Mode of delivery of twins
Perinatal mortality is five times higher in twins than in singletons. This increased risk is mostly unrelated to mode of delivery. Nevertheless, vaginal birth of twins at term is well recognised as a high-risk area. It is associated with increased rates of perinatal death and a depressed Apgar score, primarily because of intrapartum asphyxia of the second twin. It is plausible that planned caesarean section may have a protective effect on these outcomes but there is a lack of direct evidence in this area.
Perinatal mortality is five times higher in twins than in singletons.0101 This is mostly because of factors unrelated to mode of delivery, such as antepartum stillbirth, the effects of prematurity and congenital abnormality. The vaginal birth of twins is, however, recognised as an area associated with specific risks. Determining the possible protective role of interventions, in particular the use of caesarean section, necessitates isolating and measuring adequately the effects of mode of delivery. Already in UK practice, approximately 60% of twins are delivered by caesarean section. This high rate may reflect some obstetricians’ anxieties about vaginal twin delivery and may be evidence of defensive practice. The argument surrounding vaginal birth of twins still relies solely on observational data for adequately powered studies.
Levels of evidence
The major advantage of randomly controlled trials (RCTs) over other sources of comparative data is that differences between groups are unlikely to be explained by a number of types of bias; specifically, selection bias and, if blinded, observer bias.
A major problem related to RCTs in obstetrics is that serious adverse outcomes are relatively uncommon. For example, delivery-related perinatal death of the second twin at term affects approximately one in 270 births. It has, therefore, been estimated that an RCT would require 6500 twin pregnancies to determine whether caesarean section would reduce the risk of perinatal death.0201 Although RCTs have been produced, these include a small fraction of the number required to test the hypothesis and such studies will, almost inevitably, yield negative results. These theoretical issues present obstetricians with practical problems when making clinical decisions. While it is true to say that there are no data from RCTs directly addressing the question in twins, there is evidence to indicate that caesarean section is likely to reduce perinatal mortality and morbidity in this context.
Antenatal care and timing of delivery
There are several features of the antenatal management of twin pregnancies that are important when considering mode of delivery in twins.
- Accurate gestational dating is best performed in the first-trimester of pregnancy as this is important for optimal planning of timing of delivery.
- First-trimester ultrasound has the most accurate diagnosis of chorionicity and amnionicity, with close to 100% sensitivity and specificity. Current practice is to deliver monoamniotic twins by planned caesarean section because of the increased risk of cord complications.
Some centres advocate earlier delivery of all monochorionic twins. There are two main reasons for this.
- First, antepartum stillbirth is particularly common among twins and the risk may increase with advancing gestational age. If intrauterine death of one of a pair of monochorionic twins occurs, there is a high risk of death (around 25%) or serious morbidity of the other twin (around 50%).9901
- Second, monochorionic twins are at risk of twin-to-twin transfusion syndrome; early elective delivery eliminates the risk of late adverse events due to late-onset twin-to-twin transfusion syndrome. Clearly, these potential benefits of early elective delivery need to be balanced against the risks of prematurity. The exact gestational age where the balance is achieved is unclear, although some centres schedule delivery as early as 36 weeks. Twins are at increased risk of fetal anomaly and, subject to parental consent, all twin pregnancies should undergo a detailed anatomical survey. Antenatal biometry is typically performed every 4 weeks for dichorionic twins and every 2 weeks for monochorionic twins. Twins are known to have an increased risk of growth restriction. The relevance for this in the context of mode of delivery is that, in the presence of fetal compromise, planned caesarean section may be performed to avoid the stress of labour. Also, increased birthweight discordance between twins is associated with an increased risk of delivery-related perinatal death.0201
Some studies have indicated an increase of stillbirths among twins at around 38 weeks, with a greater than fivefold excess over singletons.0102 However, these analyses ignore key differences in the management of intrauterine fetal death. If a singleton dies in utero, labour will usually be induced promptly. If one of a dichorionic twin pair dies in utero, the pregnancy will typically be managed conservatively until 37–38 weeks, when labour will be induced. It follows, therefore, that there will be an apparent marked excess of antepartum stillbirths among twins at around this gestation. Nevertheless, when expressed as all deaths at or beyond 24 weeks, there is a three-fold excess of antepartum stillbirth among twins.0301 Given the much higher risk of antepartum stillbirth among twins, elective delivery before 40 weeks would seem to be a reasonable approach.
Neonatal mortality is lower for the second twin after caesarean delivery at birth before 34 weeks. At term, mortality is low irrespective of delivery mode.0801
Mode of delivery
First twin breech
Most recommend that, where the first twin is presenting by the breech, delivery should be by planned caesarean section. A large-scale RCT has demonstrated that planned caesarean section reduced the risk of neonatal morbidity or mortality among singleton pregnancies in a breech presentation.0001
First twin cephalic
For some years, UK practice has generally been that vaginal delivery will normally be considered when the first twin is in a cephalic presentation. There is some controversy when the second twin is in a noncephalic presentation. However, the presentation of the second twin may change after delivery of the first, in up to 20% of cases, depending on the gestation. Therefore, if noncephalic presentation of the second twin is seen as a contradiction to attempting vaginal birth, then either both twins should be delivered by planned caesarean or emergency caesarean section should be performed for the second twin in the event that the presentation becomes noncephalic after delivery of the first.
Most current reviews suggest that vaginal delivery should be attempted if the first twin is cephalic, irrespective of the lie and presentation of the second, provided that the estimated fetal weight is greater than 2000 g. There are, however, a number of lines of evidence that suggest that this policy may be associated with a specific increased risk of morbidity and mortality in the second twin. A study of 16 000 twin pairs in New York between 1978 and 19840202 demonstrated rates of intrapartum stillbirth and neonatal death three to four times greater than the rest of the population. Twins delivered vaginally had a four-fold risk of neonatal mortality compared with those delivered by caesarean section.
A Swedish analysis of approximately one million births at term between 1988 and 1997 demonstrated a four-fold risk of a depressed 5-minute Apgar score among second twins.9001
A Scottish study using linked databases of pregnancy and perinatal mortality demonstrated an excess of delivery-related perinatal death among second twins born at term.0201 Among 2436 twin births at or after 36 weeks of gestation by a means other than planned caesarean section, there were no deaths among first twins and nine deaths among second twins (P = 0.007). Seven of the nine second-twin births were caused by intrapartum anoxia, including five from an obstetric mechanical cause, such as cord prolapse or birth trauma. Six of these followed cephalic vaginal delivery of the first twin. The absolute risk of death of the second twin at term was one in 270 (or 3.7 per 1000 deliveries.
On the basis of the information currently available, planned caesarean section may reduce the risk of perinatal death even when the first twin is in a cephalic presentation. However, there are no adequately powered studies at present that have addressed this. Women should be informed of the small risk of perinatal death to the second twin (of around 1 in 270), the potential benefits of planned caesarean delivery and offered the choice between planned caesarean section and attempting vaginal birth. It is important to remember that, given the same information about risks, different women will make different decisions about the right choice for them. The key example of this is screening for Down syndrome. Different women will attach different degrees of importance to the consequences of their decisions. However, it is crucial that women are provided with the best estimates of absolute risk of both the advantages and disadvantages of intervention, in order to make an informed choice.
Attempting vaginal birth
All twin births should be conducted in a hospital with facilities for immediate caesarean section because of the risk of intrapartum fetal hypoxia and the necessity for immediate caesarean delivery. There is an absence of RCT evidence regarding the use of electronic fetal monitoring (EFM) in twin births. However, it is recommended by all reviewers.
Most authors recommend epidural anaesthesia for women attempting vaginal birth for twins. It may facilitate manoeuvres to deliver the non-vertex second twin and can also be used to provide immediate anaesthesia for intrapartum caesarean section. The drawback of the invasiveness of an epidural needs to be set against the risks of maternal morbidity and mortality associated with general anaesthesia. Many obstetricians prefer to deliver twins in theatre, with ultrasound immediately available. This reflects the possibility that, in some cases, caesarean section is required to deliver the second twin following vaginal delivery of the first. This used to be seen as a failure of obstetric management and reflected badly on the obstetrician who resorted to it. However, the practice has become widespread.
Delivery of the noncephalic second twin
Some authors have suggested that external cephalic version (ECV) should be attempted when the second twin presents by the breech, following vaginal delivery of the first. However, a series of studies comparing ECV with breech extraction found that the risks of both emergency caesarean section (38% versus 3%) and fetal distress (18% versus 1%) were dramatically higher among the ECV group.9501
Time interval between twin births
The standard dictum was that the second twin should ideally be delivered less than 15 minutes after the first and invariably within 30 minutes. However, these recommendations preceded the availability of EFM. Nevertheless, one study has shown that, even when continuous EFM is employed, the risk of acidosis in the second twin increases with increasing interval between births. This finding supports the view that when the interval exceeds 30 minutes delivery should probably be expedited.0103
The intrapartum management of twins is a major area of risk in obstetrics and twins are at increased risk of delivery-related death. Planned caesarean delivery could theoretically avoid some of these risks but direct evidence of a protective effect is currently lacking. The possible effects of caesarean section on short- and long-term maternal morbidity and future pregnancies should also be considered. Counselling of women should involve a frank discussion of the uncertainties and the small risk of adverse events at term.
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