title: Trisomies 18 and 13: trends in prevalence and prenatal diagnosis - population based study.
abstract: OBJECTIVE
  The objective of this study was to determine trends in prenatal detection and current estimates of prevalence for trisomies 18 (T18) and 13 (T13) and their implications for screening policy.
  
  
  METHODS
  We conducted a cohort study from a population-based regional anomaly register covering 995 003 births (1995-2009).
  
  
  RESULTS
  There were 786 affected cases. Total prevalence of T18 increased from 3.95 in 1995-1999 to 6.94 per 10 000 births in 2005-2009 (annual trend χ(2)  = 25.99, p < 0.001) and live birth prevalence, when adjusted for in utero attrition, increased from 1.47 to 2.30 per 10 000 births over the same time (annual trend χ(2)  = 6.36, p = 0.01). For T18 and T13 combined, the proportion of cases diagnosed by prenatal karyotype or suspected by ultrasound increased from 85.1% (165/194) in 1995-1999 to 95.2% (299/314) in 2005-2009 (p < 0.001). In 2005-2009, 50.3% of prenatal cytogenetic diagnoses for T18 and 38.5% of T13 were made after the discovery of first trimester ultrasound anomalies, and the majority, 56.4% (185/328), of affected pregnancies were karyotyped or had ended before 18 weeks.
  
  
  CONCLUSIONS
  T18 is increasing in prevalence because of maternal age and earlier surveillance. Prenatal diagnosis occurs mostly in the first trimester, without the intrinsic structures of a formal screening programme. These findings support the extension of first trimester combined screening to include T18 and T13.
authors.affiliation: West Midlands Perinatal Institute, Birmingham, UK. anntonks@nhs.net
authors.forename: A M
authors.initials: AM
authors.lastname: Tonks
authors.forename: A S
authors.initials: AS
authors.lastname: Gornall
authors.forename: S A
authors.initials: SA
authors.lastname: Larkins
authors.forename: J O
authors.initials: JO
authors.lastname: Gardosi
mesh_terms: D000328:Adult; D002887:Chromosomes, Human, Pair 18; D015331:Cohort Studies; D004314:Down Syndrome / Q000175:diagnosis* / Q000453:epidemiology*; D005260:Female; D006801:Humans; D007231:Infant, Newborn; D011153:Population; D011247:Pregnancy; D011296:Prenatal Diagnosis / Q000706:statistics & numerical data*; D015995:Prevalence; D012042:Registries; D014314:Trisomy / Q000175:diagnosis*; D000073842:Trisomy 18 Syndrome; D055815:Young Adult