fetal hydronephrosis

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Fetal hydronephrosis represents the abnormal dilatation of the fetal renal collecting system, with pelviureteric junction obstruction the most commonly encountered cause.

Please, refer to the article on fetal pyelectasis for a dedicated discussion on this relatively common and usually benign form of mild hydronephrosis.

Terminology

Although there is an overlap of definition between pyelectasis and hydronephrosis, the former has been widely used instead of mild hydronephrosis given that the great majority of the cases represent only a physiological incidental finding that resolves spontaneously, while the latter tends to be reserved for cases where a pathological obstruction is suspected.

Epidemiology

Depending on its definition, the estimated prevalence may range between 1-3% of pregnancies.

Pathology

Marked hydronephrosis (particularly in the 3 trimester) can result from a number of pathologies, which include:

posterior urethral valves: in males
congenital pelviureteric junction obstruction: the commonest cause of fetal hydronephrosis
congenital vesicoureteric junction obstruction
ureterocoele(s)
urethral agenesis
congenital megaloureter
megacystis microcolon syndrome
megacystis megaureter syndrome
fetal vesicoureteric reflux
prune-belly syndrome (rare)

Associations

Down syndrome: more so with mild renal pelvic dilatation (low association)

Radiographic features

Antenatal ultrasound

In the second trimester, the severity of fetal hydronephrosis may be graded as .

mild hydronephrosis: anteroposterior renal pelvic diameter measures ≥5mm (≥4mm at 16-20 weeks)
moderate/severe hydronephrosis: anteroposterior renal pelvic diameter measures ≥7 mm or if there is associated calyceal dilatation
persistent hydronephrosis: ≥10 mm in the 3 trimester

Ancillary sonographic features:

dilatation of the renal calyces
dilated ureter if the obstruction is distal
concurrent oligohydramnios if there is a bilateral obstruction (or unilateral obstruction with significantly impaired renal function in the other kidney)

Treatment and prognosis

Management will depend on the underlying pathology. The degree of renal pelvic dilatation correlates with the outcome. A renal pelvic anteroposterior diameter of 9 mm or more, and a pelvic-to-renal anteroposterior diameter ratio of 0.45 before 32 weeks of gestation and 0.52 thereafter are considered to be useful for the detection of a severe outcome postnatally .