Mild pyelectasis is diagnosed when:

         The renal pelvis anteroposterior diameter is greater than 4 or 5 mm and less than 10 mm.

         Corteville et al (1) in a study evaluating the ability of six different ultrasonographic parameters to predict postnatally confirmed congenital hydronephrosis, found that an:

1.      anteroposterior diameter of greater than or equal to 4 mm before 33 weeks and greater than or equal to 7 mm after 33 weeks had a sensitivity of 100%, but a false-positive rate of 21% to 55% (depending on the gestational age).

2.      If one is screening for Down syndrome, however, it is preferable to minimize the false-positive rate to less than 5% so as to avoid unintended fetal loss as a result of invasive procedures. Corteville et al (2), using the parameters of greater than or equal to 4 mm before 33 weeks and greater than or equal to 7 mm after 33 weeks, found the overall sensitivity of pyelectasis for the detection of Down syndrome to be 17%. This dropped to 4% when pyelectasis was an isolated finding. The false-positive rate in this series was 2%, with a positive predictive value of isolated pyelectasis for Downs of 1 in 340 (2).






Gestational age-based criteria for the diagnosis of fetal renal pyelectasisa

Study / Reference

Definition (AP diameter of renal pelvis)


Benacerraf 1990 (3)

≥ 4 mm 16–20 wk


≥ 5 mm 20–30 wk

≥ 7 mm 30–40 wk

Corteville 1991 (1)

≥ 4 mm < 33 wk


≥ 7 mm > 33 wk

Corteville 1992 (2)

≥ 4 mm < 33 wk


≥ 7 mm > 33 wk

Adra 1995 (4)

≥ 4 mm < 33 wk and ≤ 10 mm


≥ 7 mm > 33 wk and ≤ 10 mm

Langer 1996 (5)

≥ 5 mm < 28 wk


≥ 10 mm > 28 wk

Wickstrom 1996a (6)

≥ 4 mm all gestational ages


Wickstrom 1996b (6)

≥ 4 mm < 33 wk


≥ 7 mm > 33 wk

Chudleigh 2001 (7)

≥ 5 mm and ≤ 10 mm at 16–26 wk


Broadley 1999 (8)

≥ 5 mm > 18 wk


≥ 10 mm > 30 wk

Ismaili 2003 (9)

≥ 4 mm < 33 wk and ≤ 10 mm


≥ 7 mm > 33 wk and ≤ 10 mm

Bobrowski 1997 (10)

≥ 4 mm < 32 wk


≥ 7 mm ≥ 32 wk



Some pyelectasis can be commonly seen in normal fetuses, particularly after 24 weeks with dilatations of:

         1 to 2 mm noted in 41%, and

         3 to 11 mm in 18% of routine examinations (17).

         Theories as to causes include maternal hormone influence and maternal volume expansion (14).

Persutte et al (26) noted great variability in renal measurements over time among fetuses imaged over a 2-hour period.

         Fetal pyelectasis is more often bilateral,

         When unilateral it is more likely to be on the left side (11,12).

         It is more common in male fetuses both prenatally and postnatally (1,11, 7,4,13).

         The size of the fetal renal collecting system seems to be highly variable over time. Persutte et al (13) performed intermittent ultrasound measurements (every 15 minutes) on 20 fetuses with pyelectasis. Overall, the mean variation (minimum to maximum) for the transverse anteroposterior diameter was 3.8 2.49 mm. Seventy percent of cases had both normal and abnormal values during the 2-hour study period (< 4 mm and ≥ 4 mm, respectively).

         Fetuses may be more likely to have pyelectasis if they have a full bladder, or if the women themselves have concomitant pyelectasis (14,15).

         The effect of maternal hydration on fetal pyelectasis is unclear; aggressive maternal hydration did not increase the risk of fetal pyelectasis in some series but did in others (16-18).

         Laterality does not seem to be useful for prognosis.

         In one study, bilateral pyelectasis was more likely to progress to hydronephrosis than unilateral pyelectasis (26% versus 3%, respectively) (10).

         Another study showed that resolution was more likely to occur in fetuses with bilateral pyelectasis (39% versus 18% resolution in unilateral fetuses, P = .046) and that fetuses with unilateral pyelectasis had a significantly higher incidence of urinary tract pathology at birth (59% versus 34% in bilateral fetuses, P = .03) (4).

         Gender does not seem to influence rates of progression or resolution of pyelectasis (4,10). The distribution of urologic abnormalities is also similar between male and female infants, although one study showed a trend toward a higher incidence of vesicoureteral reflux in male fetuses (4).

         There are no data on whether unilateral or bilateral pyelectasis is more or less likely to be associated with Down syndrome.

18 Weeks of gestation – AP diameter of left kidney = 4.5 mm

26.5 weeks of gestation – AP diameter of left kidney = 7.8 mm





Develop postnatal uropathy

Increased risk for
chromosomal abnormalities

Regression lowers the risk but does not completely eliminate it.

Increased likelihood:
* In utero progression.
* Contralateral dilatation.
* Male gender.
* Increased renal length.
* Degree of pyelectasis.
* Visualization of ureters on any scan after the 1st
* Dilated bladder.

Down syndrome
  - 3.9 fold
All chromosomes
  - 3.3 fold increase.




         An association of pyelectasis with aneuploidy (primarily Down syndrome) was first suggested in 1990 when, in a selected high-risk population, 25% of fetuses with Down syndrome were noted to have pyelectasis compared with 2.8% of fetuses with normal karyotype (3).

         Other studies, mainly in high-risk, selected populations, have supported this finding by showing that fetal pyelectasis is associated with an increased risk for both Down syndrome and other chromosomal abnormalities, although a few studies have found no association (1,4,11,12,19-22). Pyelectasis has also been associated with an increased risk of other fetal anomalies (1). Not surprisingly, the association of pyelectasis with Down syndrome is strongest when other anomalies are present (12,23).

         The largest series of fetal pyelectasis, a multicenter, prospective, observational study of unselected fetuses examined between 16 and 26 weeks, identified 737 fetuses with mild pyelectasis in a population of 101,600 births.

         Of these 737 fetuses, 12 (1.7%) had chromosomal abnormalities (six trisomy 21, one trisomy 13, one trisomy 8, two Turner's syndrome, one unbalanced translocation, and one 47,XXX).

         Of the 12 fetuses with chromosomal abnormalities, 9 had associated sonographic abnormalities and one mother was advanced maternal age (AMA); only two chromosomal abnormalities occurred in the setting of isolated pyelectasis in low-risk women (0.3%). This study showed the risk of aneuploidy in a fetus with isolated mild pyelectasis to be 0.33% and 2.22% in women less than 36 and greater than or equal to 36 years, respectively.

The most important question is whether isolated mild pyelectasis is associated with an increased risk of aneuploidy in a low-risk population, because those who are already at a higher risk for aneuploidy are offered invasive testing whether or not mild pyelectasis is present.

         There are little data on the ability of isolated pyelectasis to predict aneuploidy in a low-risk unselected population.

         A recent retrospective study reviewed the ultrasounds of 25,586 primarily low-risk, unselected women and found 320 cases of pyelectasis (defined as ≥ 5 mm anteroposterior diameter) for an incidence of 1.25% (11). Nineteen of the fetuses with pyelectasis had associated sonographic anomalies, and in 301 (incidence 1.18%) pyelectasis was an isolated finding. None of the fetuses in this series had aneuploidy. Although the authors state that this study was primarily in a low-risk population, there were some women in this series who were AMA. More studies are needed to assess the predictive value of isolated mild pyelectasis in a low-risk population, particularly when taking into account the results of multiple marker screening and nuchal translucency.


Data on the natural history of pyelectasis, particularly of mild pyelectasis, are confusing because of the heterogeneous nature of this literature.

         For approximately 60% to 70% of fetuses, the pyelectasis remains stable, improves, or completely resolves on subsequent examinations.

         The remainder, approximately one third to one quarter of fetuses, has progression of their pyelectasis (1,4,6,11,13).

         In utero progression of pyelectasis does not necessarily predict postnatal uropathy, but it does put the fetus at increased risk for clinically significant disease. Of those kidneys showing progression, regression, or no change of isolated pyelectasis, postnatal uropathy was noted in 60%, 23%, and 32% of fetuses, respectively, in one series looking at 105 fetuses with pyelectasis (6). When progression of pyelectasis occurred prenatally in this series, the probability of corrective surgery in the infant was 50%.

         A recent series described the natural history of pyelectasis in 213 fetuses (426 kidneys) with only minor degrees of dilation (mild and moderate pyelectasis) in an unselected population (9). In this series, fetuses were examined sonographically in both the second and third trimester. Fetuses were categorized as having had pyelectasis detected in the second trimester only, the third trimester only, or in both the second and third trimesters.

         In this series, 38% of fetuses with antenatal evidence of pyelectasis at any time had normal urinary tracts on postnatal examination.

         Sixty-two percent of fetuses (42% of all kidneys) had postnatal renal abnormalities.

         Sixty-three percent of those with postnatally detected disease had significant nephrouropathies that required long-term medical care (39% of fetuses overall).

         Fetuses were more likely to have postnatal renal abnormalities if they had pyelectasis in the third trimester. The presence of pyelectasis in the second trimester only, however, did not completely rule out postnatal abnormalities, because 12% of those fetuses whose pyelectasis had resolved on the third-trimester scan had significant nephrouropathy.

Postnatal abnormalities found in 213 infants (426 kidneys)

with prenatally diagnosed mild and moderate pyelectasis

Pathologic condition

Kidneys (%)

Pelviureteric junction stenosis


Primary vesicoureteric reflux


Transitory hydronephrosis


Mild idiopathic dilation




Complicated renal duplication


Noncomplicated renal duplication


Other pathology


Total kidneys with pathology


Ismaili K, Hall M, Donner C, Thomas D, Vermeylen D, Avni FE. Results of systematic screening for minor degrees of fetal and renal pelvis dilation in an unselected population. Am J Obstet Gynecol 2003;188:242–246.


In the chromosomally normal fetus, the predictive value of prenatal findings for long-term postnatal genitourinary pathology remains uncertain. The best predictions can be made based on the presence of pyelectasis in the third trimester and the severity of the pyelectasis (4, 9, 24, 25). Overall, the prognosis for most patients with pyelectasis is good, particularly when there are no associated anomalies, and particularly when the degree of dilation is mild.

Outcome by age 4 of fetuses with prenatally diagnosed bilateral pyelectasis



Chronic renal failure (%)


death (%)

Genitourinary surgery (%)


5-10 mm






10-15 mm <30 wks

10-20 mm >30 wks






>15 mm < 30 wks

>20 mm > 30 wks





Broadley P, McHugo J, Morgan I, Whittle MJ, Kilby MD. The 4 year outcome following the demonstration of bilateral renal pelvic dilatation on pre-natal renal ultrasound. Br J Radiol 1999;72:265–270.

In the above series, a cohort of 75 fetuses with bilateral pyelectasis were identified and followed until 4 years of age. None of the fetuses with mild pyelectasis had chronic renal failure or neonatal death by age 4, although 5% of them required surgery. Of the fetuses with moderate and severe pyelectasis, 9% of the children went on to develop chronic renal failure, 15% suffered neonatal deaths, and 36% required surgery. Three of the five neonatal deaths in the moderate-severe group had severe associated anomalies; the other two children had posterior urethral valves. This study reaffirms the association between the incidence and severity of postnatal genitourinary disease with the degree of in utero pyelectasis.


A strong positive correlation has been reported (5) between renal pelvis size and fetal bladder volume. The AP size of the fetal renal pelvis diminished from 6.8 1.8mm with a full bladder to 4.5 1.6mm when the bladder emptied. Fifty-three percent of fetuses with renal pelvic measurements of 5mm with a full bladder had normal appearing renal pelvises when their bladders were emptied.


As do any other soft markers, the detection of pyelectasis should prompt a thorough evaluation for concomitant abnormalities. A fetal echocardiogram can be considered to evaluate the fetal heart comprehensively. In the absence of other anomalies, soft markers, or risk factors for aneuploidy (such as maternal age), amniocentesis for isolated mild pyelectasis does not seem to be warranted.

Because 30% of cases with mild pyelectasis advance to hydronephrosis, these evaluation in the third trimester (preferably after 28 weeks) is recommended to identify worsening or persistent cases. In the absence of oligohydramnios, patients can be delivered at term. Prenatal sonography does not seem to be sensitive enough to differentiate those cases with mild pyelectasis that develops postnatal uropathy from those that do not. Therefore, it is recommended that all infants with persistent mild fetal pyelectasis undergo some degree of postnatal evaluation or surveillance.


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