Renal hypoplasia (or kidney hypoplasia) means that part of a kidney does not fully develop in the womb.

The kidney may be only slightly smaller than usual or it may be tiny. Because of its size, it may not work as well as a normal-sized kidney.

Renal hypoplasia is relatively common – it is estimated that one baby in a few hundred will be born with one hypoplastic (small) kidney.

Renal hypoplasia may be picked up before birth on the 20 week antenatal ultrasound scan, or soon after birth. It may also be picked up in an older child who has some symptoms.

  • If this affects one kidney, it is called unilateral renal hypoplasia. Many children with one small kidney do not have long-term problems, but may need to go back to the doctor for tests.
  • If this affects both kidneys, it is called bilateral renal hypoplasia. This is more serious. These children need follow-up throughout their lifetimes to check for any long-term problems.

Doctors understand that there are some possible causes of renal hypoplasia, though it may not always be possible to identify the cause in your baby. It is not usually caused by anything that the mother does during her pregnancy.

Renal hypoplasia: tests and diagnosis

Antena​tal ultrasound scan

The 20 week antenatal ultrasound scan looks at your baby growing in the womb. Renal hypoplasia may be suspected if one or both of the kidneys look smaller than usual but otherwise normal.

The scan cannot always diagnose (identify) the problem. Although your doctor will not always know how your baby will be affected at birth, he or she is less likely to have significant problems if:

  • he or she is growing well in the womb
  • no other problems have been found, and
  • there is a normal amount of amniotic fluid.

You may need to go back to the hospital for more ultrasound scans during pregnancy.

Diagnosis later in childhood

Sometimes, renal hypoplasia is only picked up after birth or when a child is older. It is usually found during a scan that a child is having for another reason, such as a urinary tract infection (UTI) or after an accident.

Other conditions that look like renal hypoplasia

Sometimes, renal hypoplasia is thought to be another type of problem with the kidney(s).

  • Renal dysplasia – one or both kidneys are smaller than usual, but have also not developed properly and may have cysts
  • Multicystic dysplastic kidney (MCDK) – a more severe form of renal dysplasia. The whole of the affected kidney is a bundle of many cysts, and does not work.
  • Reflux nephropathy – scars on the kidney.

Other conditions associated with renal hypoplasia

Sometimes, renal hypoplasia can be seen with other conditions that happen in the womb.

  • Antenatal hydronephrosis – one or both kidneys do not drain urine properly – usually because there is a blockage affecting how urine leaves the body. The affected kidney can become stretched and swollen. Antenatal hydronephrosis often gets better at a later stage in the pregnancy, but your doctor will check how your baby is affected.
  • Vesicoureteral reflux (VUR) – when babies with VUR pass urine in the womb, some urine refluxes (goes back up) towards, and sometimes into, the kidneys. This can affect the fully working kidney and/ or the hypoplastic kidney.

Tests after birth

After your baby is born, he or she may need some imaging tests (scans). These use special equipment to get images (pictures) of the inside of the body. They are used to confirm that your child has renal hypoplasia and look for any complications.

Bilateral renal hypoplasia monitoring

In bilateral renal hypoplasia, both kidneys are smaller than usual. (‘Bilateral’ means two sides.) Some babies born with two small kidneys have no immediate complications. Others need more support at birth, including ventilation to help them breathe.

All children with bilateral renal hypoplasia need monitoring, as some may go on to develop kidney failure. This occasionally happens while a baby or child is young, but is more likely to occur later in life, especially during puberty when children’s bodies grow quickly. Children need to go back to the hospital or clinic throughout childhood to check how well their kidneys are working. Treatment can be started as soon as it is needed, to help your child grow and remain healthy. Eventually the kidneys may stop working and dialysis may be needed.

Renal hypoplasia: about the name

Renal hypoplasia can be broken down:

  • renal: to do with one or both kidneys
  • hypo: fewer than usual
  • plasia: to do with development

It is one type of congenital anomaly of the kidneys and urinary tract. ‘Congenital’ means that the problem is present at birth and ‘anomaly’ means different than normal.

Causes of renal hypoplasia

Renal hypoplasia is relatively common – it is estimated that one baby in a few hundred will be born with one hypoplastic (small) kidney.

It is not always possible to know why renal hypoplasia happens. In the majority of cases, it is not caused by anything that the mother does during her pregnancy, and it is unlikely that a future pregnancy will result in renal hypoplasia or other problems with the kidneys.

Occasionally a specific cause is found.

Genetic mutations (inherited)

Most cases of renal hypoplasia are not inherited from the baby’s mother or father. However, some cases are caused by genetic mutations. These are problems in the genes (which are in each of our body’s living cells), which are passed on by the parents.

If your doctor thinks your baby has a problem that is caused by genetic mutations, you may be referred for genetic testing and counselling. Genetic testing usually involves getting a sample of blood or body tissue, which can be checked for a specific gene. Genetic counselling is a service that can give you information and guidance about conditions caused by genetic mutations.

Occasionally, renal hypoplasia is part of a syndrome, which a collection of symptoms and signs. These children may also have other problems, such as with their digestive system, nervous system, heart and blood vessels, muscles and skeleton, or other parts of their urinary system. These may be caused by genetic mutations.

Medicines and other drugs

A few cases of renal hypoplasia are caused by some medicines taken by the pregnant woman – including some prescribed medicines for seizures (also called convulsions or fits) or high blood pressure (such as angiotensin-converting enzyme (ACE) inhibitors and angiotensin receptor blockers (ARBs)). Speak with your doctor about the medicines that you are taking.

It may be also be caused by some illicit drugs, such as cocaine.

Will it happen in future pregnancies?

Unless you have been told that your baby’s renal hypoplasia was caused by a genetic mutation, it is unlikely that a future pregnancy will result in renal hypoplasia, or other problems with the kidneys. If one of your children has renal hypoplasia, it is unlikely that another of your children or family members will get it. Your doctor or healthcare professional will be able to give you more information.

More information

  • About the urinary system and kidneys

    If your child has a health condition that affects their kidneys or another part of the urinary system, you may wish to find out more.

  • Unilateral renal hypoplasia

    In unilateral renal hypoplasia, one kidney is smaller than usual because it has not fully developed while a baby is growing in the womb. The other kidney usually looks and works as normal. Most people have no long-term problems with unilateral renal hypoplasia, and some do not know that they have one kidney that is smaller than usual.

  • Bilateral renal hypoplasia

    In bilateral renal hypoplasia, both kidneys are smaller than usual because they have not fully developed in the womb.