Your baby’s kidneys: Hydronephrosis/Pelviectasis

A common source of stress and anxiety in pregnancy is an abnormal finding on a prenatal ultrasound examination. One of the more common findings we deal with is dilatation of the renal collecting system, or hydronephrosis, in the fetus. Hydronephrosis is seen in 1-4% of all pregnancies and occurs more commonly in males. As part of a routine prenatal ultrasound, the baby’s kidneys are examined in a scan of the cross section of the abdomen. The funnel-like structure that drains the kidney is called the renal pelvis. If it appears abnormal or dilated, it is measured.  Mild dilatation of this renal pelvis, also called pyelectasis or pelviectasis is defined as a measurement between 4mm and 10mm in the second trimester. There can be confusion about the significance of this finding and what to do when it is found. Here are the facts:

  • The majority of all cases of hydronephrosis, but especially pelviectasis, will eventually be found to represent a clinically insignificant appearance of the kidney that does not represent obstruction
  • The most common cause of urinary tract obstruction is a narrowing where the ureter meets the kidney: ureteropelvic obstruction or UPJ obstruction
  • Most cases of pelviectasis do not worsen over the course of pregnancy and, therefore, do not alter pregnancy management
  • It is often recommended that pregnancies in which pelviectasis is seen at the 20 week ultrasound, have a repeat ultrasound between 34 and 37 weeks
  • If the pyelectasis is still present in the late 3rd trimester, the pediatrician should be informed so that appropriate postnatal follow up can be planned
  • Postnatal ultrasound follow up should be performed after the first week of life in a well hydrated newborn
  • 97% of newborns with pelviectasis during pregnancy will have normal kidneys or mild dilatation on the postnatal ultrasound
  • The finding of isolated hydronephrosis (pelviectasis) is no longer considered to increase the age related risk of Down’s Syndrome

We always hope that having the 20 week ultrasound will be an enjoyable experience allowing expectant parents to see their developing baby, learn the gender if they desire and feel confident that all is going well. Unfortunately, there many instances where joy can be overshadowed by unnecessary worry or concern produced by an ultrasound finding such as pelviectasis.


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