title: Estimating the effect of gestational age on test performance of combined first-trimester screening for Down syndrome: a preliminary study.
abstract: To establish how different methods of estimating gestational age (GA) affect reliability of first-trimester screening for Down syndrome.
  
  
  
  Retrospective single-center study of 100 women with a viable singleton pregnancy, who had first-trimester screening. We calculated multiples of the median (MoM) for maternal-serum free beta human chorionic gonadotropin (free beta-hCG) and pregnancy associated plasma protein-A (PAPP-A), derived from either last menstrual period (LMP) or ultrasound-dating scans.
  
  
  
  In women with a regular cycle, LMP-derived estimates of GA were two days longer (range -11 to 18), than crown-rump length (CRL)-derived estimates of GA whereas this discrepancy was more pronounced in women who reported to have an irregular cycle, i.e., six days (range -7 to 32). Except for PAPP-A in the regular-cycle group, all differences were significant. Consequently, risk estimates are affected by the mode of estimating GA. In fact, LMP-based estimates revealed ten "screen-positive" cases compared to five "screen-positive" cases where GA was derived from dating-scans.
  
  
  
  Provided fixed values for nuchal translucency are applied, dating-scans reduce the number of screen-positive findings on the basis of biochemical screening. We recommend implementation of guidelines for Down syndrome screening based on CRL-dependent rather than LMP-dependent parameters of GA.
authors.affiliation: Division of Obstetrics and Prenatal Medicine, Department of Obstetrics and Gynecology, Erasmus MC, University Medical Center, Rotterdam, The Netherlands. p.vanheesch@erasmusmc.nl
authors.forename: Peter N
authors.initials: PN
authors.lastname: van Heesch
authors.forename: Pieter C
authors.initials: PC
authors.lastname: Struijk
authors.forename: Jaqueline A M
authors.initials: JA
authors.lastname: Laudy
authors.forename: Eric A P
authors.initials: EA
authors.lastname: Steegers
authors.forename: Hajo I J
authors.initials: HI
authors.lastname: Wildschut
mesh_terms: D000293:Adolescent; D000328:Adult; D018997:Chorionic Gonadotropin, beta Subunit, Human / Q000097:blood; D018568:Crown-Rump Length; D004314:Down Syndrome / Q000175:diagnosis*; D005260:Female; D005865:Gestational Age; D006801:Humans; D008598:Menstruation; D048208:Nuchal Translucency Measurement; D011247:Pregnancy; D011266:Pregnancy-Associated Plasma Protein-A / Q000032:analysis; D011296:Prenatal Diagnosis / Q000379:methods*; D012189:Retrospective Studies; D012680:Sensitivity and Specificity; D013997:Time Factors; D016216:Ultrasonography, Prenatal
chemical_list: D018997:Chorionic Gonadotropin, beta Subunit, Human; D011266:Pregnancy-Associated Plasma Protein-A