title: Birth order, gestational age, and risk of delivery related perinatal death in twins: retrospective cohort study.
abstract: OBJECTIVE
  To determine whether twins born second are at increased risk of perinatal death because of complications during labour and delivery.
  
  
  METHODS
  Retrospective cohort study.
  
  
  METHODS
  Scotland, 1992 and 1997.
  
  
  METHODS
  All twin births at or after 24 weeks' gestation, excluding twin pairs in which either twin died before labour or delivery or died during or after labour and delivery because of congenital abnormality, non-immune hydrops, or twin to twin transfusion syndrome.
  
  
  METHODS
  Delivery related perinatal deaths (deaths during labour or the neonatal period).
  
  
  RESULTS
  Overall, delivery related perinatal deaths were recorded for 23 first twins only and 23 second twins only of 1438 twin pairs born before 36 weeks (preterm) by means other than planned caesarean section (P>0.99). No deaths of first twins and nine deaths of second twins (P=0.004) were recorded among the 2436 twin pairs born at or after 36 weeks (term). Discordance between first and second twins differed significantly in preterm and term births (P=0.007). Seven of nine deaths of second twins at term were due to anoxia during the birth (2.9 (95% confidence interval 1.2 to 5.9) per 1000); five of these deaths were associated with mechanical problems with the second delivery following vaginal delivery of the first twin. No deaths were recorded among 454 second twins delivered at term by planned caesarean section.
  
  
  CONCLUSIONS
  Second twins born at term are at higher risk than first twins of death due to complications of delivery. Previous studies may not have shown an increased risk because of inadequate categorisation of deaths, lack of statistical power, inappropriate analyses, and pooling of data about preterm births and term births.
authors.affiliation: Department of Obstetrics and Gynaecology, Rosie Maternity Hospital, Cambridge CB2 2QQ. gcss2@cam.ac.uk
authors.forename: Gordon C S
authors.initials: GC
authors.lastname: Smith
authors.forename: Jill P
authors.initials: JP
authors.lastname: Pell
authors.forename: Richard
authors.initials: R
authors.lastname: Dobbie
mesh_terms: D001722:Birth Order; D015331:Cohort Studies; D003645:Death, Sudden / Q000209:etiology*; D036861:Delivery, Obstetric / Q000009:adverse effects*; D005260:Female; D005313:Fetal Death / Q000209:etiology*; D006801:Humans; D007231:Infant, Newborn; D016015:Logistic Models; D015999:Multivariate Analysis; D007744:Obstetric Labor Complications; D011247:Pregnancy; D011272:Pregnancy, Multiple; D012044:Regression Analysis; D012189:Retrospective Studies; D012307:Risk Factors; D012606:Scotland; D014427:Twins