title: Second-trimester Down syndrome maternal serum screening in twin pregnancies: impact of chorionicity.
abstract: OBJECTIVE
  To evaluate the diagnostic value of second-trimester maternal serum screening for Down syndrome in twin pregnancies.
  
  
  METHODS
  On the basis of a prospective study of second-trimester maternal serum screening, we studied the distribution of alpha-fetoprotein (AFP) and free ss hCG in 3043 twin pregnancies with known outcome. There were 1561 dichorionic and 244 monochorionic pregnancies. The placental type was not available in 1238 cases. We compared 5 screening policies with the same risk, 1/250, cut-off: maternal age, maternal age corrected for the risk of having at least one affected twin in dichorionic pregnancies, maternal serum marker screening using observed AFP and free ss-hCG values divided by a factor of 2, by using the median values actually observed in the global twin population, or by the median values specific to mono- or dichorionic twins.
  
  
  RESULTS
  When expressed in singleton-derived MoMs, the median was 2.10 for AFP and 2.11 for free ss-hCG. The median AFP did not differ between monochorionic and dichorionic pregnancies. The distribution of free ss-hCG was significantly shifted towards greater values in monochorionic (2.16 MoM) compared to dichorionic (2.07) pregnancies (p < 0.0001). Screened-positive and detection rates were, respectively, 6.6% and 27.3% using maternal age alone, 24.6% and 54.5% using maternal age corrected for the risk of having at least one affected twin in dichorionic pregnancies, 7.75% and 54.5% using observed AFP and free beta-hCG values divided by a factor of 2, 8.05% and 54.5% using the median values actually observed in the global twin population, and 7.75% and 54.5% using the median values specific to mono- or dichorionic twins.
  
  
  CONCLUSIONS
  Trisomy 21 second-trimester maternal serum screening is feasible in twins, and is better than a policy based on maternal age alone.
authors.affiliation: Biochimie, Hôpital Ambroise Paré, Boulogne, France. francoise.muller@apr.ap-hop-paris.fr
authors.forename: Françoise
authors.initials: F
authors.lastname: Muller
authors.forename: Sophie
authors.initials: S
authors.lastname: Dreux
authors.forename: Henry
authors.initials: H
authors.lastname: Dupoizat
authors.forename: Serge
authors.initials: S
authors.lastname: Uzan
authors.forename: Marie-Françoise
authors.initials: MF
authors.lastname: Dubin
authors.forename: Jean-François
authors.initials: JF
authors.lastname: Oury
authors.forename: Bernard
authors.initials: B
authors.lastname: Dingeon
authors.forename: Marc
authors.initials: M
authors.lastname: Dommergues
mesh_terms: D000293:Adolescent; D000328:Adult; D002823:Chorion / Q000502:physiology*; D018997:Chorionic Gonadotropin, beta Subunit, Human / Q000097:blood; D004314:Down Syndrome / Q000175:diagnosis*; D005260:Female; D006801:Humans; D008403:Mass Screening / Q000379:methods; D008423:Maternal Age; D011247:Pregnancy / Q000097:blood; D011262:Pregnancy Trimester, Second; D018566:Pregnancy, High-Risk; D011272:Pregnancy, Multiple / Q000097:blood; D011296:Prenatal Diagnosis / Q000379:methods*; D011446:Prospective Studies; D012016:Reference Values; D014427:Twins; D000509:alpha-Fetoproteins / Q000032:analysis
chemical_list: D018997:Chorionic Gonadotropin, beta Subunit, Human; D000509:alpha-Fetoproteins