title: Lack of standardization in determining gestational age for prenatal screening.
abstract: OBJECTIVE
  To determine whether estimation of gestational age (GA) in the context of first-trimester Down syndrome screening is standardized in the Netherlands.
  
  
  METHODS
  This was a retrospective study, carried out between January 2005 and December 2006, of women who underwent first-trimester Down syndrome screening (n = 40,730) based on GA, maternal serum analysis and nuchal translucency (NT) measurement. Date of the last menstrual period (LMP), dating scan information including measurement of crown-rump length (CRL), NT thickness and name of the sonographer were recorded for all pregnancies. The accuracy of estimation of GA was evaluated by comparing the GA based on the LMP with that estimated from the CRL, using relevant subsets of the database. A survey of 104 sonographers was performed to further investigate the findings of the preceding analysis.
  
  
  RESULTS
  In 44% of all first-trimester combined tests the estimation of GA was based on the dating scan; the method of determination of GA was unknown in 23%. In 15% of all cases a dating scan was recorded but was not used to provide the estimation of GA at blood sampling. Detailed analysis showed that a consistent methodology for the estimation of GA from CRL was not maintained within hospitals and obstetric practices. For a single CRL, the reported GA differed by up to 10 days. Finally, it was demonstrated that individual sonographers reported different GAs for a given CRL.
  
  
  CONCLUSIONS
  Currently, estimation of GA in the first trimester in the Netherlands is not standardized. To improve the performance of prenatal screening for Down syndrome, estimation of GA should be based on ultrasound examination, with one nationally accepted CRL curve.
authors: Diagnostic Laboratory for Infectious Diseases and Perinatal Screening, National Institute for Public Health and the Environment, Bilthoven, The Netherlands. wendy.koster@rivm.nl
authors: M P H
authors: MP
authors: Koster
authors: M
authors: M
authors: Van Leeuwen-Spruijt
authors: E J
authors: EJ
authors: Wortelboer
authors: P
authors: P
authors: Stoutenbeek
authors: L H
authors: LH
authors: Elvers
authors: J G
authors: JG
authors: Loeber
authors: G H A
authors: GH
authors: Visser
authors: P C J I
authors: PC
authors: Schielen
mesh_terms: D018568:Crown-Rump Length; D004314:Down Syndrome / Q000175:diagnosis*; D005260:Female; D005865:Gestational Age; D006801:Humans; D009426:Netherlands; D048208:Nuchal Translucency Measurement; D011247:Pregnancy; D011261:Pregnancy Trimester, First; D011296:Prenatal Diagnosis / Q000592:standards*; D012015:Reference Standards; D012189:Retrospective Studies