title: An ultrasound-based fetal weight reference for twins.
abstract: OBJECTIVE
  The objective of the study was to construct an ultrasound-based estimated fetal weight-for-gestational-age reference for twin fetuses, stratified by chorionicity.
  
  
  METHODS
  We performed a retrospective cohort study of live-born nonanomalous twins delivered longer than 34 weeks at the Royal Victoria Hospital (Montreal, Canada). Fetal weight was estimated using ultrasound biometric measurements combined using Hadlock's formula. Multilevel linear regression models were used to adjust for clustering by twin pregnancy and to account for the serial ultrasound measurements taken on each fetus. Based on this model, smoothed estimates of fetal weight were made for the third, 10th, 50th, 90th, and 97th percentiles of the fetal weight distribution. Fetal weight references were stratified by fetal chorionicity.
  
  
  RESULTS
  A total of 642 twin fetuses with a total of 3078 ultrasound observations were included. Sixteen percent of the cohort was monochorionic. Fetal growth accelerated in the second trimester and continued in a linear pattern in the third trimester until term. As expected, the median weight for monochorionic twins was lighter than the median weight for dichorionic twins throughout pregnancy.
  
  
  CONCLUSIONS
  The reference values created in this study address serious methodological limitations of existing reference charts and thus provide an improved tool for assessing fetal growth in twin pregnancies. Importantly, dichorionic twins deviated from singleton reference charts at approximately 32 weeks, whereas monochorionic twins deviated at 28 weeks.
authors.affiliation: Department of Epidemiology, Biostatistics, and Occupational Health, Faculty of Medicine, McGill University, Montreal, QC, Canada.
authors.forename: Sushmita
authors.initials: S
authors.lastname: Shivkumar
authors.affiliation: Magee-Womens Research Institute and Department of Obstetrics, Gynecology, and Reproductive Sciences, School of Medicine, University of Pittsburgh, Pittsburgh, PA.
authors.forename: Katherine Park
authors.initials: KP
authors.lastname: Himes
authors.affiliation: Department of Obstetrics and Gynaecology, Faculty of Medicine, University of British Columbia, Vancouver, BC, Canada.
authors.forename: Jennifer Anne
authors.initials: JA
authors.lastname: Hutcheon
authors.affiliation: Department of Epidemiology, Biostatistics, and Occupational Health, Faculty of Medicine, McGill University, Montreal, QC, Canada; Department of Pediatrics, Faculty of Medicine, McGill University, Montreal, QC, Canada. Electronic address: robert.platt@mcgill.ca.
authors.forename: Robert William
authors.initials: RW
authors.lastname: Platt
mesh_terms: D000328:Adult; D002823:Chorion; D015331:Cohort Studies; D005260:Female; D047109:Fetal Development; D020567:Fetal Weight; D005865:Gestational Age; D006801:Humans; D007231:Infant, Newborn; D016014:Linear Models; D008297:Male; D055361:Multilevel Analysis; D011247:Pregnancy; D059285:Pregnancy, Twin; D012016:Reference Values; D012189:Retrospective Studies; D016216:Ultrasonography, Prenatal
keywords: chorionicity; estimated fetal weight; fetal growth; reference chart; twin gestation