Kidney development undergoes a complicated series of formation and regression of primitive renal structures

Final kidney is formed from interaction of the ureteric bud and specialized mesoderm, the metanephric blastema

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  Ureteric bud induces metanephric blastema to form nephrons

  
  
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  In turn, metanephric blastema induces ureteric bud to bifurcate into developing calyces

Anomalies resulting in failure of ureteric bud to interact appropriately with metanephric blastema

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  Renal agenesis

  
  
  
  
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    Failure of ureteric bud to come into contact with metanephric blastema; no kidney is formed

  
  
  
  
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  Multicystic dysplastic kidney

  
  
  
  
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    Ureteric bud does not signal appropriate development so there is disorganized stromal expansion and cyst formation

  
  

Early in development, the kidneys lie close together low in pelvis with renal hila facing anteriorly

Mechanism for “ascent” to final retroperitoneal flank position is not completely understood, but caudal embryonic growth is likely major contributing factor

Blood supply changes as kidneys “recruit” arterial blood supply from iliac arteries and aorta

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  New, more superior arterial branches form as inferior branches involute

With ascent, renal pelves rotate medially ~ 90°

Ascent complete by 9 weeks when kidneys come into contact with adrenal glands

Anomalies related to abnormal ascent

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  Renal ectopia (e.g., pelvic kidney)

  
  
  
  
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    Kidney usually low in position with abnormal rotation

  
  
  
  
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  Crossed-fused ectopia and other fusion abnormalities

  
  
  
  
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    Embryonic fusion prior to ascent leads to various appearances

  
  
  
  
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  Horseshoe kidney

  
  
  
  
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    Inferior poles fuse

    
    
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    Become “stuck” under inferior mesenteric artery

  
  
  
  
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  Accessory renal arteries

  
  
  
  
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    Persistence of normally transient renal arteries during ascent

  
  

The bladder forms from the cloaca (Latin for sewer), which is common chamber with early communication between urinary, gastrointestinal, and reproductive tracts

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  It is divided by the urorectal septum into urogenital sinus anteriorly and rectum posteriorly

  
  
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  Urogenital sinus divides into bladder and in females, the vagina

  
  
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  Failure to divide results in a cloacal malformation , 1 common communicating chamber between bladder, vagina, and rectum

  
  
  
  
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    The vagina is often septated, which may be an important clue to the diagnosis

  
  

American Institute of Ultrasound in Medicine requires documentation of the kidneys, bladder, and amniotic fluid volume (a reflexion of fetal urine output) in all 2nd- and 3rd-trimester fetuses

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  Evaluation of fetal sex is only required when medically indicated

  
  
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  These are considered the minimum requirements; if any anomaly is suspected, a detailed examination should ensue

Components of the genitourinary tract include the kidneys, ureters, bladder, urethra, adrenal glands, internal and external genitalia

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  Knowledge of the normal developmental appearance of each of these structures is needed in order to recognize pathologic processes

Kidneys can be identified by 12- to 14-weeks gestation using endovaginal sonography; internal architecture can be resolved as early as 16-18 weeks

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  External renal contour is lobular (fetal lobulation), a finding that may persist into adulthood

  
  
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  Cortex around the periphery is intermediate in echogenicity with hypoechoic medullary pyramids arranged symmetrically around the renal pelvis

  
  
  
  
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    Do not confuse normal hypoechoic pyramids with dilated calyces

  
  
  
  
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  Normative data is available for all renal dimensions, but a rule of thumb is the length in mm approximates the fetal gestational age in weeks

  
  
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  The renal circumference:abdominal circumference ratio is stable throughout pregnancy with values from 0.27-0.30

  
  
  
  
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    When you look at the abdomen in axial plane, kidneys should never take up more than 1/3 of the area

  
  

To obtain an accurate measurement of the anterior-posterior renal pelvis diameter (APRPD) , it must be measured perpendicular to the pelvis at the 12 o’clock or 6 o’clock position

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  Normal measurements are < 4 mm from 16-27 weeks and < 7 mm from 28 weeks to term

  
  
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  If the APRPD measures larger than normal, or there is calyceal dilation or parenchymal changes, then follow-up is required

Color Doppler

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  Used to identify the bladder between umbilical arteries and document 2 umbilical arteries

  
  
  
  
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    Many renal anomalies are associated with a single umbilical artery

  
  
  
  
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  Crucial when questioning renal agenesis

  
  
  
  
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    Potential pitfall is lumbar or adrenal arteries, which can appear quite prominent and be mistaken for renal arteries

  
  

Normal adrenal gland has a characteristic ice cream sandwichappearance with hyperechoic medulla (the ice cream filling) surrounded by hypoechoic cortex

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  Triangular or Y-shaped and relatively large, when compared with the kidney

Fetal sex is not required in low-risk pregnancies but is often the most pressing question for parents

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  Preferred term is “sex” not “gender,” which is self-identified

  
  
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  At 12-14 weeks, the genital tubercle has a similar appearance in males and females

  
  
  
  
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    May look at ” the angle of the dangle ,” which points caudally in females and cranially in males, but it is wise to avoid committing to sex unless clearly identified

  
  
  
  
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  If sex identification remains indeterminate (i.e., possible disorder of sexual development), a careful search should begin for other anomalies, which would suggest a syndrome or aneuploidy

Amniotic fluid volume is evaluated in every 2nd- and 3rd-trimester study and can be assessed subjectively (shown to be very accurate when performed by an experienced sonographer) or semiqualitatively by measuring fluid pockets

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  Maximum (or deepest) vertical pocket(MVP) measurement is anterior to posterior distance of the largest fluid pocket void of fetal parts and umbilical cord

  
  
  
  
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    “Vertical” implies the measurement is obtained with the transducer perpendicular to the maternal abdomen

    
    
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    Oblique measurements are not reproducible and may lead to errors in assessment of fluid volume

    
    
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    Normal is 2-8 cm with < 2 cm indicating oligohydramnios and > 8 cm polyhydramnios

  
  
  
  
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  Amniotic fluid index(AFI) measurement is the sum of the MVPs in 4 quadrants

  
  
  
  
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    Use color Doppler to exclude umbilical cord from measurement

    
    
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    AFI varies with gestational age, but values of 5-20 cm are the general normal range

  
  

Is it really the urinary tract?

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  Peritoneal boundaries are not clear in the fetus, so care must taken in deciding what organ system is involved

  
  
  
  
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    A dilated tubular structure may be either ureterectasis or obstructed bowel

    
    
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    A solid mass may be coming from the kidney (e.g., mesoblastic nephroma), adrenal gland (e.g., neuroblastoma), or separate from both (e.g., extralobar sequestration)

  
  

Are there 2 kidneys? If so, where are they?

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  If both kidneys are absent ( bilateral renal agenesis ), there will be anhydramnios by mid-2nd trimester

  
  
  
  
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    Kidneys are not a major contributor to amniotic fluid until 16-weeks gestation , so may see normal fluid before that time

    
    
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    Adrenal glands are very prominent early in gestation and may be confused with kidneys

    
    
    
    
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      Later in gestation, they have a lying-down appearance, lacking the normal triangular configuration

    
    

  
  
  
  
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  If there is normal fluid beyond 16 weeks, at least 1 kidney must be present

  
  
  
  
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    Evaluate renal fossa carefully, and if there is only 1 kidney, begin a careful search to see if the other is absent (i.e., unilateral renal agenesis) or in an aberrant location (e.g., pelvic kidney, crossed-fused ectopia)

  
  

Is the renal size and echogenicity normal?

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  Increased renal size and echogenicity may be seen in autosomal recessive polycystic kidney disease, Meckel-Gruber syndrome, or in association with aneuploidy, typically trisomy 13

  
  
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  Differential diagnosis for unilateral renal enlargement includes: Unilateral renal agenesis with compensatory hypertrophy, cross-fused ectopia, duplicated renal collecting system, and renal vein thrombosis

Are anechoic structures renal cysts or hydronephrosis?

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  Real-time evaluation is key

  
  
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  If they connect centrally with the renal pelvis, explore causes of hydronephrosis [e.g., ureteropelvic junction obstruction, ureterovesical obstruction, and lower urinary tract obstruction (LUTO)]

  
  
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  If they do not connect, the differential diagnosis is that for multiple discrete cysts (e.g., multicystic dysplastic kidney, cystic dysplasia from obstruction)

Is the bladder normal in size?

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  Bladder should fill and empty during the course of a scan

  
  
  
  
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    Always check the bladder at the beginning and end of exam to make sure the observation of too big or too small bladder is persistent

  
  
  
  
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  An absent bladder is most commonly due to failure of urine production, in which case, look for bilateral renal anomalies

  
  
  
  
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    Can also occur with decreased renal perfusion (e.g., fetal growth restriction, donor twin in twin-twin transfusion)

    
    
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    If the bladder is never seen, but there is normal amniotic fluid, then consider bladder exstrophy (bladder is open on anterior abdominal wall with urine continuously leaking into amniotic fluid)

    
    
    
    
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      Scan the abdominal wall looking carefully for irregularity; cord insertion site is often lower than normal

    
    

  
  
  
  
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  If the bladder is distended and fails to empty (i.e., LUTO), posterior urethral valves (PUV) and prune-belly syndrome should be considered

  
  
  
  
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    Perform focused scan on bladder base looking for a dilated posterior urethra, which creates the classic keyhole appearance of PUV

    
    
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    In a female fetus, consider a cloacal malformation if there is a persistent fluid-filled structure in the pelvis

    
    
    
    
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      The distended structure is actually the vagina and not the bladder

      
      
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      Look for a fluid-debris level, which is mixing of urine, vaginal secretions, and meconium

      
      
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      Look for a vertical vaginal septum, which is present in many cases

    
    

  
  

Are the genitalia normal?

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  Anomalous/ambiguous-appearing genitalia can be seen in disorders of sexual development

  
  
  
  
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    Disorders of sexual development in XY fetus

    
    
    
    
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      Hypospadias (most common), epispadias, cryptorchidism ± penile anomaly

    
    
    
    
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    Disorders of sexual development in XX fetus

    
    
    
    
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      Congenital adrenal hyperplasia is most common cause

    
    

  
  
  
  
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  Associated with aneuploidy (e.g., trisomy 13, trisomy 18, triploidy) and syndromes (e.g., Smith-Lemli-Opitz, Prader-Willi)

  
  
  
  
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    Always look for other anomalies