A pyelectasis is a dilatation of part of the kidney. At 21 weeks, the renal pelvis can often not be seen separately from the kidney. Renal Pelvis Dilation or Dilatation (RPD) is the term used to describe a visible renal pelvis, usually greater than 4 mm at or after 18 weeks.

There are several problems associated with the detection of renal pelvis dilation in the second trimester. One is an increased risk of chromosome abnormalities such as Down syndrome. The other is the possibility of kidney damage and impaired renal function after birth.

When the doctor finds RPD, a thorough scan of the fetus is warranted to see if there are other malformations. If any other structural malformations are seen, then the incidence of chromosome abnormalities is increased to about 10%. When no other abnormalities are seen, then the incidence of a chromosome anomaly is about 0.5%. Only an amniocentesis can completely exclude a chromosome anomaly.