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Key messages

  • A simple threshold of an anterior posterior diameter (APD) of the renal pelvis does not exist as severe cases of antenatal hydronephrosis (ANH) have the potential to resolve without incident while mild degrees of ANH have the potential to progress.
  • All infants with suspected urethral obstruction (urethral valves) need immediate referral and discussion with a paediatric urologist (at either the Royal Children’s Hospital or Monash Children’s).
  • It is suggested that all infants with persisting ANH have antibiotic prophylaxis until the postnatal ultrasound and clinical review.
  • The goal in evaluating children with ANH is to detect significant congenital renal and urinary tract pathology (urinary tract obstruction, vesicoureteric reflux) whilst avoiding excessive evaluation of physiological ANH.
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    In June 2023, we commenced a project to review and update the Maternity and Neonatal eHandbook guidelines with a view to completion in 2024. Please be aware that pending this review, some of the current guidelines may be out of date. In the meantime, we recommend that you also refer to more contemporaneous evidence.

    Dilation of the fetal renal collecting system, antenatal hydronephrosis (ANH) or pyelectasis, is one of the most common abnormalities detected on prenatal ultrasound affecting 1–5 per cent of all pregnancies.

    ANH represents a wide spectrum of urological conditions ranging from transient dilation of the collecting system to clinically significant urinary tract obstruction or vesico-ureteric reflux (VUR). Each kidney is considered as a single unit and it does not matter if it is bilateral or unilateral.

    Babies with ANH should be evaluated to prevent potential complications, such as urinary tract infection (UTI), kidney stones, renal scarring and loss of function.


    Pyelectasis - dilated renal pelvis with no calyceal dilation
    Hydronephrosis - dilated renal pelvis with dilated calyceal system

    Assessing ANH in utero

    Measuring the renal pelvis

    Measurement of the antero-posterior diameter (APD) of the renal pelvis as visualised in the transverse plane on ultra sound scan (USS) is the most studied parameter for assessing ANH in utero.

    ANH is defined as a renal pelvis APD greater than 4 mm on fetal USS in the second trimester and greater than 7 mm in the third trimester. If ANH in the second trimester has resolved by the third trimester, that is, renal pelvis APD less than 7 mm in the third trimester, then the fetus does not have ANH.

    There are no distinguishing features to differentiate which children will progress to obstruction on follow up; however, a renal pelvis APD of greater than 15 mm represents severe hydronephrosis with a high likelihood of progression, whereas less than 5 mm is a threshold for considering the pelvis as normal.

    A recent prospective cohort found that no baby with a renal pelvis APD of less than 6 mm progressed to treatment for ANH. This included all babies with pyelectasis and hydronephrosis.

    To assess progression of ANH in utero, the most important step is to assess at two time points, preferably in the second and third trimesters. 

    Degree of ANHSecond trimesterThird trimesterPostnatal
    Normal<5 mm<7 mm<5 mm
    Mild5–7 mm7–10 mm

    5–10 mm

    Normal calyces

    Moderate7–10 mm10–15 mm

    10–15 mm

    Calyceal dilatation

    Severe>10 mm>15 mm

    >15 mm

    Calyceal dilatation


    Appearance of the urinary tract

    Additional features of the sonographic appearance of the urinary tract that are important include:

    • calyceal dilatation: presence is concerning
    • contralateral kidney: presence, size and appearance
    • ureteric dilatation: presence and extent
    • bladder: size, wall thickness and emptying

    Risks of postnatal pathology

    The risk of disease progression after birth is as follows:

    • mild ANH – 12 per cent
    • moderate ANH 45 per cent
    • severe ANH – 88 per cent.

    Postnatal management of hydronephrosis

    The single most important postnatal investigation is a physical examination of the neonate conducted soon after birth and assessing for palpable kidney or bladder and associated anomalies. The presence of palpable kidney or bladder mandates early review by urology or nephrology. 

    Timing of other investigations is determined by the severity and laterality of ANH.  

    Timing can affect the interpretation of results, if undertaken soon after birth. Be aware of the following:

    • Early sonography (first 72 hours) of the urinary tract may be falsely reassuring during the period of relative oliguria.  Early ultrasound is appropriate in severe ANH or bilateral ANH to allow rapid response if abnormal.
    • Early serum creatinine is affected by maternal levels, but the level can be informative by day 2, in a setting of severe ANH, bilateral ANH or single kidney.
    • If not severe, ultrasound is best delayed until after day 4 or later, to allow urine production to define actual ANH.
    • Blood pressure measurement should be undertaken at least once, while the infant is calm.  
    • Diuretic renography utilising MAG3 is more useful than DTPA in infants, as the handling of isotope and response to frusomide is poor in the immature kidney.
    • Hydration status affects ultrasound appearance – it is important the baby is well hydrated for the studies.

    Investigation and follow-up

    Investigation and follow-up depends on the degree of ANH.

    • If the ANH becomes normal during the antenatal phase with no other abnormalities then follow-up is not required.
    • For mild ANH, or calyceal dilation without increased renal pelvis APD, a postnatal USS should be performed once the baby has established a good urine output after one week of age with paediatric follow-up.
    • Babies with severe ANH need to be scanned before discharge.
    • Antibiotic prophylaxis should be provided to all babies until their first postnatal ultrasound and clinical review.  An active decision regarding ongoing antibiotic prophylaxis can then be made once initial investigations have been completed.

    Prophylactic antibiotics consist of either:

    • trimethoprim 2 mg/kg once daily
    • cephalexin 10 mg/kg once daily.

    Paediatric nephrology discussion or referral would include:

    • any specific plan constructed during antenatal consultation
    • isolated antenatal APD 10–15 mm
    • isolated antenatal APD >15 mm or moderate to severe calyceal dilation with or without pelvic dilation
    • ANH with any of the following:
      • oligohydramnios
      • dilated ureter/lower urinary tract
      • renal parenchyma abnormality
      • single kidney
    • postnatal hydronephrosis (PNH):
      • persisting isolated PNH of APD of >5 mm at six months follow-up
      • after birth an APD > 10 mm and or any other abnormality of the renal tract
    • on follow-up in babies with any of the following:
      • obstructive uropathy (pelvic-ureteric junction (PUJ) obstruction, vesico-ureteric junction (VUJ) obstruction, posterior urethral valves)
      • renal asymmetry >1 cm
      • abnormal renal parenchyma
      • severe VUR (Gr IV-V)
      • reduced renal function
      • increasing hydronephrosis grade or renal pelvis dilatation (RPD) >50 per cent increase
      • solitary kidney
      • recurrent UTIs (two or more in first year of life).

    Vesico-ureteric reflux 

    VUR is a common cause of hydronephrosis; however, this does not mean all babies with hydronephrosis should undergo micturating cystourethrography (MCU).

    Recent studies have questioned the validity of antibiotic prophylaxis in asymptomatic VUR. There is no evidence that primary VUR without infection is an ongoing threat to renal status.

    There is a clear association between VUR and congenital renal dysplasia, so specialist review is recommended if renal parenchyma is sonographically abnormal.

    Role of MCU

    Micturating cystourethrography (MCU) is an invasive test, with implications regarding iatrogenic infection, trauma and radiation. It is very useful in specific circumstances, but should not form a routine part of postnatal investigation of hydronephrosis.

    All male babies with moderate-severe bilateral hydronephrosis or severe unilateral hydro-ureteronephrosis should undergo MCU to assess for posterior urethral valves.

    This study should be undertaken under antibiotic cover, ideally before the baby leaves hospital.

    All other considerations for MCU should be under guidance of a paediatric urologist or paediatric nephrologist. 


    Follow-up should include weight, blood pressure measurement, urine analysis and clinical evaluation.

    Ultrasound is the most useful longitudinal modality, being non-invasive and involving no ionising radiation. Ultrasound images need to be compared with previous images to identify changes in appearance.


    1. Postnatal evaluation and management of antenatal hydronephrosis – The Royal Women’s Hospital 2014
    2. Antenatal hydronephrosis (neonate/paediatric): postnatal investigation and management - Southern Health 2014

    Get in touch

    Clinical Guidance Team
    Safer Care Victoria

    Version history

    First published: May 2016
    Review by: May 2019

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