Fetal hydronephrosis is dilatation of the fetal renal collecting system. The term is variably defined and at times can overlap with the term fetal pyelectasis (which usually implies a milder form).
Depending on its definition, the estimated prevalence may range between 1-3% of pregnancies.
Marked hydronephrosis (particularly in the 3rd trimester) can result from a number of pathologies which include
- posterior urethral valves: in males
- congenital pelvi-ureteric junction obstruction: one of the commonest causes of fetal hydronephrosis ref
- congenital vesico-ureteric junction obstruction
- urethral agenesis
- congenital megaloureter
- megacystis microcolon syndrome
- megacystis megaureter syndrome 7
- fetal vesico-ureteric reflux
In the second trimester, severity of fetal hydronephrosis may be graded as 1-2.
- mild hydronephrosis: anteroposterior renal pelvic diameter (APRPD) measures ≥5 mm (≥4 mm at 16-20 weeks)
- moderate/severe hydronephrosis: APRPD measures ≥7 mm or if there is associated calyceal dilatation
- persistent hydronephrosis: ≥10 mm in the 3rd trimester
Ancillary sonographic features
- dilatation of the renal calyces
- dilated ureter if obstruction is distal
- concurrent oligohydramnios if there is 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 (APD) of 9 mm or more, and a pelvic-to-renal APD 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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- 7. Twining P, McHugo JM, Pilling DW. Textbook of fetal abnormalities. Elsevier Health Sciences. (2007) ISBN:044307416X. Read it at Google Books - Find it at Amazon