Hydronephrosis (hydo=water; nephron=kidney) is a descriptive term, not a diagnosis. It is the term doctors use to describe a kidney that appears “swollen” (“dilated”) because it holds more urine than normal. It is often picked up on antenatal scanning (1:500 pregnancies) and is transient and insignificant in up to 90% of cases, depending on the measurements used. This is because the fetus produces much more urine than a newborn baby and this increased flow may transiently stretch the kidney. The hydronephrosis then often resolves once the baby is born.

Babies born with prenatally-diagnosed hydronephrosis are usually started on an antibiotic called Trimethoprim once they are born. The antibiotic is given once every evening and is prescribed at a quarter of the normal dose for that antibiotic. It protects the baby from developing an infection in the hydronephrotic kidney, which can be very serious. It is therefore important that you give the baby the antibiotic until advised otherwise by the paediatric or paediatric urology team. It is a very small dose and does not harm the baby or effect its immune system. It will be stopped once the doctors are happy that the kidney is normal.

The postnatal management will depend on a number of factors, mainly on whether the prenatal hydronephrosis only involves one of two kidneys (unilateral hydronephrosis), both kidneys (bilateral hydronephrosis) or a solitary kidney. The postnatal doctor will organise a postnatal ultrasound scan and follow-up with a specialist – please ensure this is organised before you get discharged, or contact us for an appointment. The postnatal scan will determine whether the hydronephrosis is self-resolving, or whether further tests are required.

The following is a list of possible causes of hydronephrosis – hence the involvement of a specialist is essential.

Transient hydronephrosis (self-resolving)


Pelvi-ureteric junction obstruction (PUJO)


Vesico-ureteric reflux (VUR)


Vesico-ureteric junction obstruction (VUJO) & Megaureter


Multicystic dysplastic kidney (MCDK)


Duplex system +- ureterocele


Posterior urethral valves (PUV) & Urethral atresia

(Adapted from Pediatrics 2006)


It is always best to discuss further management of each condition with your paediatric urologist as each child is individual and the management will depend on whether or not the child is symptomatic as well as the kidney function and other indicators on your child’s scans.

  • Contact

  • NHS Referrals

    NHS Referrals

    Please visit your GP and request a referral to Chelsea & Westminster Hospital Paediatric Urology

  • Private Referrals

    Miss MK Farrugia

    Please e-mail [email protected] or book an appointment in Chelsea & Westminster Hospital (0203-315-8599) or BUPA Cromwell Hospital (0207-460-5700)