title: Accuracy of sonographic chorionicity classification in twin gestations.
abstract: OBJECTIVE
  To evaluate the accuracy of sonographic classification of chorionicity in a large cohort of twins and investigate which factors may be associated with sonographic accuracy.
  
  
  METHODS
  We conducted a secondary analysis of a randomized trial of preterm birth prevention in twins. Sonographic classification of chorionicity was compared with pathologic examination of the placenta. Maternal (age, body mass index, diabetes, and hypertension), obstetric (prior cesarean delivery, gestational age at the first sonographic examination, and antepartum bleeding), and sonographic (oligohydramnios, polyhydramnios, and twin-twin transfusion syndrome) factors were assessed for their possible association with accuracy.
  
  
  RESULTS
  A total of 545 twin sets in which chorionicity was classified by sonography before 20 weeks' gestation were included; 455 were dichorionic and 90 were monochorionic based on pathologic examination. Sonography misclassified 35 of 545 twin pregnancies (6.4%): 18 of 455 dichorionic twins (4.0%) and 17 of 90 monochorionic twins (19.0%). The sensitivity and specificity of sonographic diagnosis of monochorionicity were 81.1% and 96.0%, respectively. In a multivariable analysis, pregnancies with initial sonographic examinations before 14 weeks' gestation were less likely to have misclassified chorionicity than those with sonographic examinations at 15 to 20 weeks (odds ratio [OR], 0.47; 95% confidence interval [CI], 0.23-0.96). For each week increase in gestational age, the odds of misclassification rose by 10% (OR, 1.10; 95% CI, 1.01-1.2). In the multivariable analysis, maternal age, body mass index, parity, and prior cesarean delivery were not associated with sonographic accuracy.
  
  
  CONCLUSIONS
  Sonography before 20 weeks incorrectly classified chorionicity in 6.4% of twin gestations. Those with first sonographic examinations performed at earlier gestational ages had improved chorionicity diagnosis.
authors.affiliation: Department of Obstetrics and Gynecology, Stanford University School of Medicine, Stanford, California USA (Y.J.B.); George Washington University Biostatistics Center, Washington, DC USA (V.M.); Department of Obstetrics and Gynecology, University of Alabama at Birmingham, Birmingham, Alabama USA (D.J.R.); University of Pittsburgh, Pittsburgh, Pennsylvania USA (S.N.C.); Drexel University, Philadelphia, Pennsylvania USA (A.C.S.); Northwestern University, Chicago, Illinois USA (A.M.P.); Eunice Kennedy Shriver National Institute of Child Health and Human Development, Bethesda, Maryland USA (U.M.R.); University of Utah, Salt Lake City, Utah USA (M.W.V); Columbia University, New York, New York USA (F.D.M.); The Ohio State University, Columbus, Ohio USA (J.D.I); Case Western Reserve University-MetroHealth Medical Center, Cleveland, Ohio USA (B.M.M); University of North Carolina, Chapel Hill, North Carolina USA (J.M.T.); Wayne State University, Detroit, Michigan USA (Y.S); Brown University, Providence, Rhode Island USA (M.W.C.); University of Texas Southwestern Medical Center, Dallas, Texas USA (J.L); University of Texas Health Science Center, Houston, Texas USA (S.M.R.); and Wake Forest University Health Sciences, Winston-Salem, North Carolina USA (M.H.). yairb@stanford.edu.
authors.forename: Yair J
authors.initials: YJ
authors.lastname: Blumenfeld
authors.affiliation: Department of Obstetrics and Gynecology, Stanford University School of Medicine, Stanford, California USA (Y.J.B.); George Washington University Biostatistics Center, Washington, DC USA (V.M.); Department of Obstetrics and Gynecology, University of Alabama at Birmingham, Birmingham, Alabama USA (D.J.R.); University of Pittsburgh, Pittsburgh, Pennsylvania USA (S.N.C.); Drexel University, Philadelphia, Pennsylvania USA (A.C.S.); Northwestern University, Chicago, Illinois USA (A.M.P.); Eunice Kennedy Shriver National Institute of Child Health and Human Development, Bethesda, Maryland USA (U.M.R.); University of Utah, Salt Lake City, Utah USA (M.W.V); Columbia University, New York, New York USA (F.D.M.); The Ohio State University, Columbus, Ohio USA (J.D.I); Case Western Reserve University-MetroHealth Medical Center, Cleveland, Ohio USA (B.M.M); University of North Carolina, Chapel Hill, North Carolina USA (J.M.T.); Wayne State University, Detroit, Michigan USA (Y.S); Brown University, Providence, Rhode Island USA (M.W.C.); University of Texas Southwestern Medical Center, Dallas, Texas USA (J.L); University of Texas Health Science Center, Houston, Texas USA (S.M.R.); and Wake Forest University Health Sciences, Winston-Salem, North Carolina USA (M.H.).
authors.forename: Valerija
authors.initials: V
authors.lastname: Momirova
authors.affiliation: Department of Obstetrics and Gynecology, Stanford University School of Medicine, Stanford, California USA (Y.J.B.); George Washington University Biostatistics Center, Washington, DC USA (V.M.); Department of Obstetrics and Gynecology, University of Alabama at Birmingham, Birmingham, Alabama USA (D.J.R.); University of Pittsburgh, Pittsburgh, Pennsylvania USA (S.N.C.); Drexel University, Philadelphia, Pennsylvania USA (A.C.S.); Northwestern University, Chicago, Illinois USA (A.M.P.); Eunice Kennedy Shriver National Institute of Child Health and Human Development, Bethesda, Maryland USA (U.M.R.); University of Utah, Salt Lake City, Utah USA (M.W.V); Columbia University, New York, New York USA (F.D.M.); The Ohio State University, Columbus, Ohio USA (J.D.I); Case Western Reserve University-MetroHealth Medical Center, Cleveland, Ohio USA (B.M.M); University of North Carolina, Chapel Hill, North Carolina USA (J.M.T.); Wayne State University, Detroit, Michigan USA (Y.S); Brown University, Providence, Rhode Island USA (M.W.C.); University of Texas Southwestern Medical Center, Dallas, Texas USA (J.L); University of Texas Health Science Center, Houston, Texas USA (S.M.R.); and Wake Forest University Health Sciences, Winston-Salem, North Carolina USA (M.H.).
authors.forename: Dwight J
authors.initials: DJ
authors.lastname: Rouse
authors.affiliation: Department of Obstetrics and Gynecology, Stanford University School of Medicine, Stanford, California USA (Y.J.B.); George Washington University Biostatistics Center, Washington, DC USA (V.M.); Department of Obstetrics and Gynecology, University of Alabama at Birmingham, Birmingham, Alabama USA (D.J.R.); University of Pittsburgh, Pittsburgh, Pennsylvania USA (S.N.C.); Drexel University, Philadelphia, Pennsylvania USA (A.C.S.); Northwestern University, Chicago, Illinois USA (A.M.P.); Eunice Kennedy Shriver National Institute of Child Health and Human Development, Bethesda, Maryland USA (U.M.R.); University of Utah, Salt Lake City, Utah USA (M.W.V); Columbia University, New York, New York USA (F.D.M.); The Ohio State University, Columbus, Ohio USA (J.D.I); Case Western Reserve University-MetroHealth Medical Center, Cleveland, Ohio USA (B.M.M); University of North Carolina, Chapel Hill, North Carolina USA (J.M.T.); Wayne State University, Detroit, Michigan USA (Y.S); Brown University, Providence, Rhode Island USA (M.W.C.); University of Texas Southwestern Medical Center, Dallas, Texas USA (J.L); University of Texas Health Science Center, Houston, Texas USA (S.M.R.); and Wake Forest University Health Sciences, Winston-Salem, North Carolina USA (M.H.).
authors.forename: Steve N
authors.initials: SN
authors.lastname: Caritis
authors.affiliation: Department of Obstetrics and Gynecology, Stanford University School of Medicine, Stanford, California USA (Y.J.B.); George Washington University Biostatistics Center, Washington, DC USA (V.M.); Department of Obstetrics and Gynecology, University of Alabama at Birmingham, Birmingham, Alabama USA (D.J.R.); University of Pittsburgh, Pittsburgh, Pennsylvania USA (S.N.C.); Drexel University, Philadelphia, Pennsylvania USA (A.C.S.); Northwestern University, Chicago, Illinois USA (A.M.P.); Eunice Kennedy Shriver National Institute of Child Health and Human Development, Bethesda, Maryland USA (U.M.R.); University of Utah, Salt Lake City, Utah USA (M.W.V); Columbia University, New York, New York USA (F.D.M.); The Ohio State University, Columbus, Ohio USA (J.D.I); Case Western Reserve University-MetroHealth Medical Center, Cleveland, Ohio USA (B.M.M); University of North Carolina, Chapel Hill, North Carolina USA (J.M.T.); Wayne State University, Detroit, Michigan USA (Y.S); Brown University, Providence, Rhode Island USA (M.W.C.); University of Texas Southwestern Medical Center, Dallas, Texas USA (J.L); University of Texas Health Science Center, Houston, Texas USA (S.M.R.); and Wake Forest University Health Sciences, Winston-Salem, North Carolina USA (M.H.).
authors.forename: Anthony
authors.initials: A
authors.lastname: Sciscione
authors.affiliation: Department of Obstetrics and Gynecology, Stanford University School of Medicine, Stanford, California USA (Y.J.B.); George Washington University Biostatistics Center, Washington, DC USA (V.M.); Department of Obstetrics and Gynecology, University of Alabama at Birmingham, Birmingham, Alabama USA (D.J.R.); University of Pittsburgh, Pittsburgh, Pennsylvania USA (S.N.C.); Drexel University, Philadelphia, Pennsylvania USA (A.C.S.); Northwestern University, Chicago, Illinois USA (A.M.P.); Eunice Kennedy Shriver National Institute of Child Health and Human Development, Bethesda, Maryland USA (U.M.R.); University of Utah, Salt Lake City, Utah USA (M.W.V); Columbia University, New York, New York USA (F.D.M.); The Ohio State University, Columbus, Ohio USA (J.D.I); Case Western Reserve University-MetroHealth Medical Center, Cleveland, Ohio USA (B.M.M); University of North Carolina, Chapel Hill, North Carolina USA (J.M.T.); Wayne State University, Detroit, Michigan USA (Y.S); Brown University, Providence, Rhode Island USA (M.W.C.); University of Texas Southwestern Medical Center, Dallas, Texas USA (J.L); University of Texas Health Science Center, Houston, Texas USA (S.M.R.); and Wake Forest University Health Sciences, Winston-Salem, North Carolina USA (M.H.).
authors.forename: Alan M
authors.initials: AM
authors.lastname: Peaceman
authors.affiliation: Department of Obstetrics and Gynecology, Stanford University School of Medicine, Stanford, California USA (Y.J.B.); George Washington University Biostatistics Center, Washington, DC USA (V.M.); Department of Obstetrics and Gynecology, University of Alabama at Birmingham, Birmingham, Alabama USA (D.J.R.); University of Pittsburgh, Pittsburgh, Pennsylvania USA (S.N.C.); Drexel University, Philadelphia, Pennsylvania USA (A.C.S.); Northwestern University, Chicago, Illinois USA (A.M.P.); Eunice Kennedy Shriver National Institute of Child Health and Human Development, Bethesda, Maryland USA (U.M.R.); University of Utah, Salt Lake City, Utah USA (M.W.V); Columbia University, New York, New York USA (F.D.M.); The Ohio State University, Columbus, Ohio USA (J.D.I); Case Western Reserve University-MetroHealth Medical Center, Cleveland, Ohio USA (B.M.M); University of North Carolina, Chapel Hill, North Carolina USA (J.M.T.); Wayne State University, Detroit, Michigan USA (Y.S); Brown University, Providence, Rhode Island USA (M.W.C.); University of Texas Southwestern Medical Center, Dallas, Texas USA (J.L); University of Texas Health Science Center, Houston, Texas USA (S.M.R.); and Wake Forest University Health Sciences, Winston-Salem, North Carolina USA (M.H.).
authors.forename: Uma M
authors.initials: UM
authors.lastname: Reddy
authors.affiliation: Department of Obstetrics and Gynecology, Stanford University School of Medicine, Stanford, California USA (Y.J.B.); George Washington University Biostatistics Center, Washington, DC USA (V.M.); Department of Obstetrics and Gynecology, University of Alabama at Birmingham, Birmingham, Alabama USA (D.J.R.); University of Pittsburgh, Pittsburgh, Pennsylvania USA (S.N.C.); Drexel University, Philadelphia, Pennsylvania USA (A.C.S.); Northwestern University, Chicago, Illinois USA (A.M.P.); Eunice Kennedy Shriver National Institute of Child Health and Human Development, Bethesda, Maryland USA (U.M.R.); University of Utah, Salt Lake City, Utah USA (M.W.V); Columbia University, New York, New York USA (F.D.M.); The Ohio State University, Columbus, Ohio USA (J.D.I); Case Western Reserve University-MetroHealth Medical Center, Cleveland, Ohio USA (B.M.M); University of North Carolina, Chapel Hill, North Carolina USA (J.M.T.); Wayne State University, Detroit, Michigan USA (Y.S); Brown University, Providence, Rhode Island USA (M.W.C.); University of Texas Southwestern Medical Center, Dallas, Texas USA (J.L); University of Texas Health Science Center, Houston, Texas USA (S.M.R.); and Wake Forest University Health Sciences, Winston-Salem, North Carolina USA (M.H.).
authors.forename: Michael W
authors.initials: MW
authors.lastname: Varner
authors.affiliation: Department of Obstetrics and Gynecology, Stanford University School of Medicine, Stanford, California USA (Y.J.B.); George Washington University Biostatistics Center, Washington, DC USA (V.M.); Department of Obstetrics and Gynecology, University of Alabama at Birmingham, Birmingham, Alabama USA (D.J.R.); University of Pittsburgh, Pittsburgh, Pennsylvania USA (S.N.C.); Drexel University, Philadelphia, Pennsylvania USA (A.C.S.); Northwestern University, Chicago, Illinois USA (A.M.P.); Eunice Kennedy Shriver National Institute of Child Health and Human Development, Bethesda, Maryland USA (U.M.R.); University of Utah, Salt Lake City, Utah USA (M.W.V); Columbia University, New York, New York USA (F.D.M.); The Ohio State University, Columbus, Ohio USA (J.D.I); Case Western Reserve University-MetroHealth Medical Center, Cleveland, Ohio USA (B.M.M); University of North Carolina, Chapel Hill, North Carolina USA (J.M.T.); Wayne State University, Detroit, Michigan USA (Y.S); Brown University, Providence, Rhode Island USA (M.W.C.); University of Texas Southwestern Medical Center, Dallas, Texas USA (J.L); University of Texas Health Science Center, Houston, Texas USA (S.M.R.); and Wake Forest University Health Sciences, Winston-Salem, North Carolina USA (M.H.).
authors.forename: Fergal D
authors.initials: FD
authors.lastname: Malone
authors.affiliation: Department of Obstetrics and Gynecology, Stanford University School of Medicine, Stanford, California USA (Y.J.B.); George Washington University Biostatistics Center, Washington, DC USA (V.M.); Department of Obstetrics and Gynecology, University of Alabama at Birmingham, Birmingham, Alabama USA (D.J.R.); University of Pittsburgh, Pittsburgh, Pennsylvania USA (S.N.C.); Drexel University, Philadelphia, Pennsylvania USA (A.C.S.); Northwestern University, Chicago, Illinois USA (A.M.P.); Eunice Kennedy Shriver National Institute of Child Health and Human Development, Bethesda, Maryland USA (U.M.R.); University of Utah, Salt Lake City, Utah USA (M.W.V); Columbia University, New York, New York USA (F.D.M.); The Ohio State University, Columbus, Ohio USA (J.D.I); Case Western Reserve University-MetroHealth Medical Center, Cleveland, Ohio USA (B.M.M); University of North Carolina, Chapel Hill, North Carolina USA (J.M.T.); Wayne State University, Detroit, Michigan USA (Y.S); Brown University, Providence, Rhode Island USA (M.W.C.); University of Texas Southwestern Medical Center, Dallas, Texas USA (J.L); University of Texas Health Science Center, Houston, Texas USA (S.M.R.); and Wake Forest University Health Sciences, Winston-Salem, North Carolina USA (M.H.).
authors.forename: Jay D
authors.initials: JD
authors.lastname: Iams
authors.affiliation: Department of Obstetrics and Gynecology, Stanford University School of Medicine, Stanford, California USA (Y.J.B.); George Washington University Biostatistics Center, Washington, DC USA (V.M.); Department of Obstetrics and Gynecology, University of Alabama at Birmingham, Birmingham, Alabama USA (D.J.R.); University of Pittsburgh, Pittsburgh, Pennsylvania USA (S.N.C.); Drexel University, Philadelphia, Pennsylvania USA (A.C.S.); Northwestern University, Chicago, Illinois USA (A.M.P.); Eunice Kennedy Shriver National Institute of Child Health and Human Development, Bethesda, Maryland USA (U.M.R.); University of Utah, Salt Lake City, Utah USA (M.W.V); Columbia University, New York, New York USA (F.D.M.); The Ohio State University, Columbus, Ohio USA (J.D.I); Case Western Reserve University-MetroHealth Medical Center, Cleveland, Ohio USA (B.M.M); University of North Carolina, Chapel Hill, North Carolina USA (J.M.T.); Wayne State University, Detroit, Michigan USA (Y.S); Brown University, Providence, Rhode Island USA (M.W.C.); University of Texas Southwestern Medical Center, Dallas, Texas USA (J.L); University of Texas Health Science Center, Houston, Texas USA (S.M.R.); and Wake Forest University Health Sciences, Winston-Salem, North Carolina USA (M.H.).
authors.forename: Brian M
authors.initials: BM
authors.lastname: Mercer
authors.affiliation: Department of Obstetrics and Gynecology, Stanford University School of Medicine, Stanford, California USA (Y.J.B.); George Washington University Biostatistics Center, Washington, DC USA (V.M.); Department of Obstetrics and Gynecology, University of Alabama at Birmingham, Birmingham, Alabama USA (D.J.R.); University of Pittsburgh, Pittsburgh, Pennsylvania USA (S.N.C.); Drexel University, Philadelphia, Pennsylvania USA (A.C.S.); Northwestern University, Chicago, Illinois USA (A.M.P.); Eunice Kennedy Shriver National Institute of Child Health and Human Development, Bethesda, Maryland USA (U.M.R.); University of Utah, Salt Lake City, Utah USA (M.W.V); Columbia University, New York, New York USA (F.D.M.); The Ohio State University, Columbus, Ohio USA (J.D.I); Case Western Reserve University-MetroHealth Medical Center, Cleveland, Ohio USA (B.M.M); University of North Carolina, Chapel Hill, North Carolina USA (J.M.T.); Wayne State University, Detroit, Michigan USA (Y.S); Brown University, Providence, Rhode Island USA (M.W.C.); University of Texas Southwestern Medical Center, Dallas, Texas USA (J.L); University of Texas Health Science Center, Houston, Texas USA (S.M.R.); and Wake Forest University Health Sciences, Winston-Salem, North Carolina USA (M.H.).
authors.forename: John M
authors.initials: JM
authors.lastname: Thorp
authors.affiliation: Department of Obstetrics and Gynecology, Stanford University School of Medicine, Stanford, California USA (Y.J.B.); George Washington University Biostatistics Center, Washington, DC USA (V.M.); Department of Obstetrics and Gynecology, University of Alabama at Birmingham, Birmingham, Alabama USA (D.J.R.); University of Pittsburgh, Pittsburgh, Pennsylvania USA (S.N.C.); Drexel University, Philadelphia, Pennsylvania USA (A.C.S.); Northwestern University, Chicago, Illinois USA (A.M.P.); Eunice Kennedy Shriver National Institute of Child Health and Human Development, Bethesda, Maryland USA (U.M.R.); University of Utah, Salt Lake City, Utah USA (M.W.V); Columbia University, New York, New York USA (F.D.M.); The Ohio State University, Columbus, Ohio USA (J.D.I); Case Western Reserve University-MetroHealth Medical Center, Cleveland, Ohio USA (B.M.M); University of North Carolina, Chapel Hill, North Carolina USA (J.M.T.); Wayne State University, Detroit, Michigan USA (Y.S); Brown University, Providence, Rhode Island USA (M.W.C.); University of Texas Southwestern Medical Center, Dallas, Texas USA (J.L); University of Texas Health Science Center, Houston, Texas USA (S.M.R.); and Wake Forest University Health Sciences, Winston-Salem, North Carolina USA (M.H.).
authors.forename: Yoram
authors.initials: Y
authors.lastname: Sorokin
authors.affiliation: Department of Obstetrics and Gynecology, Stanford University School of Medicine, Stanford, California USA (Y.J.B.); George Washington University Biostatistics Center, Washington, DC USA (V.M.); Department of Obstetrics and Gynecology, University of Alabama at Birmingham, Birmingham, Alabama USA (D.J.R.); University of Pittsburgh, Pittsburgh, Pennsylvania USA (S.N.C.); Drexel University, Philadelphia, Pennsylvania USA (A.C.S.); Northwestern University, Chicago, Illinois USA (A.M.P.); Eunice Kennedy Shriver National Institute of Child Health and Human Development, Bethesda, Maryland USA (U.M.R.); University of Utah, Salt Lake City, Utah USA (M.W.V); Columbia University, New York, New York USA (F.D.M.); The Ohio State University, Columbus, Ohio USA (J.D.I); Case Western Reserve University-MetroHealth Medical Center, Cleveland, Ohio USA (B.M.M); University of North Carolina, Chapel Hill, North Carolina USA (J.M.T.); Wayne State University, Detroit, Michigan USA (Y.S); Brown University, Providence, Rhode Island USA (M.W.C.); University of Texas Southwestern Medical Center, Dallas, Texas USA (J.L); University of Texas Health Science Center, Houston, Texas USA (S.M.R.); and Wake Forest University Health Sciences, Winston-Salem, North Carolina USA (M.H.).
authors.forename: Marshall W
authors.initials: MW
authors.lastname: Carpenter
authors.affiliation: Department of Obstetrics and Gynecology, Stanford University School of Medicine, Stanford, California USA (Y.J.B.); George Washington University Biostatistics Center, Washington, DC USA (V.M.); Department of Obstetrics and Gynecology, University of Alabama at Birmingham, Birmingham, Alabama USA (D.J.R.); University of Pittsburgh, Pittsburgh, Pennsylvania USA (S.N.C.); Drexel University, Philadelphia, Pennsylvania USA (A.C.S.); Northwestern University, Chicago, Illinois USA (A.M.P.); Eunice Kennedy Shriver National Institute of Child Health and Human Development, Bethesda, Maryland USA (U.M.R.); University of Utah, Salt Lake City, Utah USA (M.W.V); Columbia University, New York, New York USA (F.D.M.); The Ohio State University, Columbus, Ohio USA (J.D.I); Case Western Reserve University-MetroHealth Medical Center, Cleveland, Ohio USA (B.M.M); University of North Carolina, Chapel Hill, North Carolina USA (J.M.T.); Wayne State University, Detroit, Michigan USA (Y.S); Brown University, Providence, Rhode Island USA (M.W.C.); University of Texas Southwestern Medical Center, Dallas, Texas USA (J.L); University of Texas Health Science Center, Houston, Texas USA (S.M.R.); and Wake Forest University Health Sciences, Winston-Salem, North Carolina USA (M.H.).
authors.forename: Julie
authors.initials: J
authors.lastname: Lo
authors.affiliation: Department of Obstetrics and Gynecology, Stanford University School of Medicine, Stanford, California USA (Y.J.B.); George Washington University Biostatistics Center, Washington, DC USA (V.M.); Department of Obstetrics and Gynecology, University of Alabama at Birmingham, Birmingham, Alabama USA (D.J.R.); University of Pittsburgh, Pittsburgh, Pennsylvania USA (S.N.C.); Drexel University, Philadelphia, Pennsylvania USA (A.C.S.); Northwestern University, Chicago, Illinois USA (A.M.P.); Eunice Kennedy Shriver National Institute of Child Health and Human Development, Bethesda, Maryland USA (U.M.R.); University of Utah, Salt Lake City, Utah USA (M.W.V); Columbia University, New York, New York USA (F.D.M.); The Ohio State University, Columbus, Ohio USA (J.D.I); Case Western Reserve University-MetroHealth Medical Center, Cleveland, Ohio USA (B.M.M); University of North Carolina, Chapel Hill, North Carolina USA (J.M.T.); Wayne State University, Detroit, Michigan USA (Y.S); Brown University, Providence, Rhode Island USA (M.W.C.); University of Texas Southwestern Medical Center, Dallas, Texas USA (J.L); University of Texas Health Science Center, Houston, Texas USA (S.M.R.); and Wake Forest University Health Sciences, Winston-Salem, North Carolina USA (M.H.).
authors.forename: Susan M
authors.initials: SM
authors.lastname: Ramin
authors.forename: Margaret
authors.initials: M
authors.lastname: Harper
mesh_terms: D000328:Adult; D002140:California / Q000453:epidemiology; D002823:Chorion / Q000000981:diagnostic imaging*; D004311:Double-Blind Method; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D010920:Placenta / Q000000981:diagnostic imaging*; D011247:Pregnancy; D059285:Pregnancy, Twin / Q000706:statistics & numerical data*; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity; D016216:Ultrasonography, Prenatal / Q000706:statistics & numerical data*; D055815:Young Adult
keywords: accuracy; chorionicity; obstetric ultrasound; sonography; twins