Williams Manual of Pregnancy Complications, 23 ed.

CHAPTER 41. Complications Unique to Twins

A number of complications are unique to multiple gestations. Described below are discordant twin growth, twin–twin transfusion syndrome, twin reversed arterial perfusion (TRAP) sequence, monoamnionic twinning, and conjoined twins. Although these have been best described in twins, they also occur in higher-order multiple gestations.


Size inequality of twin fetuses, which may be a sign of pathological growth restriction in one fetus, is calculated using the larger twin as the index. Generally, as the weight difference within a twin pair increases, perinatal mortality increases proportionately. Size discordancy between twins can be determined in several ways. One common method uses all fetal measurements to compute the estimated weight of each twin and then the weight of the smaller twin is compared with that of the larger twin (weight of larger twin minus weight of smaller twin, divided by weight of larger twin). Considering that growth restriction is the primary concern and the abdominal circumference reflects fetal nutrition, some authors diagnose discordancy when abdominal circumferences differ by more than 20 mm.


The cause of discordant size often differs in monochorionic and dichorionic twins. Discordancy in monochorionic twins is usually attributed to placental vascular anastomoses that cause hemodynamic imbalance between the twins (see twin–twin transfusion syndrome below). Occasionally, monochorionic twins are discordant in size because they are discordant for structural anomalies. Discordancy in dichorionic twins is likely due to a variety of factors. Dizygotic fetuses may have different genetic growth potential, especially if they are of opposite genders. Alternatively, because the placentas are separate and require more implantation space, there is a greater chance that one of the placentas would have a suboptimal implantation site. The observation that the incidence of discordance doubles in triplets compared with twins suggests that in utero crowding plays a role in discordant growth.

Accumulated data suggest that greater than 25 percent weight discordance, usually with growth restriction in one or both twins, most accurately predicts an adverse perinatal outcome. The incidence of respiratory distress, intraventricular hemorrhage, periventricular leukomalacia, sepsis, and necrotizing enterocolitis all increase with the degree of discordance. When discordance exceeds 30 percent, the relative risk of fetal death increases more than fivefold.


Ultrasonographic monitoring of growth within a twin pair has become a mainstay in the management of twin gestations. Other ultrasonographic findings, such as oligohydramnios, may be helpful in gauging fetal risk. Depending on the degree of discordance and gestational age, fetal surveillance may be indicated, especially if one or both fetuses exhibit growth restriction (see Chapter 12). Delivery is usually not performed for size discordance alone, except when fetal maturity is likely.


Twin–twin transfusion syndrome occurs when vascular communications within a monochorionic placenta permit transfusion of blood between one twin, the donor, and its recipient sibling. The donor becomes anemic and its growth may be restricted, whereas the recipient becomes polycythemic and may develop circulatory overload manifest as hydrops. As many as 25 percent of monochorionic twins have some clinical features of this syndrome; however, relatively few are severely affected.


Virtually all monochorionic twin placentas contain vascular anastomoses, most of which are hemodynamically balanced and of little fetal consequence. However, approximately half of monochorionic placentas also contain deep artery-to-vein communications extending through the capillary bed of the villous tissue, creating a common villus compartment. Although most of these vascular communications are hemodynamically balanced and of little consequence, unidirectional flow through arteriovenous anastomoses may result in significant vascular volume differences between the twins and lead to chronic twin–twin transfusion syndrome (Figure 41-1).


FIGURE 41-1 Anastomoses between twins may be artery-to-venous (AV), artery-to-artery (AA), or vein-to-vein (VV). Schematic representation of an AV anastomosis in twin–twin transfusion syndrome that forms a “common villous district” or “third circulation” deep within the villous tissue. Blood from a donor twin may be transferred to a recipient twin through this shared circulation. This transfer leads to a growth-restricted discordant donor twin with markedly reduced amnionic fluid, causing it to be “stuck.” (Reproduced, with permission, from Cunningham FG, Leveno KJ, Bloom SL, et al (eds). Williams Obstetrics. 23rd ed. New York, NY: McGraw-Hill; 2010.)

Chronic twin–twin transfusion syndrome typically presents in midpregnancy, when the donor fetus becomes oliguric from decreased renal perfusion. Virtual absence of amnionic fluid in the donor sac prevents fetal motion, giving rise to the “stuck twin” (i.e., immobilized fetus). Meanwhile, the recipient fetus develops severe hydramnios, presumably due to increased urine production. This hydramnios–oligohydramnios combination can lead to growth restriction, contractures, and pulmonary hypoplasia in one twin, with premature rupture of the membranes and heart failure in the other.


Classically, weight discordance and hemoglobin differences were used to make the diagnosis of twin–twin transfusion syndrome; however, it has been appreciated that these are late findings. Instead, the following sonographic findings are used for the diagnosis: (1) monochorionicity, (2) same-sex gender, (3) hydramnios of the larger twin, defined as a largest vertical pocket of amnionic fluid more than 8 cm, and (4) oligohydramnios of the smaller twin, defined as a largest vertical pocket of less than 2 cm. Additional sonographic findings that support the diagnosis include significant growth discordance, cardiac dysfunction of the twin with hydramnios, and abnormal Doppler studies of the umbilical cord vessels or ductus venosus. Once identified, twin–twin transfusion is typically staged by the Quintero staging system, shown in Table 41-1. In addition to these criteria, there is evidence that cardiac function of the recipient twin correlates with fetal outcome.

TABLE 41-1. Quintero Staging System for Twin–Twin Transfusion Syndrome


Therapy and Outcome

Several therapies are currently used to treat twin–twin transfusion, including amnioreduction, laser ablation of vascular anastomosis, selective feticide, and septostomy. The majority of randomized trials have demonstrated improved twin survival following laser ablation of anastomoses as compared with amniocentesis. At this time, laser ablation is the preferred treatment for severe twin–twin transfusion, although optimal therapy for stage I and stage II diseases is controversial.

Generally, the earlier in gestation that the diagnosis is made, the worse the prognosis. Severe twin–twin transfusion syndrome often presents between 18 and 26 weeks’ gestation, and the reported survival rate for those diagnosed before 28 weeks varies widely, from 7 to 75 percent. Twin–twin transfusion syndrome can also result in microcephaly, porencephaly, and multicystic encephalomalacia, sequelae of ischemic necrosis. In the donor twin, ischemia results from hypotension and/or anemia. In the recipient, ischemia is due to blood pressure instability and episodes of severe hypotension. Death of one twin in utero has been associated with a 40-fold risk for the development of cerebral palsy in the survivor, likely from acute hypotension. Importantly, because of the acute nature of hypotension after death of one twin, it is nearly impossible to prevent damage to the survivor, even with delivery immediately after the cotwin demise is recognized.


Approximately 1 percent of monozygotic twins are monoamnionic. The fertilized ovum does not divide until after formation of the amnion, resulting in two embryos that share the same amnionic sac (see Chapter 40). The diagnosis is typically made sonographically, when there is failure to visualize a dividing membrane between twins.

Monoamnionic twins are at greatly increased risk for morbidity and mortality. Umbilical cord intertwining, a common cause of fetal death, is estimated to complicate at least half of cases. Other causes include congenital anomalies, preterm birth, and twin–twin transfusion syndrome. Management of monoamnionic twins is problematic, due to the unpredictability of fetal death from cord entanglement and lack of an effective means of monitoring for it. Data suggest that fetal death from cord entanglement may be more common earlier in gestation, with a lower incidence among monoamnionic pregnancies that have reached 30 to 32 weeks. Timing of delivery is controversial.


A complication unique to monoamnionic gestations is conjoined twinning, which occurs when division of the fertilized ovum does not take place until after the embryonic disc has begun to form (see Chapter 40). The estimated frequency is one per 60,000 pregnancies. Varying degrees of fusion may be present, involving any number of organs, with the most common site of attachment being the chest and/or abdomen (see Figure 41-2). The diagnosis is frequently made sonographically by midpregnancy. Fetal MR imaging later in pregnancy may provide useful anatomical information in selected cases. Conjoined twins may be further complicated by being discordant for structural anomalies. Surgical separation may be successful if essential organs are not shared, and when it can be performed on a planned basis, as opposed to an emergency basis. Prenatal consultation with a pediatric surgeon often assists parental decision making.


FIGURE 41-2 Types of conjoined twins. (Reproduced, with permission, from Cunningham FG, Leveno KJ, Bloom SL, et al (eds). Williams Obstetrics. 23rd ed. New York, NY: McGraw-Hill; 2010. Redrawn from Spencer R: Theoretical and analytical embryology of conjoined twins: Part 1: Embryogenesis. Clin Anat 13:36, 2000.)


Acardiac twinning, also called twin reversed arterial perfusion sequence, is a rare—1 in 35,000 births—but serious complication of monochorionic multifetal gestation. In the TRAP sequence, there is usually a normally formed donor twin and a recipient twin who lacks a normal heart (acardius) and other structures. It has been hypothesized that the TRAP sequence is caused by a large artery-to-artery placental shunt, often also accompanied by a vein-to-vein shunt (see Figure 41-3). With the single, shared placenta, arterial perfusion pressure of the donor twin exceeds that of the recipient twin, who thus receives reverse blood flow of deoxygenated arterial blood from its cotwin. This “used” arterial blood reaches the recipient twin through its umbilical arteries and preferentially goes to its iliac vessels. Thus, only the lower body is perfused, and disrupted growth and development of the upper body results. Failure of head growth is called acardius acephalus; a partially developed head with identifiable limbs is called acardius myelacephalus; and failure of any recognizable structure is called acardius amorphous.


FIGURE 41-3 In the TRAP sequence, there is usually a normally formed donor twin, who has features of heart failure, and a recipient twin, who lacks a heart. It has been hypothesized that the TRAP sequence is caused by a large artery-to-artery placental shunt, often also accompanied by a vein-to-vein shunt. Within the single, shared placenta, perfusion pressure of the donor twin overpowers that in the recipient twin, who thus receives reverse blood flow from its twin sibling. The “used” arterial blood that reaches the recipient twin preferentially goes to its iliac vessels and thus perfuses only the lower body. This disrupts growth and development of the upper body. (Reproduced, with permission, from Cunningham FG, Leveno KJ, Bloom SL, et al (eds). Williams Obstetrics. 23rd ed. New York, NY: McGraw-Hill; 2010.)

Because of this vascular connection, the normal donor twin must not only support its own circulation but also pump its blood through the underdeveloped acardiac recipient. This may lead to cardiomegaly and high-output heart failure in the normal twin. Without treatment, the death rate of a donor or “pump” twin ranges from 50 to 75 percent. The goal of treatment is interruption of aberrant vascular communication between the twins. Survival rates of 90 percent have been reported following radiofrequency ablation of umbilical vessels in the malformed recipient twin.

For further reading in Williams Obstetrics, 23rd ed.,

see Chapter 39, “Multifetal Gestation.”