title: Effect of routine screening for Down's syndrome on the significance of isolated fetal hydronephrosis.
abstract: OBJECTIVE
  To determine the risk of Down's syndrome in fetuses with isolated hydronephrosis at 18-23 weeks in an unselected general population after routine screening for Down's syndrome, using first trimester nuchal translucency measurement and second trimester maternal serum biochemistry.
  
  
  METHODS
  All pregnant women undergoing a routine 18-23 week ultrasound scan, from a population who had been offered screening for Down's syndrome.
  
  
  METHODS
  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 syndrome was calculated for different ultrasound findings.
  
  
  RESULTS
  10,971 women were scanned at 18-23 weeks during the study period. Down's syndrome was diagnosed in 14 of 20 cases before this stage using first trimester nuchal translucency measurement and second trimester maternal serum biochemistry. Isolated fetal hydronephrosis was diagnosed in 423 pregnancies (3.9%); none of these pregnancies were affected by Down's syndrome. The relative risk for Down's syndrome was 0.18 (95% CI 0.06-0.53) for women with a normal scan (n = 9983). When multiple ultrasound markers were found (n = 565), the relative risk for Down's syndrome was 2.00 (95% CI 0.18-22.10) and 9.00 (95% CI 1.14-71.30) for all other aneuploidies.
  
  
  CONCLUSIONS
  The finding of isolated fetal hydronephrosis does not significantly increase the age-related risk for Down's syndrome. The presence of multiple ultrasound markers is associated with an increased risk of aneuploidies other than Down's syndiome. These findings are explained by the reduced prevalence of Down's syndrome as a result of prior screening and diagnosis of this condition.
authors.affiliation: Feto-Maternal Medicine Unit, Homerton Hospital, St Bartholomew's and the Royal London School of Medicine and Dentistry, UK.
authors.forename: M O
authors.initials: MO
authors.lastname: Thompson
authors.forename: B
authors.initials: B
authors.lastname: Thilaganathan
mesh_terms: D000328:Adult; D004314:Down Syndrome / Q000175:diagnosis*; D005260:Female; D005315:Fetal Diseases / Q000175:diagnosis*; D006801:Humans; D006869:Hydronephrosis / Q000175:diagnosis*; D008423:Maternal Age; D009333:Neck / Q000000981:diagnostic imaging / Q000196:embryology; D011247:Pregnancy; D011296:Prenatal Diagnosis / Q000379:methods*; D011446:Prospective Studies; D012307:Risk Factors; D016216:Ultrasonography, Prenatal / Q000379:methods