Antenatal hydronephrosis is quite common, and may affect one in 100 pregnancies. In most cases, it is not inherited from the mother or father. It is not usually caused by anything that the mother does during her pregnancy.

Sometimes, your doctor will not know why your baby has antenatal hydronephrosis and whether it will lead to problems later in the pregnancy or after birth. You may need more scans during the rest of your pregnancy to check whether the problem is changing. Your baby may need tests after birth to find out the cause and whether treatment is needed.

Which parts of the urinary tract are affected?

The kidney has several distinct parts. After the kidney makes urine, it collects in the renal pelvis before passing into the ureter.

In antenatal hydronephrosis, the renal pelvis holds on to urine and it swells up. Sometimes, the ureter that is connected to the kidney also swells up.

No other problems

In most cases, there are no other problems with the urinary system, and the antenatal hydronephrosis usually resolves (gets better) before birth. These children will probably not have any long-term problems and will not need follow-up after birth.  Doctors think antenatal hydronephrosis with no other problems happens as the baby’s urinary system develops – especially if one or both ureters do not fully develop before the kidneys start making urine. 


Some cases of antenatal hydronephrosis are caused by vesicoureteral reflux (VUR). In VUR, some urine refluxes (goes back up) towards one or both kidney. In severe VUR, the reflux reaches into the kidney, causing the antenatal hydronephrosis. 


Some cases are caused by a blockage (or obstruction) that partially or fully stops the flow of urine out of the kidney. If this is suspected, you may be referred to a paediatric urologist (a surgeon who treats children with problems of the urinary system), who will monitor your child. 

There are different types of problems that cause a blockage. These may be diagnosed after a baby is born, and may need treatment. 

  • Posterior urethral valves (PUV): some boys are born with extra flaps of tissue in part of the urethra, the tube that carries urine out of the body. A long, thin tube (catheter) is inserted into the urethra to drain urine. PUV are removed by surgery. 
  • Pelviureteric junction (PUJ) dysfunction (or PUJ obstruction): a blockage between the renal pelvis (in the kidney) and the ureter, the tube that carries urine from the kidney to the bladder. In some cases, surgery is needed. 
  • Vesicoureteric junction (VUJ) dysfunction (or VUJ obstruction): a blockage between the ureter and the bladder. Many cases get better on their own, but in some cases, surgery is needed.
  • Duplex kidney: sometimes a kidney and ureter on one side of the body develop in two parts. In some cases, one of the ureters dilates (swells) where it goes into the bladder – this is called a ureterocele. In many cases, this will not need follow-up or treatment, but if this causes blockage or reflux, children may need more tests and occasionally surgery.

Will antenatal hydronephrosis affect other family members?

Antenatal hydronephrosis does not usually run in families, and it is not likely to happen in future pregnancies. A few conditions that cause antenatal hydronephrosis, such as VUR, may run in families and may be inherited from the mother or father. 

More information

  • Vesicoureteral reflux (VUR) and reflux nephropathy

    In VUR, some urine refluxes back up towards the kidneys; sometimes associated with scars on the kidneys (reflux nephropathy).

  • Posterior urethral valves (PUV)

    Some boys are born with posterior urethral valves (PUV). These are extra flaps of tissue that are in the tube that carries urine out of the body. PUV need to be removed by surgery.

  • Pelvi-ureteric junction (PUJ) dysfunction and obstruction

    PUJ dysfunction means a narrowing, hold up or blockage at the point where the pelvis of the kidney joins the tube from kidney to bladder (ureter).