EFFECT OF ROUTINE SCREENING FOR DOWNS-SYNDROME ON THE SIGNIFICANCE OFISOLATED FETAL HYDRONEPHROSIS

Objective To determine the risk of Down's syndrome in fetuses with iso lated hydronephrosis at 18-23 weeks in an unselected general populatio n after routine screening for Down's syndrome, using first trimester n uchal translucency measurement and second trimester maternal serum bio chemistry. Population All pregnant women undergoing a routine 18-23 we ek ultrasound scan, from a population who had been offered screening f or Down's syndrome. Setting A district general hospital serving a low risk obstetric population. Methods Prospective study of all routine 18 -23 weeks ultrasound scans. The prevalence of isolated hydronephrosis and Down's syndrome was determined and the relative risk for Down's sy ndrome was calculated for different ultrasound findings. Results 10,97 1 women were scanned at 18-23 weeks during the study period. Down's sy ndrome was diagnosed in 14 of 20 cases before this stage using first t rimester nuchal translucency measurement and second trimester maternal serum biochemistry. Isolated fetal hydronephrosis was diagnosed in 42 3 pregnancies (3.9%); none of these pregnancies were affected by Down' s syndrome. The relative risk for Down's syndrome was 0.18 (95% Cl 0.0 6-0.53) for women with a normal scan (n = 9983). When multiple ultraso und markers were found (n = 565), the relative risk for Down's syndrom e was 2.00 (95% Cl 0.18-22.10) and 9.00 (95% Cl 1.14-71.30) for all ot her aneuploidies. Conclusion The finding of isolated fetal hydronephro sis does not significantly increase the age-related risk for Down's sy ndrome. The presence of multiple ultrasound markers is associated with an increased risk of aneuploidies other than Down's syndrome. These f indings are explained by the reduced prevalence of Down's syndrome as a result of prior screening and diagnosis of this condition.