title: Temporal changes in maternal serum biochemical markers of trisomy 21 across the first and second trimester of pregnancy.
abstract: BACKGROUND
  Many maternal serum markers show concentration changes in Down's syndrome pregnancies but the magnitude of the change in median marker levels varies with gestation. To date these changes have not been accurately specified.
  
  
  METHODS
  The trends in marker median levels between 6 and 20 weeks of gestation were examined for alphafetoprotein (AFP), free beta human chorionic gonadotrophin (Fbeta-hCG), total human chorionic gonadotrophin (ThCG) and pregnancy-associated plasma protein A (PAPP-A) by a meta-analysis of data obtained from our collaborative studies and routine screening programmes for Down's syndrome over a 10-year period. Data were available from between 709 and 1082 Down's syndrome pregnancies and from between 14607 and 153909 unaffected pregnancies for each marker. The median multiple of the median (MoM) and mean log10MoM for each marker at each completed week of gestation were estimated and the trend with gestation smoothed using a weighted least squares regression model.
  
  
  RESULTS
  The gestational ages at which maximum separation of marker levels occurred, comparing affected and unaffected pregnancies, and the respective regressed median MoMs and mean log10MoMs, were: for AFP at 16 weeks, 0.72 MoM, -0.14288log10MoM; for Fbeta-hCG at 15 weeks, 2-24MoM, 0.35034 log10MoM; for ThCG at 16 weeks, 1.93 MoM, 0.28548 log10MoM, as well as before 8 weeks (<0.65 MoM, -0.18853 log10MoM); and for PAPP-A before 8 weeks, <0.33 MoM, -0.47727 log10MoM.
  
  
  CONCLUSIONS
  There is significant temporal variation in mean log10MoM values for the screening markers investigated. Screening algorithms, modified to take account of this variation, should allow more accurate gestation-specific risks to be calculated in individual pregnancies.
authors.affiliation: Endocrine Unit, Clinical Biochemistry Department, Harold Wood Hospital Gubbins Lane Romford Essex RM3 0BE, UK. KevinSpencer1@aol.com
authors.forename: K
authors.initials: K
authors.lastname: Spencer
authors.forename: J A
authors.initials: JA
authors.lastname: Crossley
authors.forename: D A
authors.initials: DA
authors.lastname: Aitken
authors.forename: A B J
authors.initials: AB
authors.lastname: Nix
authors.forename: F D J
authors.initials: FD
authors.lastname: Dunstan
authors.forename: K
authors.initials: K
authors.lastname: Williams
mesh_terms: D015415:Biomarkers / Q000097:blood; D018997:Chorionic Gonadotropin, beta Subunit, Human / Q000097:blood; D004314:Down Syndrome / Q000097:blood* / Q000175:diagnosis*; D010450:Endopeptidases / Q000097:blood; D005260:Female; D006801:Humans; D011247:Pregnancy; D011257:Pregnancy Proteins / Q000097:blood; D011261:Pregnancy Trimester, First / Q000097:blood*; D011262:Pregnancy Trimester, Second / Q000097:blood*; D011266:Pregnancy-Associated Plasma Protein-A / Q000032:analysis; D013997:Time Factors; D000509:alpha-Fetoproteins / Q000032:analysis
chemical_list: D015415:Biomarkers; D018997:Chorionic Gonadotropin, beta Subunit, Human; D011257:Pregnancy Proteins; D000509:alpha-Fetoproteins; D010450:Endopeptidases; C430889:PAPPA2 protein, human; D011266:Pregnancy-Associated Plasma Protein-A