Significant Chromosomal Aberrations in Fetuses with Isolated Hydronephrosis: A Nationwide Register-Based Study Including a Low-Risk Population of 580,090 Pregnancies

Introduction: In cases of isolated fetal hydronephrosis, guidance on genetic testing remains unstructured due to lack of evidence. We aimed to assess the prevalence of chromosomal aberrations in fetuses with isolated hydronephrosis detected at the second-trimester anomaly scan in a low-risk population. Methods: A register-based nationwide cohort study of all singleton pregnancies with prenatal second-trimester ultrasound screening and due date between 2009 and 2018, retrieved from the Danish Fetal Medicine Database, which includes pregnancy, child and genetic data (karyotypes and chromosomal microarray). We included fetuses with isolated hydronephrosis (renal pelvis ≥5 mm) before gestational age 23+0. Fetuses without a genetic test performed were considered genetically healthy. Chromosomal aberrations were interpreted by a clinical geneticist. Pathogenic and likely pathogenic findings were considered significant. Results: Of 580,090 singleton pregnancies, 1,899 fetuses had isolated hydronephrosis. A total of 270 (14.22%) cases underwent chromosomal analysis Chromosomal aberrations were found in 11 cases. Seven of these were classified as significant, resulting in an overall prevalence of significant chromosomal aberrations in 0.37% (95% CI 0.10–0.64). Conclusion: In relation to former results of background risk, our results do not imply an increased prevalence of chromosomal aberrations in case of isolated hydronephrosis before gestational age 23+0.