Supplementary Material for: Significant chromosomal aberrations in fetuses with isolated hydronephrosis – a nationwide register-based study including a low-risk population of 580,090 pregnancies

Objective
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 2009-2018, 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 to have no pathogenic chromosomal aberrations. Chromosomal aberrations were interpreted by a clinical geneticist, and 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, leading to a detection of chromosomal aberration 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 of chromosomal aberrations, our results do not imply an increased prevalence of chromosomal aberrations in case of isolated hydronephrosis before gestational age 23+0.