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.
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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.
Associations
Down syndrome: more so with mild renal pelvic dilatation (low association)
Pathology
Etiology
Marked hydronephrosis (particularly in the 3rd trimester) can result from a number of pathologies, which include:
posterior urethral valves: in males
congenital pelviureteric junction obstruction: the most common cause of fetal hydronephrosis 8
congenital vesicoureteric junction obstruction
fetal vesicoureteric reflux
prune-belly syndrome (rare)
Radiographic features
The degree of fetal hydronephrosis can be graded by urinary tract dilatation classification.
Ultrasound
In the second trimester, the severity of fetal hydronephrosis may be graded as 1,2.
mild hydronephrosis: anteroposterior renal pelvic diameter measures ≥5 mm (≥4 mm 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 third 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 3.
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