PERINATOLOGY.com Information for you about your pregnancy
Patient Handout

Isolated Pyelectasis

The renal pelvis is the part of the kidney where urine collects before it travels to the bladder. When this area measures 4 to 10 millimeters wide, it is called fetal pyelectasis or renal pelvic dilation. When it measures more than 10 millimeters, it is called severe pyelectasis or hydronephrosis.

Isolated means no other problem was seen on the detailed ultrasound.

Medical illustration of the fetal kidneys, ureters, and bladder, showing a widened renal pelvis caused by a narrowing at the ureteropelvic junction.
Example of urine backing up above a narrowing at the ureteropelvic junction, called UPJ obstruction.
Most mild cases are not serious. Many babies improve before birth or after birth. Most babies with isolated pyelectasis are healthy.
Important: Pyelectasis is an ultrasound finding. It does not always mean there is a blockage. UPJ obstruction is one possible cause, but the cause is often not known before birth.

What is isolated pyelectasis?

  • The renal pelvis is a little wider than usual.
  • It may be seen in one kidney or both kidneys.
  • No other concern was found on the ultrasound.
  • It does not mean your baby's kidneys have stopped working.

How the urinary tract works

  • The kidneys make urine.
  • Urine moves down tubes called ureters.
  • The bladder stores urine.
  • Amniotic fluid around the baby is mostly made from urine.

What can cause it?

  • Often, no clear cause is found.
  • Urine may drain more slowly from the kidney.
  • A narrowing may be present where the kidney joins the ureter. This is called UPJ obstruction.
  • Less often, urine may flow backward from the bladder.

What happens during pregnancy?

  • Your care team may repeat the ultrasound later in pregnancy.
  • They will check the renal pelvis, kidneys, bladder, and amniotic fluid.
  • They will watch to see if the widening gets better, stays the same, or gets larger.

Do I need chromosome screening?

Mild pyelectasis can be a soft marker for Down syndrome. If your cell-free DNA or other chromosome screening was low risk, isolated mild pyelectasis usually does not need more chromosome testing.

If you have not had screening, your care team may offer cell-free DNA screening or another screening test.

What happens after birth?

  • Your baby may need a kidney ultrasound after birth.
  • The timing depends on how wide the renal pelvis is and whether one or both kidneys are involved.
  • Some babies may need to see a pediatric kidney or urinary specialist.
  • Only a small number need surgery or other treatment.

When should I call my care team?

  • Your baby is moving less than usual.
  • You have bleeding, leaking fluid, or painful contractions.
  • You have questions about your ultrasound or follow-up plan.
What should I remember? Isolated mild pyelectasis is common and often gets better. Your care team will watch your baby's kidneys and explain the next steps.

Updated July 2026

References
  1. Society for Maternal-Fetal Medicine. Consult Series #57: Evaluation and management of isolated soft ultrasound markers for aneuploidy in the second trimester.
  2. Society for Maternal-Fetal Medicine. Consult Series #4: Genitourinary anomalies.
  3. Nguyen HT, Benson CB, Bromley B, et al. Multidisciplinary consensus on the classification of prenatal and postnatal urinary tract dilation. J Pediatr Urol. 2014.