Twins account for 3.3 percent of all births in the United States. This near doubling of the incidence since 1980 is attributable to advancing maternal age at conception and increasing use of assisted reproductive technology (American College of Obstetricians and Gynecologists, 2014a). During roughly the same period, the cesarean delivery rate for twins has increased to 75 percent, thereby reducing the overall experience with vaginal delivery of twins (Lee, 2011). Germane to this, an obstetric care consensus developed jointly by the American College of Obstetricians and Gynecologists (2014b) and the Society for Maternal-Fetal Medicine concluded that perinatal outcomes when the first twin presents cephalic are not improved by cesarean delivery. This document states that for twins with cephalic/cephalic or cephalic/noncephalic presentations, obstetric care providers should counsel women to attempt vaginal delivery. Such a strong recommendation assumes that residents are being trained to perform vaginal twin deliveries and that practicing clinicians are sufficiently competent and confident to manage the labor and delivery of such women (Carroll, 2006). The goal of this chapter is to present a well-illustrated approach to the vaginal delivery of twins.
Twin gestation presents a broad array of maternal, fetal, and neonatal complications, some of which are related to the intrapartum period and some that are not. Gestational hypertension or preeclampsia complicates up to 20 percent of twin pregnancies (Fox, 2015). Postpartum hemorrhage due to uterine atony is more common with multifetal gestations because the uterus is overdistended. Compared with those for singletons, rates for stillbirth are nearly threefold higher, for neonatal death are sevenfold higher, and for very low birthweight (<1500 grams) are 11-fold higher in twins (Cunningham, 2014).
Of twins, 16 percent have weight discordance of at least 20 percent (Miller, 2012). Discordance is expressed as a percent and defined by the formula below.
Some cases of discordance are caused by fetal growth restriction of the smaller twin, and severe growth restriction increases the risk of mortality. If twin B is more than 25-percent larger than twin A, the planned route of delivery may need to be reconsidered.
Arguably, the most significant complication is preterm birth, occurring in more than 50 percent of twin gestations. Clearly, preterm birth of twins is a major contributor to overall neonatal morbidity and mortality. Congenital anomalies are more frequent in twins and constitute a second important contributor to adverse outcomes.
For this chapter, the appropriate focus is on intrapartum complications. This was the approach taken in the Twin Birth Study (Barrett, 2013). This large randomized trial defined a composite outcome that incorporated serious neonatal morbidity and fetal or neonatal mortality. Serious morbidity was birth trauma, birth asphyxia, neonatal seizures, requirement for resuscitation or intubation, intraventricular hemorrhage, and admission to a neonatal intensive care unit. Notably, in this large trial, composite outcome between planned cesarean and planned vaginal delivery did not differ for all cases in which twin A was in cephalic presentation. Cesarean delivery itself was not considered maternal morbidity, but perhaps it should be if the neonatal outcome is not improved by cesarean.
The diagnosis of “twins” is imprecise, and these gestations should be specifically classified as either dichorionic or monochorionic (Fig. 22-1) (Moise, 2010). Using sonography, the assignment of chorionicity is at least 98-percent accurate in the first trimester (Emery, 2015a). Sonography can help identify the twin peak sign, also called the lambda sign, which indicates apposition of two amnions and intervening chorions. This can be distinguished from the T sign, which reflects two apposed amnions without intervening chorions (Fig. 22-2). With advancing gestation beyond the first trimester, the accuracy of determining chorionicity diminishes.
FIGURE 22-1
Mechanism of monozygotic twinning. Dichorionic twinning is not shown here. Black boxing and blue arrows in columns A, B, and C indicate timing of division. A. At 0 to 4 days postfertilization, an early conceptus may divide in two. Division at this early stage creates two chorions and two amnions (dichorionic, diamnionic). Placentas may be separate or fused. B. Division between 4 and 8 days leads to formation of a blastocyst with two separate embryoblasts (inner cell masses). Each embryoblast will form its own amnion within a shared chorion (monochorionic, diamnionic). C. Between 8 and 12 days, the amnion and amnionic cavity form above the germinal disc. Embryonic division leads to two embryos with a shared amnion and shared chorion (monochorionic, monoamnionic). D. Differing theories explain conjoined twin development. One describes an incomplete splitting of one embryo into two. The other describes fusion of a portion of one embryo from a monozygotic pair onto the other. (Reproduced with permission from Cunningham F, Leveno K, Bloom S, et al: Multifetal pregnancy. In Williams Obstetrics, 24th ed. New York, McGraw-Hill Education, 2014.)
FIGURE 22-2
A. Sonographic image of the “twin-peak” sign in a dichorionic, diamnionic gestation. Twins A and B are separated by a membrane created by the juxtaposed amnion and chorion of each twin. A triangular portion of placenta is seen insinuating between the amniochorion layers and creates the peak (arrow). (Used with permission from Dr. Jamie Morgan.) B. Sonographic image of the “T” sign in a monochorionic, diamnionic gestation. Twins are separated by a membrane created by the juxtaposed amnion of each twin. A “T” is formed at the point at which amnions meet the placenta. (Used with permission from Jason C. McWhirt, ARDMS.) In both images, twins are differentiated by A and B labels.
Several of the more serious complications of twin gestations are limited to monochorionic twins. Twin-twin transfusion syndrome (TTTS) and twin reversed arterial perfusion are two examples, and their treatment is described in Chapter 16 (p. 261). With monochorionic twinning, monoamnionic twins and their uncommon subset of conjoined twins are other complications. Although delivery of monochorionic twins may be planned a week earlier than that of dichorionic twins (at 37 weeks as opposed to 38 in the absence of complications), only for the rare occurrence of monoamnionic twins is cesarean favored over vaginal delivery. Recently, the North American Fetal Therapy Network issued two consensus statements on the management of uncomplicated and complicated monochorionic twin gestations, respectively (Emery 2015a,b). They suggested that uncomplicated monochorionic twins can be offered elective delivery between 360/7 and 376/7 weeks’ gestation (Emery, 2015a). Moreover, they emphasized that vaginal delivery is preferred for monochorionic diamnionic twins, assuming that providers have sufficient expertise in twin vaginal delivery. At most centers in the United States, monochorionic, monoamnionic twins are delivered by cesarean at 32 to 34 weeks’ gestation after providing a course of antenatal corticosteroids. Worldwide some centers still offer vaginal delivery for selected cases of monochorionic, monoamnionic twins (Anselem, 2015).
For dichorionic twins with the first twin presenting cephalic, vaginal delivery is also preferred if appropriate expertise is available. In the absence of complications, spontaneous onset of labor can be awaited. At Parkland Hospital, twin gestations have empirically been considered to be prolonged at 40 weeks’ gestation (Cunningham, 2014). Others, including the author, prefer to induce women at 380/7 weeks if labor has not begun spontaneously by then.
As previously mentioned, fewer than half of twin gestations will reach 37 weeks’ gestation undelivered. Categories of preterm birth in twins are the same as for singletons, but the proportionate contributions to preterm birth are different. In singletons, idiopathic preterm labor, preterm premature rupture of membranes (PPROM), and indicated preterm delivery each account for approximately one third of preterm births. Conversely, for twins, more than half of preterm births are medically indicated. For preterm birth, a wide array of preventive interventions have been studied and found to be ineffective. These include bed rest, progesterone administration, cerclage, tocolysis, and pessary placement, to name a few. Last, the management of PPROM in twins is the same as that for singletons. However, the latency period in twin gestations is somewhat shorter.
Sonography is an important tool for antepartum surveillance of twin gestations. The recommended interval for monochorionic twins is 2 weeks and for dichorionic twins is 4 weeks (Emery, 2015a). One recent report suggests reducing the interval to 2 weeks for dichorionic twins as well (Corcoran, 2015). The purported advantage of more frequent imaging is earlier detection of fetal growth restriction and abnormal umbilical artery Doppler velocimetry. Such findings may lead to earlier indicated delivery.
Antepartum sonography is also used to detect anomalies, confirm fetal presentation, estimate fetal weights, and diagnose complications such as TTTS or death of one twin. Melamed and associates (2015) analyzed data from one large randomized trial to determine the likelihood of fetal presentation change after 32 weeks’ gestation (Barrett, 2013). The presenting fetus (twin A) tended to persist in vertex presentation, but twin B underwent spontaneous version and change of polarity 25 percent of the time. Clearly, this can affect delivery planning, and thus repeat sonography is advisable for gravidas presenting in labor.
No consensus guides the necessity, frequency, or method of antepartum fetal surveillance in twin pregnancies. One group recommends weekly biophysical profile evaluation starting at 32 weeks’ gestation and suggests that it should be considered as the primary mode of testing (Booker, 2015). However, the American College of Obstetricians and Gynecologists (2014a) found no evidence that such testing was associated with improved perinatal outcomes.
For women with twin gestations who choose to attempt planned vaginal delivery, astute labor management can reduce the incidence of intrapartum cesarean. The Twin Birth Study, described earlier, randomly assigned women with twins at 32 to 39 weeks’ gestation to planned vaginal delivery or scheduled cesarean delivery (Barrett, 2013). Of women in the planned vaginal delivery group, 44 percent ultimately underwent cesarean delivery, and 10 percent of surgeries in this subgroup were performed after vaginal delivery of twin A. This contrasts with data from Schmitz (2008) and Fox (2010) and their coworkers, who reported intrapartum cesarean delivery rates of only 15 to 20 percent. These examples illustrate the potential effect of intrapartum management on delivery route.
Leftwich and colleagues (2014) demonstrated slower progression of active labor for both nulliparas and multiparas with twins. Accordingly, they suggest that allowing an additional 2 to 3 hours in labor could lead to fewer cesarean deliveries for failure to progress.
Evidence supports that oxytocin can be safely used for either induction or augmentation of labor in women with twins. When women who required oxytocin with twins were matched to a group of singletons in regard to parity, gestational age, and cervical dilation, the maximum oxytocin dosage, time from initiation to delivery, and rate of successful vaginal delivery did not differ (Fausett, 1997). Thus, it is reasonable to consider a trial of oxytocin prior to deciding on cesarean delivery of twins for failure to progress in labor.
Misinterpretation of electronic fetal heart rate tracings may also contribute to intrapartum cesarean delivery of twins. For this reason, providers should be willing to attempt intrauterine resuscitation before opting for cesarean delivery.
Last, more than 10 percent of intrapartum cesarean deliveries in the Twin Birth Study were performed for malpresentation of twin B (Barrett, 2013). However, adopting the management of twin B outlined on page 356, it may be possible to minimize or even eliminate malpresentation of twin B as the sole indication for cesarean delivery. In sum, lowering the overall cesarean rate for twins requires reducing both prelabor and intrapartum performance of cesarean delivery.
For delivery, it is advisable to move laboring women from the labor room to a well-equipped and spacious operating room (OR). For women with one or more prior vaginal deliveries, transfer to the OR should be accomplished once the cervix is completely dilated. If the cervix is changing rapidly, transfer before complete dilation may be prudent. Conversely, nulliparas can be allowed to push in the labor room unless the head of twin A is already at low station. Transfer can be safely delayed until the head has descended to +2 station.
Once the decision for transfer is made, anesthesia and pediatric personnel are notified to allow them time to arrive at the OR and prepare for delivery. On arrival to the OR, electronic fetal monitoring should be resumed, as well as oxytocin infusion if one was running in the labor room. Suspending the legs in candy-cane stirrups has practical value. It creates room for operative vaginal delivery, intrauterine manipulation, and application of Piper forceps to the aftercoming head if indicated.
Assembled equipment includes forceps, vacuum extractor, and a cesarean delivery instrument set. Also, medications such as nitroglycerin, magnesium sulfate, and various uterotonic agents should all be immediately available in the OR. As previously noted, atony risk is elevated in twins, sometimes necessitating prompt administration of oxytocin (Pitocin), methylergonovine (Methergine), and 15-methyl prostaglandin F2α (Hemabate), which are described further in Chapter 29 (p. 470). The use of a checklist that includes these is recommended (Table 22-1). For twin vaginal delivery in a residency training program, a resident should be the primary operator. An experienced faculty obstetrician should be gowned and gloved to provide supervision and assistance. Training the next generation of obstetricians is essential to developing the necessary technical skills and confidence to maintain the option of twin vaginal delivery in the future and lower cesarean delivery rates.
Equipment: stirrups, forceps, vacuum extractor, electronic fetal monitor, two neonatal resuscitation beds, cesarean delivery instrument set, anesthesia machine. Portable sonography machine (optional) Setting: spacious OR with cesarean delivery capability Medications: uterine relaxants, uterotonics Resuscitation: large-bore IV access, blood for transfusion readily available Personnel: Obstetricians—one trainee or junior provider and one senior provider with skills for intrauterine fetal manipulation, breech extraction, and operative vaginal delivery Anesthesiologist—capable of endotracheal intubation and induction of general anesthesia Pediatricians—two resuscitation teams Circulating nurse Scrub technician |
The goal of planned vaginal delivery is vaginal delivery of both twins. On the surface, this statement appears to be a tautology. However, if the intertwin delivery interval is not shortened, the net result is an increase in combined delivery. This is defined as a vaginal delivery of twin A followed by cesarean delivery of twin B. Thirty years ago Rayburn and associates (1984) showed that intervals of more than 2 hours were not associated with adverse neonatal outcome, as long as electronic fetal monitoring was used and was reassuring. That said, intervals of greater than 30 minutes are associated with a sixfold rise in combined delivery rates (Cruikshank, 2007). Persad and coworkers (2001) found that an intertwin delivery interval of more than 1 hour produced an eightfold higher risk of combined delivery. Leung and associates (2002) concluded that intervals between twins of more than 30 minutes increased the risk of fetal distress and acidosis in the second twin. Interestingly, operative vaginal delivery of the first twin was associated with a decreased risk of cesarean delivery for the second twin (Wen, 2004).
A unique secondary analysis of data collected by the Maternal-Fetal Medicine Network described outcomes for combined delivery of twins (Alexander, 2008). In one group of 179 pregnancies, twin A was delivered vaginally followed by cesarean for twin B. In the comparator group of 849 cases, both twins were delivered by cesarean after labor onset. Intertwin delivery intervals were not presented. The authors concluded that serious neonatal sequelae were not affected by the route of delivery of the second twin. Notably, a very high percentage of women (17 percent) underwent combined delivery. This suggests a lesser degree of technical skill on the part of the managing obstetricians.
Obstetricians should make every effort to avoid delivering the first twin vaginally followed by urgent or emergent delivery of the second twin (Cruikshank, 2007). However, if the operator’s skill is insufficient to complete the delivery vaginally, he or she must make a timely decision to perform a cesarean to optimize neonatal outcome.
The weight of the foregoing evidence suggests that shortening the interval between twins is a meritorious strategy. Such a strategy has been labeled “immediate delivery of twin B” or “active second stage management” in twin pregnancies and is detailed next (Carroll, 2006; Fox, 2010).
