of the amniotic fluid is always recommended.

Renal Cysts

Isolated renal cysts may occur. In such cases the renal tissue is completely normal, which enables this condition to be distinguished from dysplastic kidney disease. The isolated cysts (there may be several of them) are not connected to the urinary system. This condition can be difficult to distinguish from pronounced urinary obstruction. The prognosis is good [2] ([Fig. 20]).

Fig. 20 Illustration of a fetus with renal cysts: a Transverse view, GW 23 + 2. b The same fetus in longitudinal view. c Transverse view. On the contralateral side, the renal pelvis shows minimal urinary retention. d The cyst extends far into the ventral area. e Fetus at GW 28 + 2, transverse view. f GW 28 + 2, longitudinal view.

Duplex Kidney

Duplex kidney can be either unilateral or bilateral. If both parts of the duplex kidney are structurally normal and there is no drainage impairment, the diagnosis is challenging. You may occasionally notice a kidney that appears enlarged, with significant enlargement confirmed through biometry. Both renal pelvises are then significantly more marked in appearance, especially in the later weeks of pregnancy. In these cases, the ureteropelvic junction can be seen twice. It is also possible to detect two renal arteries, although there are a large number of normal variants. Furthermore, the suprarenal arteries are also located in the area adjacent to the adrenal glands, so that the usefulness of this for diagnostic purposes is hard to assess.

Because a duplex kidney has two ureters, the development of a ureterocele is often observed; this is similar in appearance to a cystic spatial growth in the fetal urinary bladder, and is virtually conclusive for the diagnosis of duplex kidney.

If a portion of the duplex kidney is affected by urinary retention, often due to incorrect joining of the ureter and the urinary bladder, the diagnosis is much easier. This often affects the cranial pole of the kidney, which then shows varying degrees of urinary retention. As a rule, especially in the later weeks of pregnancy, it is possible to visualize the kinked and distended ureter.

Both parts of a duplex kidney on the same side may also be affected.

The prognosis is fairly favorable. As a rule, the contralateral kidney and/or second part of the duplex kidney is unaffected, so there is no need for treatment until after birth [2] [29] ([Fig. 21], [22], [23], [24]).

Fig. 21 Fetuses with duplex kidney: a GW 24. There is massive urinary retention in the cranial pole; the path of the associated ureter can be visualized. b GW 22 + 2, unremarkable duplex kidney, no signs of urinary retention. c GW 22 + 3, urinary retention in the cranial pole. d The same fetus from c at GW 28. No progression of findings.

Fig. 22 Fetus with duplex kidney over time: a GW 22, urinary retention in the cranial pole. b The same fetus in transverse view with visible ureter. c GW 23, clearly visible ureter. d Detection of ureterocele in the urinary bladder. e) GW 27 + 6, unchanged urinary retention in the cranial pole. f Detection of ureterocele at GW 27 + 6. g GW 33 + 3, detection of a massively dilated ureter.

Fig. 23 Fetus with duplex kidney, with various ultrasound techniques: a B-scan, GW 23, illustration of the ureter affected by urinary retention. b Static 3 D image (minimum mode), GW 27, spatial image of the path of the ureter. c Inversion of the image from the B-scan with digital contrast, so to speak (inversion mode).

Fig. 24 Unremarkable duplex kidney at GW 22 (a) and bilateral duplex kidney with bilateral urinary retention at GW 28 (b).

Urinary Retention

With urinary retention, there is a drainage obstruction which leads to a backlog of urine in the urinary system. The cause may be found in the area of the urethra (LUTO – always bilateral!) or in the area where the ureter joins the urinary bladder (may be unilateral or bilateral), or may be due to valves in the area of the Ureter (either unilateral or bilateral), or due to an incorrect outflow of the ureter from the renal pelvis (either unilateral or bilateral). In the literature, urinary retention is synonymous with pyelectasia and hydronephrosis. Its severity is determined based on the anterior-posterior diameter of the renal pelvis (largest diameter), the description of the renal tissue (typical or echogenic), and possibly the ureter if visible (diameter!) ([Fig. 25], [26]) [2] [3] [30]. The normal values according to gestational age are shown in [Table 2] [31].

Fig. 25 Illustration of fetuses with urinary retention: a Mild bilateral pylelectasia at GW 28. b Mild bilateral pylelectasia at GW 23 + 2. c Massive enlargement of the renal pelvic-calyceal system at GW 33. d Massive unilateral urinary retention at GW 36 with markedly dilated ureter. e Pronounced unilateral pyelectasia at GW 27. f Significant enlargement of the renal pelvic-calyceal system at GW 27.

Fig. 26 a Mild bilateral urinary retention with correct kidney dimensions. b Pronounced unilateral urinary retention at GW 27. c+d Massive unilateral grade IV urinary retention with dilated ureter (c), with normal contralateral kidney (d). e Bilateral urinary retention with correct kidney dimensions.

Table 2 Normal values for distention of the urinary tract.

Ultrasound findings	GW 16–27	>/ = GW 28	Postnatal > 48 hours
Renal pelvic-calyceal system – anterior-posterior diameter	< 4 mm	< 7 mm	< 10 mm
Central enlargement of calyces	No	No	No
Peripheral enlargement of calyces	No	No	No
Parenchymal thickness	Normal	Normal	Normal
Parenchymal structure	Normal	Normal	Normal
Ureter	Normal/not visible	Normal/not visible	Normal/not visible
Urinary bladder	Filled	Filled	Filled
Oligohydramnios	No	No

Male fetuses are more likely to have a urinary obstruction than female fetuses.

Depending on the cause, more or fewer parts of the urinary system may be affected by urinary retention. Unilateral findings are always located in the area between the kidney and urinary bladder. As the fetus grows larger and urine production increases, in the case of a permanent drainage obstruction, there is always an increase in urinary retention during the course of the pregnancy. If the finding remains stable or decreases, this is more suggestive of a temporary problem, such as an external pressure, or restriction of the renal function on the affected side.

As part of the broader diagnosis, the possibility of genetic diagnostic testing of the fetus should be discussed. Serial follow-up examinations (approx. 4–6 weeks apart) should be performed to record these dynamics. The condition of both kidneys, the filling state of the urinary bladder, and the quantity of amniotic fluid should be documented. Furthermore, any visible signs of urinary retention in the ureter should be looked for.

The earlier the drainage impairment becomes clinically relevant, the earlier dysplastic restructuring with impairment of renal function may occur in the area of the affected kidney. These dysplastic changes appear on ultrasound as a hyperechoic texture in the affected kidney, which is reduced in size and features cortical cysts.

Note

Multicystic renal degeneration may appear similar on ultrasound, but the kidney is always enlarged during the prenatal stage.

In very rare cases, the pressure caused by the backlog is so great that urine leaks into the surrounding tissue; this is called urinoma. Urinomas appear on ultrasound as hypoechoic space-occupying lesions of varying size in the area of one of the kidneys (or both in the case of a bilateral finding). These space-occupying lesions can compress the kidney and displace it from its normal position. Urinoma should be considered an indication of severe functional impairment of the affected kidney. Due to previous damage to the kidney, shunt insertion is not useful in these cases [2].

Fetuses benefit from full-term delivery, regardless of the severity of the finding; induction of premature birth is not recommended. In the case of very pronounced bilateral findings, prenatal shunt insertion may be useful. The basic prognosis depends on the severity of the urinary retention, but is usually very good. Very pronounced findings usually require prompt postnatal pediatric surgery [2] [3] [30] [31].

Late Onset LUTO

Urethral urinary retention may also occur for the first time late in pregnancy. In contrast to LUTO that is diagnosed early in the pregnancy, the prognosis is significantly better here.

Occasionally, if LUTO persists over a prolonged period, the fetus develops hypertrophy of the bladder wall. This can be diagnosed based on a clearly increased distance between the umbilical arteries and the lumen of the urinary bladder. This can also be measured to assess the dynamics [2] [3] ([Fig. 27]).

Fig. 27 a Bilateral urinary retention in the presence of a known left diaphragm hernia, frontal view. b Detection of dilated ureters in the fetus from a at GW 21. c Same fetus at GW 29 with signs of urinary retention in the kidney and ureter. d 3 D representation (minimum mode) of the ureter affected by urinary retention in the same fetus. e Initial diagnosis of LUTO at GW 31 with hypertrophy of the bladder wall, enlargement of the urethra, and bilateral urinary retention affecting the renal pelvis (f).

Renal Hypoplasia

Renal hypoplasia can be unilateral or bilateral. It may be caused by an early and severe urinary flow disorder with subsequent dysplastic remodeling of the kidney. However, even with completely normal renal structure, the kidney may fail to increase in size. In the course of the disease, oligohydramnios often develops as an expression of impaired renal function. This condition is diagnosed through renal biometry. The cause is unclear and the prognosis is difficult to assess ([Fig. 28]).

Fig. 28 Fetus with mosaic trisomy 20: a Empty renal bed in unilateral renal agenesis. b Hypoplastic pelvic kidney adjacent to the filled urinary bladder. c Illustration of the biometry values over time in a fetus with bilateral renal hypoplasia and postnatal death. d Illustration of the amniotic fluid quantities over time in the fetus from c).

Decreased Amniotic Fluid

The quantity of amniotic fluid during the 2nd and 3 rd trimesters is mostly a function of the fetus, especially the fetal kidneys. This is a dynamic situation, and accordingly there is a high degree of variability; however, there are methods for quantification using the single deepest pocket method and the amniotic fluid index.

A decrease in the quantity of amniotic fluid in conjunction with typical renal findings on ultrasound may be an expression of fetal renal failure. As differential diagnoses, a rupture of membranes (normal fetal kidneys, normal urinary bladder filling) and intrauterine growth restriction with pathological Doppler (abnormal biometry, abnormal Doppler, normal kidneys, normal urinary bladder filling) should be excluded.

Note

Changes in the kidneys and urinary system in the second half of pregnancy are noticeable due to changes in the quantity of amniotic fluid (oligo-anhydramnios), structural changes, or due to fluid-filled areas of varying size. Both kidneys may be affected.

Diagnostic algorithm for malformations of the kidney and urinary system:

First trimester:

Urinary bladder filled	There must be at least one kidney present
Urinary bladder empty	Look for kidneys
Megacystis	Visualize kidneys (echogenicity, urinary retention)
	Urinary bladder biometry
	Concomitant malformations?
	Follow-up (7 mm after a week, transiently larger diameter)
	Diagnostic puncture recommended

Second/third trimester:

Renal pelvic-calyceal system – Distention:	Unilateral/bilateral?
	Bladder filling?
	Quantity of amniotic fluid?
	Visualize ureter(s)
	Concomitant malformations?
	Consultation on genetic diagnostic testing
	Progress monitoring

MCDK	Unilateral/bilateral?
	Bladder filling
	Quantity of amniotic fluid?
	Concomitant malformations?
	Consultation on genetic diagnostic testing?
	Progress monitoring/management

PCKD	Bladder filling?
	Quantity of amniotic fluid?
	Concomitant malformations
	Consultation on genetic diagnostic testing
	Progress monitoring/management

Kidney cannot be visualized	Unilateral/bilateral?
	Pelvic kidney?
	Quantity of amniotic fluid?
	Structure?
	Cardiac muscle
	With pelvic kidney and in female fetuses: Look out for urogenital tract malformations

Duplex kidney	Kidney biometry in 3 planes
	Number of renal pelvises?
	Unilateral/bilateral?
	Ureterocele in the urinary bladder? – Look for it again
	Quantity of amniotic fluid?
	Follow-up examinations

Conflict of Interest

Declaration of financial interests

Receipt of research funding: no; receipt of payment/financial advantage for providing services as a lecturer: no; paid consultant/internal trainer/salaried employee: no; patent/business interest/shares (author/partner, spouse, children) in company: no; patent/business interest/shares (author/partner, spouse, children) in sponsor of this CME article or in company whose interests are affected by the CME article: no.

Declaration of non-financial interests

The authors declare that there is no conflict of interest.

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Publication History

Article published online:
28 September 2023