Papers

Birth order, gestational age, and risk of delivery related perinatal death in twins: retrospective cohort study

Gordon C S Smith, professor, ^a Jill P Pell, consultant, ^b Richard Dobbie, senior statistician. ^c

^a Department of Obstetrics and Gynaecology, Rosie Maternity Hospital, Cambridge CB2 2QQ, ^b Department of Public Health, Greater Glasgow Health Board, Glasgow G3 8YU, ^c Information and Statistics Division, Common Services Agency, Edinburgh EH5 3SE

Correspondence to: G C S Smith gcss2{at}cam.ac.uk

Objective: To determine whether twins born second are at increased risk of perinatal death because of complications during labour and delivery.
Design: Retrospective cohort study.
Setting: Scotland, 1992 and 1997.
Participants: 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.
Main outcome measure: 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.

What is already known on this topic It is difficult to assess the wellbeing of second twins during labour Deliveries of second twins are at increased risk of mechanical problems, such as cord prolapse and malpresentation, after vaginal delivery of first twins Increased risks of perinatal death in second twins have not been shown, but the methods of these studies were flawed What this study adds Second twins delivered at term are at increased risk of delivery related perinatal deaths Intrapartum anoxia caused 75% of these deaths in second twins, and most of these resulted from mechanical problems after vaginal delivery of first twins Planned caesarean section of twins at term may prevent perinatal deaths