Fetal hydronephrosis; prevalence, natural history and postnatal consequences in an unselected population

Background. The aim of this study was to estimate the incidence of fetal hydronephrosis, the antenatal development and the postnatal findings in a low risk population. Methods. A 2-year retrospective study included almost 9,000 patients, and all cases found to have antenatal hydronephrosis, defined as an AP diameter >= 5 mm, had a structuralised ante- and postnatal follow-up. Results. Some 85 cases were found in the antenatal period, and 32 of these cases had a pathologic follow-up. Of these, 13 had an increase in severity of dilatation during pregnancy, 10 had stable findings, but no case where the dilatation decreased had any postnatal pathology. Furthermore, half of the cases found in the third trimester ( 18 of 32) also had an abnormal follow-up. Two cases of Down's syndrome were diagnosed after birth. Conclusion. Antenatal hydronephrosis was found in almost 1% (n = 85) of the study population. There was an almost 2% risk of Down's syndrome in this group. The majority of cases had an increase in severity and some were stable in the antenatal period, but it was not possible to predict the individual postnatal outcome. In the group where the finding diminished in the antenatal period, no patient had an abnormal postnatal follow-up. Female fetuses with hydronephrosis seem to have an adverse postnatal outcome compared to male fetuses.