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Renal Dilation
of the Renal Tract and Hyronephephrosis

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    Renal tract
    dilation and
      Upper renal
      tract obst.
      Lower renal
      tract obst.
    dysplastic kidney
    Polycystic kidney
    Renal agenesis


Introduction Antenatal Postnatal West Midlands Data

Any part of the renal tract can become dilated and common sites are the collecting system within the kidney (hydronephrosis), the ureter (megaloureter), or the bladder (megacystis).

Blockage of the renal/urinary tract can be physical, where there is a narrowing (stenosis) or complete block (atresia) to the flow of urine, or functional, where the tubes are open (patent) but are not working properly to propel the urine along. A failure of function of the pelvi-ureteric junction (PUJ obstruction), reflux of urine back up the ureter from the bladder, a blockage at the uretero-vesicle junction or blockage along the urethra.

Complete obstruction in early fetal life causes renal hypoplasia and dysplasia. Intermittent obstruction may allow normal renal development. Obstruction in later pregnancy will result in hydronephrosis and severity of renal impairment depends on the degree and duration of obstruction.


Mild hydronephrosis, sometimes called dilated renal pelvis (DRP) or pyelectesis, is a common finding on antenatal ultrasound scans. There is some evidence that this finding on a mid-pregnancy scan increases the chances of the fetus being affected by Down's syndrome, and some authorities have recommended karyotyping for this reason. Most fetal medicine specialist in this country would not recommend this approach unless there is some other risk factor for Down's syndrome, such as advanced maternal age, other structural anomaly, or abnormal serum screening results.

Hydronephrosis is the most common fetal renal anomaly. Mild degrees of transient hydronephrosis are present in many fetuses during pregnancy, possibly due to the high levels of maternal hormones that cause the smooth muscle of the urinary tract to relax.
Dilation of the urinary tract signifies obstruction. The size of the renal pelves is dependent on gestational age but calyceal dilation is more significant and usually indicates significant hydronephrosis. It is important to delay postnatal assessment until 48 hours as mild to moderate hydronephrosis may not be present in the first day of life due to dehydration.


© Perinatal Institute 2011