title: Determination of gestational age in twin pregnancy: Which fetal crown-rump length should be used?
abstract: OBJECTIVE
  For twin pregnancy, discrepancies in the crown-rump length (CRL) between two fetuses often exist. Evidence is lacking regarding which fetal CRL should be used for estimation of gestational age (GA). Our aim was to determine whether the larger, smaller or the mean CRL is more accurate in determining the GA in the first trimester of pregnancy.
  
  
  METHODS
  This is a retrospective study of twin pregnancies conceived by assisted reproduction. The oocyte retrieval date was used for determination of the true gestational age. CRL dating charts by Robinson, Hadlock and Chitty were used for reference. The values of the larger, smaller and mean CRL were compared with the reference CRL for the corresponding GA, which was obtained from each of the three reference charts. The differences between the reference CRL and measured CRL were calculated. The percentages of which CRL, the larger, smaller or the mean, was closest to the expected reference values were calculated.
  
  
  RESULTS
  A total of 52 pairs of twins were included in the study. According to Robinson's chart, the proportion of larger, smaller and mean CRL values that were closest to the reference value was found in 11.5%, 75.0% and 5.8% of cases respectively. The larger, smaller and the mean CRLs were closest to the reference CRL in the Hadlock chart for 28.9%, 44.2% and 19.2% of cases, respectively, and closest to the reference CRL in the Chitty chart for 17.3%, 59.6% and 15.4% of cases, respectively.
  
  
  CONCLUSIONS
  The smaller CRL is more accurate in the estimation of the GA for twin pregnancy compared to the larger or mean CRL values.
authors.affiliation: Department of Obstetrics and Gynaecology, National University Hospital, Singapore, Singapore. kanikaojha@yahoo.com.sg
authors.forename: Kanika
authors.initials: K
authors.lastname: Chaudhuri
authors.forename: Lin-Lin
authors.initials: LL
authors.lastname: Su
authors.forename: Peng-Cheang
authors.initials: PC
authors.lastname: Wong
authors.forename: Yiong-Huak
authors.initials: YH
authors.lastname: Chan
authors.forename: Mahesh A
authors.initials: MA
authors.lastname: Choolani
authors.forename: Dawn
authors.initials: D
authors.lastname: Chia
authors.forename: Arijit
authors.initials: A
authors.lastname: Biswas
mesh_terms: D000328:Adult; D000465:Algorithms; D015331:Cohort Studies; D018568:Crown-Rump Length; D005260:Female; D005307:Fertilization in Vitro; D047109:Fetal Development; D005500:Follow-Up Studies; D005865:Gestational Age; D057238:Growth Charts; D006801:Humans; D007247:Infertility, Female / Q000628:therapy; D011247:Pregnancy; D011261:Pregnancy Trimester, First; D059285:Pregnancy, Twin; D012189:Retrospective Studies; D016216:Ultrasonography, Prenatal