title: Selective miscarriage of Down's syndrome fetuses in women aged 35 years and older.
abstract: OBJECTIVE
  To estimate the fetal loss of Down's syndrome fetuses between the time of chorionic villus sampling (10 weeks gestation) and the time of amniocentesis (16 weeks gestation) and term in women aged 35 years and older.
  
  
  METHODS
  The age specific prevalence rates in the first trimester of Down's syndrome were estimated using the Danish cytogenetic register in combination with results from four published studies. These were compared with the reported prevalence at the time of amniocentesis and at birth.
  
  
  METHODS
  5927 singleton pregnancies undergoing chorionic villus sampling (71 cases of Down's syndrome and 5856 unaffected cases). This was combined with published data on a further 231 cases of Down's syndrome and 16,620 unaffected cases.
  
  
  METHODS
  Age specific prevalences at the time of chorionic villus sampling. Proportion of pregnancies lost between the time of chorionic villus sampling and the time of amniocentesis and term.
  
  
  RESULTS
  Thirty-two percent of Down's syndrome pregnancies are lost between the time of chorionic villus sampling (10 weeks) and the time of amniocentesis (16 weeks) and 54% are lost by term.
  
  
  CONCLUSIONS
  The high fetal loss rates of Down's syndrome between the time of chorionic villus sampling and term introduce problems when evaluating first trimester screening tests with respect to their effective detection rates at term. A recommendation for quoting term risks is made.
authors.affiliation: Academic Department of Reproductive Physiology, Obstetrics and Gynaecology, St. Bartholomew's Hospital Medical College, London, UK.
authors.forename: M C
authors.initials: MC
authors.lastname: Macintosh
authors.forename: N J
authors.initials: NJ
authors.lastname: Wald
authors.forename: T
authors.initials: T
authors.lastname: Chard
authors.forename: J
authors.initials: J
authors.lastname: Hansen
authors.forename: M
authors.initials: M
authors.lastname: Mikkelsen
authors.forename: A J
authors.initials: AJ
authors.lastname: Therkelsen
authors.forename: G B
authors.initials: GB
authors.lastname: Petersen
authors.forename: C
authors.initials: C
authors.lastname: Lundsteen
mesh_terms: D000022:Abortion, Spontaneous / Q000453:epidemiology*; D000328:Adult; D003718:Denmark / Q000453:epidemiology; D004314:Down Syndrome / Q000453:epidemiology*; D005260:Female; D006801:Humans; D008423:Maternal Age; D008875:Middle Aged; D011247:Pregnancy; D011261:Pregnancy Trimester, First; D018566:Pregnancy, High-Risk; D015995:Prevalence; D012307:Risk Factors