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The second twin seems more likely to survive when an absolute risk is calculated
- Christopher H Cheetham (19 March 2007)
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Experienced accoucheur for twin vaginal delivery
- Bode Williams (21 March 2007)
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Perinatal death in twins: Term vaginal delivery vs. Caesarean Section:
- Shachi mudgal, Chris Spencer, Consultant Obstetrician & Gynaecologist, St John's Hospital, Chelmsford (23 March 2007)
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Twins, birth order and the risk of perinatal death
- Gordon CS Smith (2 April 2007)
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Interpret study sample with caution
- Deirdre J Murphy (3 April 2007)
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Birth order and the risk of perinatal death in twins
- Gordon CS Smith (5 April 2007)
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Christopher H Cheetham, Consultant Paediatrician (retired) 6 Pretoria Road High Wycombe HP13 6QW
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Sir Drs Smith, Fleming and White (BMJ 17th March p.576) are to be congratulated on their analysis of nearly 100,000 twin pregnancies. Their conclusion of three to four fold increased mortality in a second twin when born vaginally at term is a relative rather than an absolute risk. There was an excess 2nd twin all cause mortality of 73 infants, and an excess 2nd twin anoxic mortality of 60 infants. Altogether there were nearly 100,000 births, but the preterm rate (higher in twin births) was not stated, so there might have been 90,000 full term confinements. This gives an increased mortality for the second twin of 0.8 per thousand births, and an increased anoxic mortality for the second twin of 0.67 per thousand births. If Caesarean Section were to be successful in avoiding the total second twin mortality, 1250 Caesarean Sections would be needed to save one infant. If Caesarean Section were only successful in preventing anoxic deaths, 1500 Caesarean Sections would be needed to save one infant. Though this large number of extra Sections might be justifiable, the low possible statistical benefit might persuade some mothers or their obstetricians that a vaginal delivery is a reasonable option for full term confinements. The absolute rather than the relative risk is the correct guide to decision making. Professor Steer in his accompanying leading article (p. 545) rightly states that the figures do not prove that Caesarean Sections would prevent deaths in the second twin. It is possible that foetal behaviour affects birth order in twin deliveries, and that the more vulnerable infant is delivered second in vaginal, but not in Caesarean deliveries. The benefit of Caesarean Section can only be proved by a comparison of total foetal mortality rates in vaginal and Caesarean deliveries. Christopher Cheetham FRCPCH Competing interests: None declared |
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Bode Williams, Consultant obstetrician and gynaecologist Frimley Park Hospital Foundation Trust, Portsmouth Road, Frimley, Surrey GU16 7UJ
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Dear Sir, The interesting study by Smith and colleagues show that second twins have a four fold increased risk of intrapartum anoxic death following vaginal delivery. In addition, the authors suggest that a planned caesarean delivery for all twins may have a protective effect. The authors also feel that a randomized study regarding the optimum method of delivery for twin pregnancy is required to address this question. I, however feel that the single most important and independent factor in achieving optimum outcomes in twin vaginal delivery is the skill of the attending obstetrician. This important variable is difficult to measure and may not be taken into account in a multi centre randomised study. I would advocate the presence of an experienced accoucheur for all twin vaginal deliveries as the major intrapartum risk factors for death of the second twin including prolonged intertwin interval and the associated birth trauma with manipulation of the second twin can be avoided with skilful management of the second stage of labour. Competing interests: None declared |
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Shachi mudgal, Specialist registrar, Obstetrics & Gynaecology St Mary's Hospital Portsmouth PO3 6AD, Chris Spencer, Consultant Obstetrician & Gynaecologist, St John's Hospital, Chelmsford
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We read this study with interest because of its potential implications on intrapartum management of twin pregnancies. Based on the information provided, it appears that, at term, vaginally delivered second twins have a four-fold risk of death caused by intra-partum hypoxia, when compared to twins delivered by elective caesarean section. We believe that this study has misleading conclusions because precise data regarding these deliveries are lacking. These data include risk factors affecting the safe delivery of the second twin such as presentation, birth weight and chorionicity. In addition, maternal parity is of some importance as this is relevant to the success of a vaginal twin delivery. The authors suggest a randomised study to shed light on the place of elective Caesarean section for all twin pregnancies but this will only create more misleading information, as all studies comparing vaginal birth with Caesarean delivery will favour abdominal delivery. This is because Caesarean delivery is faster than vaginal birth and will usually give a better fetal outcome than a labour lasting several hours. If we are to go down the route of favouring elective Caesarean section for all twin pregnancies, then we will effectively de-skill our obstetric medical staff and if a woman requests a vaginal birth, there will no skilled attendant at hand to acquiesce to her wishes. Competing interests: None declared |
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Gordon CS Smith, Professor of Obstetrics & Gynaecology Dept. Obstetrics & Gynaecology, Cambridge University, The Rosie Hospital, Cambridge, CB2 2SW, UK.
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In response to Cheetham,[1] as stated, our study was able only to address the relative risk to the second twin given the nature of the data. We have previously addressed both absolute risk and the effect of caesarean section using record linkage of the Scottish Morbidity Record and the Scottish Stillbirth and Infant Death Enquiry. This linkage provides both the numerator and denominator required for the comparisons proposed. The absolute risk of perinatal death for second twins born at term was approximately 1 in 270 for all causes, 1 in 350 for death due to intrapartum anoxia, and 1 in 500 for anoxic death due to a mechanical cause.[2] Planned caesarean section is associated with a lower risk of perinatal death and, if causality is assumed, the number needed to treat is 264 caesarean deliveries to prevent each death.[3] These numbers reflect higher absolute risks than calculated by Dr Cheetham. This may reflect flaws in the assumptions of his calculations. It may also reflect the observation that, although the second twin was at increased risk of death at term in both populations, the relative risk was higher in Scotland. We cannot address whether this reflects a greater absolute risk to the second twin in Scotland or to the first twin in England, Northern Ireland and Wales. In response to Williams, we agree that an experienced accoucheur is ideal for all twin births. The purpose of our paper was to demonstrate that, whatever provision is made for twin births at present, there is still an excess risk to the second twin. It is necessary to define a problem prior to coming up with a solution. In response to Mudgal and Spencer, they do not appear to have considered the question being addressed. There are two stages to the question: (1) is there an association between the risk of death and birth order? (2) what is the cause of the association with birth order? We primarily addressed the first of these questions and, as outlined in the paper, there remained significant dispute in the literature. Their assertion that "all studies comparing vaginal birth with Caesarean delivery will favour abdominal delivery" lacks rational justification. Their point about obstetricians become deskilled is interesting. Would they propose to a woman with twins that she should attempt vaginal birth in order to maintain the skills of the accoucheur? Women may have very many motivations to prefer vaginal birth to caesarean section and these should be discussed and respected. But we suspect there are very few women who would be prepared to consider contributing to their medical attendant's experience as a major motivation for deciding on mode of delivery. References 1. Cheetham C. Absolute risk: better basis for decision making. BMJ 2007;334:651-2. 2. Smith GC, Pell JP, Dobbie R. Birth order, gestational age, and risk of delivery related perinatal death in twins: retrospective cohort study. BMJ 2002;325:1004. 3. Smith GCS, Shah I, White IR, Pell JP, Dobbie R. Mode of delivery and the risk of delivery-related perinatal death among twins at term: a retrospective cohort study of 8073 births. BJOG 2005;112:1139-44. Competing interests: None declared |
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Deirdre J Murphy, Professor of Obstetrics & Gynaecology, Consultant Obstetrician Trinity College Dublin & Coombe Women's Hospital
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The increased risks of labour for the second twin have long been known and are mentioned in most undergraduate textbooks. It is also entirely logical that avoiding labour will avoid the risk of an adverse intrapartum event - no complex analyses are required to establish this. The study by Smith et al is useful in that it aims to quantify the risk of perinatal mortality in twin pregnancies. [1] However, the analyses presented and the interpretation warrant closer scrutiny. The study is based exclusively on pairs of twins discordant for perinatal death. The absence of a true denominator precludes the calculation of absolute risks and therefore limits interpretation and the study's usefulness for patient counselling. The study sample does not appear to be representative of the overall twin population. Only 12.9% of babies were born at term (defined as >35 weeks gestation) and the median birth weight was 970g. This strongly suggests that discordant growth and specifically fetal growth restriction may be involved in the aetiology of the anoxic deaths rather than vaginal delivery per se. It is also important to note that data on the mode of delivery were missing for 29.3% of the study sample. The sub-group analyses stratified by gestational age would also suggest a trend towards increased perinatal death among preterm first born twins compared to second born twins. What is the biologically plausible mechanism for this? Both the authors and Steer in his editorial [2] suggest that elective caesarean section is likely to be the preferred mode of delivery for all twin pregnancies. They need to reflect that while the Term Breech Trial reported an important reduction in perinatal mortality and morbidity there was no difference in mortality or neuro-developmental outcomes at two years and in fact there was an excess of childhood medical problems reported among those delivered by caesarean section. [3] In a previous cohort study of a complete population of twin pregnancies in Bristol, we found that vaginal delivery had a greater effect on reducing rates of respiratory distress syndrome (RDS) than the use of antenatal corticosteroids. [4] Vaginal birth for twins may have potential advantages worth considering. There is also no mention of the author's previous study reporting an increased risk of unexplained term stillbirth following birth by caesarean section. [5] Women may wish to consider the overall reproductive consequences of opting for caesarean section. Unlike Smith, I draw some comfort from the finding that there was no increased risk of perinatal death for preterm second twins, albeit with caveats about the study design. With the unrelenting increase in rates of caesarean section, further research is long overdue on how to facilitate safe vaginal delivery in the context of complex obstetric care including preterm birth, twin pregnancy, breech presentation and failure to progress in the second stage of labour. Obstetricians should retain their critical abilities when addressing the evidence on caesarean section, even if the reported findings support their personal preferences. Women deserve informed choice when making decisions about birth but this will only be possible if skilled obstetric care is available to them. References 1. Smith GC, Fleming KM, White IR. Birth order of twins and risk of perinatal death related to delivery in England, Northern Ireland and Wales, 1994-2003: retrospective cohort study. BMJ Mar 17:334(7593):576. Epub 2007 Mar 2. 2. Steer P. Comment. BMJ Mar 17:334(7593): 3. Hofmeyr, GJ. Hannah, ME. Planned caesarean section for term breech delivery. [Systematic Review] Cochrane Pregnancy and Childbirth Group Cochrane Database of Systematic Reviews. 1, 2007. 4. Murphy DJ, Caukwell S, Joels L, Wardle PW. Cohort study of the neonatal outcome of twin pregnancies that were treated with prophylactic or rescue antenatal corticosteroids. Am J Obstet Gynecol 2002;187:483-8. 5. Smith G, Pell J, Dobbie R. Caesarean section and the risk of unexplained stillbirth in subsequent pregnancy. Lancet 2002;362:1779-84. Competing interests: None declared |
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Gordon CS Smith, Professor of Obstetrics & Gynaecology Dept of Obstetrics & Gynaecology, Cambridge University, The Rosie Hospital, Cambridge, CB2 2SW
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In response to Prof Murphy, multiple previous studies had stated that there was no difference in the risk of perinatal death between first and second twins.[1-5] The only recent studies that have shown this are our previous analyses,[6;7] including the most recent which she refers to.[8] In fact, if non-paired statistical tests are applied to a population including infants at all gestational ages, there is no difference.[6] The analysis of a dichotomous outcome in paired data only utilises pairs which are discordant.[9] In terms of assessing relative risk in relation to birth order, having information on the concordant pairs is non- informative. We agree that our most recent analysis lacked data on absolute risks. However, we have previously published these.[6;7] The primary motivation for performing this study was that our previous findings had been dismissed as "probably an artefact of mortality comparisons".[5] In fact, the failure to demonstrate a difference in other populations probably reflects the failure to use paired statistical tests, the failure to stratify by gestational age and the failure to define truly delivery-related events.[9] It is commented that the study group is unrepresentative of the whole population of twins. Given that the study group consists of twin pairs where one died and the other survived, this is hardly surprising. However, when comparing the relative risk of death for second twins, this is the appropriate group to study. We agree that the short term reduction in the risk of mortality should be weighed against the increased risk of unexplained stillbirth in subsequent pregnancies,[10] a finding which we have recently confirmed.[11] We explicitly discussed this point when we published the absolute risks of death associated with vaginal birth of twins and caesarean delivery.[7] It is probably best, when counselling women about mode of delivery, for obstetricians to leave their personal preferences behind, whether they favour caesarean section or vaginal birth. I am neither for nor against caesarean section. I have previously highlighted both its risks[10;12] and benefits.[7;13] Ultimately, the individual who carries the consequences of the decision is the pregnant woman. For many women, vaginal birth may be more appropriate, for others a caesarean section may be more appropriate. The decision to have a vaginal birth or caesarean section rests on the balance of the risks and benefits, in relation to the individual woman's circumstances, attitudes and priorities. References 1. McCarthy BJ, Sachs BP, Layde PM, Burton A, Terry JS, Rochat R. The epidemiology of neonatal death in twins. Am J Obstet Gynecol 1981;141:252-6. 2. Zhang J, Bowes WA, Jr., Grey TW, McMahon MJ. Twin delivery and neonatal and infant mortality: a population-based study. Obstet Gynecol 1996;88:593-8. 3. Newman RB, Mauldin JG, Ebeling M. Risk factors for neonatal death in twin gestations in the state of South Carolina. Am J Obstet Gynecol 1999;180:757-62. 4. Hartley RS, Emanuel I, Hitti J. Perinatal mortality and neonatal morbidity rates among twin pairs at different gestational ages: optimal delivery timing at 37 to 38 weeks' gestation. Am J Obstet Gynecol 2001;184:451-8. 5. Sheay W, Ananth CV, Kinzler WL. Perinatal mortality in first- and second-born twins in the United States. Obstet Gynecol 2004;103:63-70. 6. Smith GCS, Pell JP, Dobbie R. Birth order, gestational age, and risk of delivery related perinatal death in twins: retrospective cohort study. BMJ 2002;325:1004. 7. Smith GCS, Shah I, White IR, Pell JP, Dobbie R. Mode of delivery and the risk of delivery-related perinatal death among twins at term: a retrospective cohort study of 8073 births. BJOG 2005;112:1139-44. 8. Smith GCS, Fleming KM, White IR. Birth order of twins and risk of perinatal death related to delivery in England, Northern Ireland, and Wales, 1994-2003: retrospective cohort study. BMJ 2007;334:576. 9. Smith GCS. Estimating risks of perinatal death. Am J Obstet Gynecol 2005;192:17-22. 10. Smith GCS, Pell JP, Dobbie R. Caesarean section and risk of unexplained stillbirth in subsequent pregnancy. Lancet 2003;362:1779-84. 11. Smith GCS, Shah I, White IR, Pell JP, Dobbie R. Previous pre- eclampsia, preterm delivery and delivery of a small for gestational age infant and the risk of unexplained stillbirth in the second pregnancy: a retrospective cohort study, Scotland 1992-2001. Am J Epidemiol 2007;165:194-202. 12. Smith GCS, Wood AM, White IR, Pell JP, Cameron AD, Dobbie R. Neonatal respiratory morbidity at term and the risk of childhood asthma. Arch.Dis.Child 2004;89:956-60. 13. Smith GCS, Pell JP, Cameron AD, Dobbie R. Risk of perinatal death associated with labor after previous cesarean delivery in uncomplicated term pregnancies. JAMA 2002;287:2684-90. Competing interests: None declared |
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