ETIOLOGY AND CLASSIFICATION OF TWINNING

Multiple gestation occurs as the result of either the splitting of an embryo (i.e., identical or monozygotic twinning) or the fertilization of two or more eggs produced in a single menstrual cycle (i.e., fraternal or dizygotic twinning). Because dizygotic twins arise from separate eggs, they are structurally distinct pregnancies coexisting in a single uterus, each with its own amnion, chorion, and placenta. Monozygotic twins arise from cleavage of a single fertilized egg at various stages during embryogenesis, and thus the arrangement of the fetal membranes and placentas will depend on the time at which the embryo divides (Table 13-1). The earlier the embryo splits, the more separate the membranes and placentas will be. If division occurs within the first 72 hours of fertilization, the membranes will be dichorionic, diamniotic with a thick, four-layered intervening membrane. If division occurs after 4 to 8 days of development, when the chorion has already formed, monochorionic, diamniotic twins will evolve with a thin, two-layer septum. If splitting occurs after 8 days, when both amnion and chorion have already formed, the result will be monochorionic, monoamniotic twins residing in a single sac with no septum. Of all monozygotic twins, 30% are dichorionic, diamniotic, and 69% are monochorionic, diamniotic. Only 1% of twins are monoamnionic. Because twins share a sac in this type, without an intervening membrane, the risk for umbilical cord entanglement is high, resulting in a net mortality in these twins of almost 50% (Figure 13-1).

TABLE 13-1 RELATIONSHIP BETWEEN TIMING OF CLEAVAGE AND NATURE OF MEMBRANES IN TWIN GESTATIONS

∗ Time interval between ovulation and cleavage of the egg.

FIGURE 13-1 Diagrammatic representation of the major types of twin placentas found with monozygotic twins.

(Redrawn from Benirschke K, Driscoll SG: Pathology of the Human Placenta. New York, Springer-Verlag, 1974, p 263.)

INCIDENCE AND EPIDEMIOLOGY

Twins account for about 3.5% of all U.S. births. The frequency of monozygotic twinning, which depends on a very infrequent biologic event (embryo splitting), is constant in all populations studied at about 1 in 250 births. However, the frequency of dizygotic twinning, which arises from multiple ovulations in the mother, is strongly influenced by family history, ethnicity, and maternal age. A family history of dizygotic but not monozygotic twins in the maternal pedigree increases the likelihood of dizygotic twinning in subsequent generations. In western Nigeria, twinning occurs in 1 in 22 gestations, whereas in the Native American and Inuit populations, twinning is less than one fifth of that rate. Twins are twice as common in women over 35 years of age as in women at 25 years of age. Given these statistics, about two thirds of spontaneously conceived twins are fraternal, and one third are identical (monozygotic). However, in recent years, the incidence of multizygotic multifetal gestation has increased markedly with the more widespread use of ovulation induction agents and the practice of transferring multiple embryos after in vitro fertilization. The incidence of multiple gestation with the use of clomiphene is about 6% to 8%, and it is about 20% to 30% following gonadotropin therapy.

DETERMINATION OF ZYGOSITY

The prognosis and expected morbidity with twins is strongly dependent on zygosity: monozygotic twins are more likely to involve congenital anomalies, weight discordancy, twin-twin transfusion syndrome (TTTS), neurologic morbidity, premature delivery, and fetal death. Thus, determination of zygosity is the most important next step after multifetal pregnancy has been first diagnosed.

Ultrasonographic evaluation of the pregnancy is frequently very helpful in determining zygosity. Imaging of discordant fetal gender confirms a dizygotic gestation. Visualization of a thick amnion-chorion septum is suggestive of dizygotic twins, as is the presence of a “peak” or inverted “V” at the base of the membrane septum (Figure 13-2A). Conversely, in monochorionic gestation, the dividing membrane is fairly thin (Figure 13-2B). Because an early embryonic split can infrequently result in dichorionic, diamniotic twins with separate placentas, these findings are not definitive. Similarly, in rare cases of postzygotic genetic events, monochorionic twins may be gender discordant. Thus, confident diagnosis of zygosity may require detailed examination of the placenta after delivery. Thirty percent of twins will be of different sex and are, therefore, dizygotic. Twenty-three percent have monochorionic placentas and are, therefore, monozygotic. Twenty-seven percent have the same sex, dichorionic placentas, but different blood groupings, and must be, therefore, dizygotic. Twenty percent have the same sex, dichorionic placentas, and identical blood groupings. For the latter group, further studies, such as human leukocyte antigen (HLA) typing or DNA analysis, allow determination of zygosity.

FIGURE 13-2 A: Real-time ultrasound with a thick vertical amnion-chorion septum (membrane) separating one twin on the left side from the second twin on the right. The arrow points to a “peak or inverted V” suggesting dizygotic twins. B: Ultrasound of a thin vertical membrane separating one twin on the left side from the second twin on the right, suggesting a monochorionic gestational sack.

ABNORMALITIES OF THE TWINNING PROCESS

Among monozygotic multiple gestations, abnormalities in the twinning process are relatively common and include conjoined twins, interplacental vascular anastomoses, TTTS, fetal malformations, and umbilical cord abnormalities.

Twin-Twin Transfusion Syndrome

The presence of unbalanced anastomoses in the placenta (typically arterial-venous connections) leads to a syndrome in which one twin’s circulation perfuses the other (i.e., TTTS) in about 10% of monozygotic twins. In this syndrome, arterial blood from the “donor twin” enters the placenta (through the umbilical artery) and is taken up by the umbilical venous system belonging to the “recipient twin,” which results in a net transfer of blood from the donor to the recipient twin. Fetal complications include hypovolemia, hypotension, anemia, oligohydramnios, and growth restriction in the donor twin, and hypervolemia, hydramnios, hyperviscosity, thrombosis, hypertension, cardiomegaly, polycythemia, edema, and congestive heart failure in the recipient twin. Both twins are at risk for demise from the circulatory derangement, and the pregnancy is predisposed further for preterm delivery due to uterine overdistention with hydramnios.

TTTS is diagnosed using ultrasound. Typically the donor twin is smaller and may have oligohydramnios, absent bladder, and anemia. The recipient, on the other hand, is larger with possible polyhydramnios, cardiomegaly, and ascites or hydrops (Figure 13-3).

FIGURE 13-3 Ultrasound of a twin-twin transfusion syndrome, with one twin (upper left) in an amniotic cavity with a reduced fluid volume and a membrane (memb) separating this fetus from the second twin in an amniotic cavity with an excessive amount of fluid (right and lower half of scan image).

Given the poor prognosis of untreated TTTS (about 50% survival of either twin), treatment with either serial amniocentesis and fluid reduction from the recipient twin’s sac or laser photocoagulation of the anastomotic vessels on the surface of the placenta is performed in specialized centers.

ALTERED MATERNAL PHYSIOLOGIC ADAPTATION WITH MULTIPLE FETUSES

A number of normal maternal physiologic responses to pregnancy are exaggerated with multiple gestation. Whereas in normal pregnancy, maternal blood volume is augmented by 40% (2 L) over the nonpregnant baseline, in twins this increase may be 3 L or more. The increased blood volume and demand for iron and folate increase the risk for anemia in the mother and makes the patient less able to tolerate the stresses of infection, labor, and premature labor therapy. Preeclampsia and gestational hypertension are almost doubled in multifetal gestation. The increased uterine size associated with multiple fetuses can cause maternal respiratory embarrassment, orthostatic hypotension due to compression of the vena cava and aorta, and compromise of renal function due to compression of the ureters.

DIAGNOSIS

Historical factors such as a maternal family history of dizygotic twinning, the use of fertility drugs, a maternal sensation of feeling larger than with previous pregnancies, or a sensation of excessive fetal movements should raise the suspicion of twins. Physical signs, including excessive weight gain, excessive uterine fundal growth, and auscultation of fetal heart rates in separate quadrants of the uterus are suggestive but not diagnostic. An obstetric ultrasound should be performed when multiple gestation is suspected. The diagnosis of multiple gestation requires a sonographic examination demonstrating two separate fetuses and heart activities and can be made as early as 6 weeks of gestation.

ANTEPARTUM MANAGEMENT

Because of the high risk for preterm birth, intensive antepartum management schemes are directed at prolonging gestation and increasing birth weight in order to decrease perinatal morbidity and mortality. The complications of multiple gestation are shown in Box 13-1.

TREATMENT OF PRETERM LABOR

The treatment of preterm labor is discussed elsewhere, but multiple gestations present special challenges. Relative contraindications to tocolysis in these pregnancies include a gestational age of 34 weeks or more, growth failure of one or more fetuses, concerning fetal status on biophysical monitoring, and preeclampsia. Aggressive tocolysis typically involves use of agents with adverse cardiovascular effects in the mother, such as β-mimetics and magnesium sulfate and calcium channel blockers. These agents, particularly when combined with antenatal corticosteroid therapy, have been associated with maternal volume overload and congestive heart failure. Box 13-2 provides a list of necessary prerequisites for the management of labor in pregnancies complicated by multiple gestation.

In the special case of monoamniotic twins, delivery by cesarean birth is usually accomplished by 34 to 36 weeks because of the increased risk for lethal cord entanglement. For diamniotic twin pregnancies, delivery management is outlined later.

VERTEX-VERTEX PRESENTATIONS

To choose the safest route of delivery for mother and babies, the presentations of the fetuses must be accurately known. By convention, the presenting twin is designated as twin A and the second twin as twin B. Vertex (twin A)-vertex (twin B) presentation occurs most frequently (50% of the time), followed by vertex-breech, breech-vertex, and breech-breech presentations.

Vertex-vertex twins are managed similarly to a singleton vertex presentation. Both fetal heart rates should be monitored continuously during labor. Oxytocin (Pitocin) can be used to manage hypotonic contractions. After delivery of the first twin, the cord is clamped (identified as twin A) and cut, but cord blood samples are not obtained until the second fetus has been delivered to prevent potential hemorrhage from the undelivered fetus through placental vascular anastomoses. A vaginal examination is then performed to assess the presentation and station of the second twin. If the second twin is still in a vertex presentation, spontaneous delivery is expected. If necessary, forceps or vacuum can be used to assist delivery of a vertex second twin. Because the second twin is at increased risk for cord prolapse, abruptio placentae, and malpresentation, careful attention to fetal heart monitoring is necessary.

After delivery of the second fetus, the cord blood samples are obtained, and the placenta is delivered. Care should be taken not to disrupt the fetal membranes because these will often reveal the zygosity of the twins. Following delivery of the placenta, uterine tone should be closely monitored because the incidence of postpartum atony and hemorrhage is increased in multiple gestations.

MANAGEMENT OF OTHER PRESENTATIONS

Increased risk for fetal injury exists with delivery of a breech fetus. For this reason, breech-breech and breech-vertex twins are usually delivered by cesarean birth. When delivery of vertex-breech or vertex-transverse twins is contemplated, informed consent by the mother and skill of the obstetrician are determining factors in choosing between cesarean and vaginal delivery. Although there is presently no scientific evidence that cesarean birth is superior for the vertex-breech presentation, difficulty in extracting the breech second twin can result in umbilical cord prolapse, head entrapment, neck injury, and asphyxia. To avoid head entrapment an anesthesiologist can give IV or sublingual nitroglycerine to relex the uterus. Unless the obstetrician is comfortable with managing these problems, planned cesarean is the only reasonable choice.

PERINATAL OUTCOME

The high perinatal mortality rate in twin gestations (30 to 50 per 1000 births), which is about 5 times that in singleton gestations, is largely attributable to prematurity and congenital anomalies (Box 13-3). Birth asphyxia is also a significant factor, and thus it is not surprising that second twins have twice the perinatal mortality of first-born twins. Compared with singletons, death from complications of birth trauma is 4 times more frequent with second-born twins and twice as frequent in first-born twins. Congenital anomalies and stillbirths account for about one third of the perinatal mortality rate. Stillbirths occur twice as frequently in twins as in singletons. Cerebral hemorrhage, asphyxia, and anoxia account for one tenth of the overall perinatal mortality rate.

Twin gestations experience a fourfold increase in cerebral palsy. The increased morbidity in multiple gestations is related to placental, anatomic, and delivery abnormalities. Low birth weight (mean birth weight in twins is 2395 g vs. 3377 g for singletons), prematurity, and IUGR may predispose to permanent brain injury. The increased frequency of congenital anomalies and injuries during delivery (with both cesarean and vaginal routes) contributes to the increase in suboptimal outcome in newborns from multiple gestations. Postnatally, twins on average are shorter and lighter than singletons of similar birth weight until 4 years of age.

MULTIPLE GESTATION WITH MORE THAN TWO FETUSES

Although higher-order multiple gestations (triplets and higher) can result from embryo splitting and polyovulation, today the most frequent cause is iatrogenic from the use of ovulation induction agents. The incidence of spontaneous triplets is 1 in 8000 and that of spontaneous quadruplets 1 in 700,000 births. However, because of the widespread use of assisted reproductive technologies, current estimate of the incidence of triplets is 1 in 3000 births. This rate has tripled in the past two decades.

Prematurity increases as the number of fetuses increases. The average length of gestation is 33 weeks for triplets but only 29 weeks for quadruplets, with mean birth weights 1818 g and 1395 g, respectively. Theoretically, delivery of higher-order multiples can follow the principles outlined above for twins. However, in contemporary practice, almost all high-order multiples are delivered by cesarean birth to decrease the risk for morbidity in these very premature pregnancies. The perinatal mortality rate for triplets and quadruplets is 50 to 100 per 1000 births, a rate that is twice that of twins.

BREECH PRESENTATION

Breech presentation occurs when the fetal buttocks or lower extremities present into the maternal pelvis. The incidence of breech presentation is 4% of all deliveries. Before 28 weeks, about 25% of fetuses are in a breech presentation position. As the fetus grows and occupies more of the uterus, it tends to assume a vertex presentation to accommodate best to the confines and shape of the uterus. By 34 weeks’ gestation, most fetuses have assumed the vertex presentation position.

Classification

There are three types of breech presentation: frank, complete, and incomplete or footling (Figure 13-4). Frank breech occurs when both fetal thighs are flexed and both lower extremities are extended at the knees. A complete breech has both thighs flexed and one or both knees flexed (sitting in a “squat” position). An incomplete (or footling) breech has one or both thighs extended and one or both knees or feet lying below the buttocks. At term, 65% of breech fetuses are frank, 25% are complete, and 10% are incomplete.

FIGURE 13-4 Types of breech presentation.

Labor Management

VAGINAL DELIVERY

Until the publication of randomized trials demonstrating that vaginal breech delivery is associated with increased perinatal mortality compared with planned cesarean birth, vaginal breech deliveries were performed in selected centers in patients who met strict criteria. These criteria are summarized in Box 13-4. The standard of care now in most practices is to deliver all breeches by cesarean birth to avoid the potential morbidities of umbilical cord prolapse, head entrapment, birth asphyxia, and birth trauma.

BOX 13-4 Criteria for Vaginal Delivery of a Breech Presentation.

Fetus must be in a frank or complete breech presentation.

Gestational age should be at least 36 weeks.

Estimated fetal weight should be between 2500 and 3800 g.

Fetal head must be flexed.

Maternal pelvis must be adequately large, as assessed by x-ray pelvimetry,^∗ or tested by prior delivery of a reasonably large baby.

There must be no other maternal or fetal indication for cesarean delivery.

Anesthesiologist must be in attendance.

Obstetrician must be experienced.

Assistant must be scrubbed and prepared to guide the fetal head into the pelvis.

^∗ Inlet: anteroposterior (AP) diameter ≥ 11.0 cm; transverse diameter ≥ 10.0 cm. Midpelvis: AP diameter ≥ 11.5 cm; transverse diameter ≥ 10.0 cm.

ASSISTED BREECH DELIVERY

Because the breech presentation can present in a setting in which cesarean birth is impossible or unsafe, vaginal delivery of the breech continues to be an important practitioner skill. Once the fetus has delivered spontaneously to the umbilicus (Figure 13-5A), gentle downward traction is exerted until the scapulae appear at the introitus (see Figure 13-5B). After delivery of the scapulae, the shoulders are delivered by sweeping each arm in turn across the fetal chest until only the fetal head remains undelivered (see Figure 13-5C). Once the shoulders have been delivered, the head is delivered by manual flexion of the fetal head with one hand flexing the head at the base of the skull while the operator’s other hand is applied to the fetal maxilla for downward flexion (see Figure 13-5D). Some obstetricians use Piper forceps routinely because this method has been shown to result in delivery of the head with the least amount of trauma to the fetus (see Figure 13-5E).

FIGURE 13-5 Partial breech extraction. A: After spontaneous delivery to the umbilicus, traction is applied to the infant’s pelvis. When the scapulae are visible, rotation of the trunk allows delivery of the anterior shoulder. B: Delivery of the anterior shoulder by downward traction. C: Delivery of the posterior shoulder by upward traction. The posterior arm is freed digitally by splinting the fetal humerus (inset). D: Delivery of the aftercoming head using the Mauriceau-Smellie-Veit maneuver. Abdominal pressure is applied to maintain flexion of the fetal head.E: Delivery of the aftercoming head using Piper forceps.

FACE PRESENTATION

Face presentation occurs when the fetal head is hyperextended such that the fetal face, between the chin and orbits, is the presenting part. The incidence is about 1 in 500 deliveries.

Mechanism of Labor

The position of the presenting face is classified according to the location of the fetal chin (mentum). About 60% of face presentations are mentum anterior at the time of diagnosis, whereas 15% are mentum transverse and 25% mentum posterior. The mechanism of labor with a face presentation is similar to the vertex presentation in that the longest diameter (mentum to brow) enters the pelvis transversely. As labor proceeds and the face descends to the midplane, internal rotation occurs into the vertical axis. If the mentum rotates anteriorly under the symphysis pubis, vaginal delivery should be expected. Forceps, but not vacuum, can be applied to assist if prerequisites are met. However, if the mentum rotates posteriorly, the fetal head will be unable to extend farther to complete the expulsive process. Thus, mentum posterior cases and those with persistent mentum transverse must be delivered by cesarean birth. However, because final rotation from mentum transverse may occur only after a significant period of maternal pushing, patience is necessary. About half of the mentoposterior and mentotransverse presentations spontaneously rotate to a mentoanterior position. When delivered by spontaneous vaginal delivery (Figure 13-6) or low forceps (Figure 13-7), perinatal morbidity and mortality for face presentations are similar to those for vertex presentations.

FIGURE 13-6 Spontaneous delivery of a mentum anterior face presentation. Note the flexion of the head under the symphysis pubis. The chin appears first, followed by the nose, brow, vertex, and occiput.

FIGURE 13-7 Simpson forceps applied to a mentum anterior face presentation.