title: Definition of intertwin birth weight discordance.
abstract: OBJECTIVE
  To establish the level of birth weight discordance at which perinatal morbidity increases in monochorionic and dichorionic twin pregnancy.
  
  
  METHODS
  This prospective multicenter cohort study included 1,028 unselected twin pairs recruited over a 2-year period. Participants underwent two weekly ultrasonographic surveillance from 24 weeks of gestation with surveillance of monochorionic twins two-weekly from 16 weeks. Analysis using Cox proportional hazards compared a composite measure of perinatal morbidity (including any of the following: mortality, respiratory distress syndrome, hypoxic-ischemic encephalopathy, periventricular leukomalacia, necrotizing enterocolitis, or sepsis) at different degrees of birth weight discordance with adjustment for chorionicity, gestational age, twin-twin transfusion syndrome, birth order, gender, and growth restriction.
  
  
  RESULTS
  Perinatal outcome data were recorded for 977 patients (100%) who continued the study with both fetuses alive beyond 24 weeks, including 14 cases of twin-twin transfusion syndrome. Adjusting for gestation at delivery, twin order, gender, and growth restriction, perinatal mortality, individual morbidity, and composite perinatal morbidity were all seen to increase with birth weight discordance exceeding 18% for dichorionic pairs (hazard ratio 2.2, 95% confidence interval [CI] 1.6-2.9, P<.001) and 18% for monochorionic twins without twin-twin transfusion syndrome (hazard ratio 2.6, 95% CI 1.6-4.3, P<.001). A minimum twofold increase in risk of perinatal morbidity persisted even when both twin birth weights were appropriate for gestational age.
  
  
  CONCLUSIONS
  : The threshold for birth weight discordance established by this prospective study is 18% both for dichorionic twin pairs and for monochorionic twins without twin-twin transfusion syndrome. This threshold is considerably lower than that defined by many retrospective series as pathologic. We suggest that an anticipated difference of 18% in birth weight should prompt more intensive fetal monitoring.
authors.affiliation: From the Royal College of Surgeons in Ireland, the UCD School of Medicine and Medical Science, University College Dublin, the National Maternity Hospital, the Rotunda Hospital, Coombe Women and Infants' University Hospital, Dublin, Ireland; Anu Research Centre, University College Cork, Cork, Ireland; Royal Victoria Maternity Hospital, Belfast, Northern Ireland, United Kingdom; the National University of Ireland, Galway, Ireland; Mid-Western Regional Maternity Hospital, Limerick, Ireland; and Our Lady of Lourdes Hospital, Drogheda, Ireland.
authors.forename: Fionnuala M
authors.initials: FM
authors.lastname: Breathnach
authors.forename: Fionnuala M
authors.initials: FM
authors.lastname: McAuliffe
authors.forename: Michael
authors.initials: M
authors.lastname: Geary
authors.forename: Sean
authors.initials: S
authors.lastname: Daly
authors.forename: John R
authors.initials: JR
authors.lastname: Higgins
authors.forename: James
authors.initials: J
authors.lastname: Dornan
authors.forename: John J
authors.initials: JJ
authors.lastname: Morrison
authors.forename: Gerard
authors.initials: G
authors.lastname: Burke
authors.forename: Shane
authors.initials: S
authors.lastname: Higgins
authors.forename: Patrick
authors.initials: P
authors.lastname: Dicker
authors.forename: Fiona
authors.initials: F
authors.lastname: Manning
authors.forename: Rhona
authors.initials: R
authors.lastname: Mahony
authors.forename: Fergal D
authors.initials: FD
authors.lastname: Malone
mesh_terms: D001724:Birth Weight; D005260:Female; D005323:Fetal Monitoring; D005330:Fetofetal Transfusion / Q000000981:diagnostic imaging; D006801:Humans; D011247:Pregnancy; D011256:Pregnancy Outcome; D016016:Proportional Hazards Models; D011446:Prospective Studies; D014427:Twins / Q000502:physiology*; D016216:Ultrasonography, Prenatal