title: Second-trimester assessment of gestational age in twins: validation of singleton biometry charts.
abstract: OBJECTIVE
  To determine the accuracy of dating twin pregnancies, of between 16 and 26 weeks' gestation, using singleton head circumference (HC) formulae.
  
  
  METHODS
  This was a retrospective study of 269 singleton and 119 twin non-anomalous pregnancies conceived by in-vitro fertilization (IVF) with a known embryo transfer date. Fetal ultrasound biometry data for HC, obtained using different formulae, were compared with expected fetal HC size for gestation calculated from the date of conception. Similar comparisons were undertaken for femur length (FL) and for transverse cerebellar diameter.
  
  
  RESULTS
  The mean differences in HC between observed ultrasound measurements and those expected from the IVF history were small (1-4 mm) and within the measurement error for both singletons and twins for all formulae. All measurements from the larger and the smaller twins straddled those of singletons, regardless of biometry and formula used. Negative skewing of FL measurements in the smaller twin suggests that fetal growth restriction may occur at this gestation and supports the practice of dating using the HC of the larger twin.
  
  
  CONCLUSIONS
  Singleton pregnancy HC charts can be used to date reliably twin pregnancies. The data of the study also suggest that the HC of the larger twin is the most reliable measurement for use in dating.
authors.affiliation: Fetal Medicine Unit, Academic Department of Obstetrics and Gynaecology, St George's, University of London, London, UK.
authors.forename: T
authors.initials: T
authors.lastname: Dias
authors.forename: T
authors.initials: T
authors.lastname: Arcangeli
authors.forename: A
authors.initials: A
authors.lastname: Bhide
authors.forename: S
authors.initials: S
authors.lastname: Mahsud-Dornan
authors.forename: A
authors.initials: A
authors.lastname: Papageorghiou
authors.forename: B
authors.initials: B
authors.lastname: Thilaganathan
mesh_terms: D001699:Biometry; D005260:Female; D005307:Fertilization in Vitro; D005865:Gestational Age; D006257:Head / Q000033:anatomy & histology / Q000000981:diagnostic imaging* / Q000196:embryology; D006801:Humans; D011247:Pregnancy; D011262:Pregnancy Trimester, Second; D012016:Reference Values; D012189:Retrospective Studies; D014427:Twins; D016216:Ultrasonography, Prenatal / Q000379:methods*