|Enlargement of the part of the kidney that collects urine
(the renal pelvis) from 4 to 10 millimeters in diameter
is called fetal pyelectasis or “renal pelvic dilatation”.
Dilation of the renal pelvis to greater than 10 mm is called severe
pyelectasis or hydronephrosis. The illustration at right shows a normal
kidney compared to a kidney with pyelectasis.
Pyelectasis is detected
in 1 per 100 babies before birth, and is seen more commonly in male
fetuses. About 1 in 500 cases develops significant disease. Most mild cases resolve before
birth or within a few months after delivery. .
A fetus with severe pyelectasis or
hydronephrosis is at increased risk of
developing problems such as low amniotic fluid and kidney disease. In addition
pyelectasis has a weak association with chromosome abnormalities, in
particular Down syndrome. The severity of isolated pyelectasis does
not appear to increase the risk for chromosome
What Causes It?
The most common causes of severe pyelectasis or hydronephrosis are a blockage of the ureter (the tube that carries urine from the kidney to
the bladder) where it joins the kidney and backward
flow of urine from the bladder (vesicoureteric reflux).
Less common causes of hydronephrosis include posterior urethral valves, urethral
atresia, ectopic ureteroceles, duplication of the collecting system,
megacystis-microcolon-intestinal-hypoperistalsis syndrome, prune belly syndrome,
and cloacal malformation.
Does Pyelectasis Need Treatment?
Pylectasis involving only one kidney does not need intervention, but requires
followed up ultrasound evaluation after 28 weeks for possible worsening of the
Severe hydronephrosis involving both kidneys is associated with increased risk
of poor outcome, and is monitored for the development of low amniotic fluid.
An ultrasound is usually done 5 to 7 days after
birth to reevaluate the infant’s kidneys. Most cases of mild pyelectasis where
the renal pelvis measures less than 8 mm and involves only one kidney tend to
resolve spontaneously either in utero or within a few months after delivery.
Pyelectasis greater than 8 mm is more likely to need surgical correction during
What are the Chances of Having Pyelectasis Again
in a Future Pregnancy?
In a study of 64 women with the finding of pyelectasis in their fetuses, 43 (67%) had a recurrence
of this finding in their next pregnancy.
What is the Risk of Down Syndrome in a Fetus with Pyelectasis?
Presently genetic testing of the fetus (amniocentesis) is offered if the risk of
Down syndrome in the fetus is ~ 1 in 200 or greater. For a 34 year old woman the
isolated finding of pyelectasis at midtrimester increases the risk of Down
syndrome from 1 in 365 to 1 in 192.
For a woman 34 years of age or less with a normal triple marker screen for Down
syndrome the risk benefit ratio does not favor amniocentesis when the only
ultrasound finding is pyelectasis.