The incidence of multiple pregnancies is rising across many parts of the world. The Australian Bureau of Statistics reported 4316 sets of twins, representing 1.5 per cent of all births and 65 sets of triplets and higher order multiples, making up 0.02 per cent of all births in Australia for 2014.¹ During the same reporting period there were 10 989 maternities with multiple births in England. This represents 1.6 per cent of all maternities and in excess of 22 000 babies born.² The increase in higher order pregnancies has been attributed to the widespread use of assisted reproduction techniques resulting from delayed childbirth and advanced maternal age at conception.³ The management of women with higher order multiple pregnancies brings unique challenges to healthcare providers. One of the key challenges is the decision regarding the most appropriate mode of birth. Both timing and mode of birth for twins is a subject of considerable debate and the safety of term vaginal birth (VB) for twins has long been of concern.

The appropriate intrapartum management of twin pregnancy remains a controversial issue in obstetric practice. The Twin Birth Study has provided level-one evidence that at least most twins after 32 weeks should be delivered by planned VB provided the leading twin is cephalic and there is an experienced operator in attendance, with facilities to carry out an immediate lower segment caesarean section (CS) if the need arises.⁴ In addition, this study provided clearer information about the optimal methods of delivering the second, non-vertex twin. The time interval between the twin VB in this study was 10 to 16 minutes.⁵ This seems to be associated with the best health outcomes.

It is common clinical practice for uncomplicated monochorionic twins to be born at 36–37^{6 7} weeks and dichorionic twins at 37–38 weeks.⁸ This is based on the risk at this gestation of intrauterine mortality from placental insufficiency of twin pregnancies which is stated to equal that of post-term singleton pregnancies.^{9 10} The timing of birth, either by induction or elective CS, is an important consideration in twin birth. The purpose of this discussion is to review the evidence around different modes of birth in the management of twin pregnancies.

Vaginal delivery for twin births

The Twin Birth Study, published in 2013, was a large, prospective, international randomised controlled trial of 1398 women (2795 fetuses) who were randomly assigned to planned cesarean delivery or 1406 women (2812 fetuses) to planned vaginal delivery between 32 and 38 weeks gestation, where the presenting twin was cephalic. The rate of cesarean delivery was 90.7 per cent in the planned-cesarean-delivery group and 43.8 per cent in the planned-VB group.¹¹ This study reported that planned CS did not reduce the risk of fetal or neonatal death or serious neonatal morbidity when compared with planned VB. This supports the practice of planned VB in women with an uncomplicated pregnancy when the first twin is in a cephalic presentation. These findings were confirmed by Castro et al who found that VB of twin pregnancies can be successful, especially for women who have had previous VB.¹²

The intrapartum management of twins presents difficulties and challenges. Often, external monitoring of the heart rate of the fetuses can be difficult and performing fetal scalp sampling is only feasible for the leading twin. Other requirements for managing labour for women with a twin gestation include the presence throughout labour and delivery of a skilled obstetrician, midwife, obstetric anaesthetist and paediatric specialist. Intravenous access is required, as are immediate availability of blood products and uterotonic agents and rapid access to operating theatres.¹³ Monochoronic twins have a 1.5–2.5  per cent risk of intrapartum twin-to-twin transfusion syndrome.¹⁴ If this occurs, both twins require immediate delivery. Of great importance throughout labour and delivery is that the obstetrician has a carefully considered plan regarding what he or she will do in the event of cord prolapse, placental separation before birth of the second twin, change in position of the second twin during birth of the first, or immediate postpartum heamorrhage. The use of epidural analgesia during twin labour may enable prompt abdominal delivery of the second twin if this becomes indicated following VB of the leading twin. The increasing number of women with a twin pregnancy requires there be sufficient appropriately trained birth attendants to manage the twin births.

Elective CS for twin births

Obstetricians are performing an increasing number of CS for non-vertex presentations. Absolute indications for elective CS are few, and there are no level-one evidenced clinical studies on which to base strong recommendations.¹⁵ From the available literature, CS without a trial of labour should be carried out in cases of conjoined twins and mono-amniotic twins.¹⁶ Level-two evidence from Vendettii also confirmed that, under these circumstances, a policy of planned cesarean delivery from 34 weeks gestation is indicated.¹⁷ An advantage of delivering twins by elective CS is it can be done during daylight hours when appropriate staffing levels and support from paediatrics are more readily available. The risk of fetal distress associated with labour, especially for the second twin, is obviated and the woman knows when her babies will be born.¹⁸ The maternal risks associated with an elective twin caesarean include bleeding, infection, visceral organ damage, urinary tract infection, wound dehiscence, pneumonia, deep vein thrombosis and pulmonary embolus. There are also implications for any future pregnancies, such as placenta accreta, that need to be taken into consideration when counselling women about a CS. The fetal risk of transient tachypnoea of the newborn, respiratory distress syndrome, and even persistent fetal circulation associated with elective CS is likely to be higher in twin pregnancies and should be discussed and documented when counselling women about twin birth. Planned cesarean section may decrease the risk of a low five-minute Apgar score, particularly if the first twin is breech.¹⁹ Otherwise, there is no evidence to support planned cesarean section for uncomplicated twin pregnancies.²⁰

The retained second twin

The retained second twin presents a unique challenge in labour. After the birth of the first twin, the position and presentation of the second twin should be carefully monitored.²¹ Intravenous oxytocin infusion should be ready so that uterine contractions can be re-established promptly. Retention of the second twin occurs when its delivery has not taken place for approximately 15 to 30 minutes or more following the birth of the first. In the Twin Birth Study, the mean interval between the twins was eight minutes, with a range from one to 33 minutes. The longer the second twin remains in utero the greater the risks, since the birth of the first twin is followed by reduction in the utero-placental blood flow, thus compromising the oxygen and nutritional supplies to the second twin.²²

The predisposing causes of retention of the second twin are uterine atony, inadvertent administration of ergometrine following the delivery of the first in an undiagnosed twin pregnancy, a constriction ring clamping down on a larger second twin, obstruction from malpresentation or malposition, rupture of forewaters in a monochorionic twin pregnancy, and retention of the second twin in a horn of a congenitally malformed uterus.²³ Frequent maternal complications are chorioamnionitis, postpartum haemorrhage, retained placenta and placental abruption. Fetal complications include fetal distress and malpresentation. There are three delivery options in such circumstances:

1. Breech extraction (coupled with internal version in the case of a transverse lie) and vaginal delivery of the second twin
2. External version and delivery of the second twin vaginally from vertex presentation
3. Combined vaginal-caesarean delivery.²⁴

The decision to carry out breech extraction, operative vacuum or forceps will be aided by some assessment of the estimated fetal weight of the second twin and any other maternal or fetal risk factors. The optimal conditions to perform a breech extraction or instrumental delivery are: cervix fully dilated, presenting part engaged, appropriate maternal analgesia, strong regular contractions and no fetal distress. There is significant published literature suggesting breech extraction of the non-vertex second twin is preferable to external cephalic version because it appears to be associated with a significantly lower incidence of fetal distress and abdominal delivery with comparable neonatal outcome.²⁵ Such extraction may require internal podalic version if the fetal feet are more readily accessible to the birth attendant than the vertex.

Emergency CS for the retained twin

Although CS of a second twin after VB of the first twin is rare, it has clinical importance as an acute obstetric emergency.²⁶ Various problems mandate that obstetricians deliver a second twin quickly after the first has been born. Common indications are imminent uterine asphyxia and suspected fetal distress, transverse lie with or without prolapse of umbilical cords or limbs, and when rapid VB seems unlikely.²⁷ Risk factors for emergency cesarean section of the second twin are preterm delivery, previous CS, placental abruption and breech presentation.²⁸ Nevertheless, short-term perinatal outcomes are comparable to twins born vaginally.

In conclusion, the mode of birth for women with a twin pregnancy remains controversial. The Twin Birth Study has given clinicians reassurance that it is safe to undertake a trial of labour when the leading twin is in a cephalic presentation. There are currently no absolute indications for a CS in the absence of other obstetric complications. Management of the second twin will continue to challenge obstetricians and midwives. Therefore, appropriate patient counselling and high-quality multidisciplinary team management of women with twin pregnancies will help ensure good outcomes and the safety of women and their babies.

1. Australian Bureau of Statistics Births. 2014. Available from: www.abs.gov.au/ausstats/[email protected]/Latestproducts/3301.0Main%20 Features32014?opendocument&tabname= Summary&prodno=3301.0&issue=2014&n um=&view=.
2. Office for National Statistics. Birth Characteristics in England and Wales.2014. [cited 30 September 2016] www.ons.gov.uk/peoplepopulationandcommunity/birthsdeathsandmarriages/livebirths/bulletins/birthcharacteristicsinenglandandwal es/2015-10-08#multiple-births.
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