Over the past 25 years, the rate and number of twin pregnancies in the United States has increased at an unprecedented pace (Figure 40-1). The extraordinary increase, largely a result of assisted reproductive technologies, is a major public health concern. Twin infants are less likely to survive and are more likely to suffer lifelong disability due to preterm delivery. At Parkland Hospital from 2002 to 2006, twins represented only 1 in 100 delivered neonates and yet accounted for nearly 1 in 10 perinatal deaths (Table 40-1). Moreover, maternal complications such as preeclampsia, postpartum hemorrhage, and maternal death are increased twofold.
TABLE 40-1. Selected Outcomes in Singleton and Twin Pregnancies Delivered at Parkland Hospital from 2002 through 2006
FIGURE 40-1 Number of twin births in the United States, 1980–2005. (Reproduced, with permission, from Cunningham FG, Leveno KJ, Bloom SL, et al (eds). Williams Obstetrics. 23rd ed. New York, NY: McGraw-Hill; 2010. Data from Martin JA, Park MM: Trends in twin and triplet births: 1980–97. Natl Vital Stat Rep 47:1, 1999; Martin JA, Hamilton BE, Sutton PD, et al: Births: Final data for 2005. Nat Vital Stat Rep 56:1, 2007.)
ETIOLOGY OF MULTIPLE FETUSES
Monozygotic twins arise from a single fertilized ovum that subsequently divides into two similar structures, each with the potential for developing into a separate individual. Monozygotic pregnancies may be dichorionic and diamnionic, monochorionic and diamnionic, or monochorionic and monoamnionic, depending on when the division occurs (Figure 40-2). The frequency of monozygotic twin births is relatively constant worldwide, at approximately one set per 250 births, and is largely independent of race, heredity, age, and parity. Although monozygotic twins are often called identical, they are usually not. Monozygotic twins may be discordant for genetic mutations as a result of a postzygotic mutation, or may have the same genetic disease but with marked variability in expression.
FIGURE 40-2 Mechanism of monozygotic twinning. Black boxes 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 into two. Division at this early stage creates two chorions and two amnions (dichorionic, diamnionic). Placentas may be separate or fused. B. Division between 4 to 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 FG, Leveno KJ, Bloom SL, et al (eds). Williams Obstetrics. 23rd ed. New York, NY: McGraw-Hill; 2010.)
Dizygotic, or fraternal, twins result from fertilization of two separate ova. Dizygotic twins occur twice as frequently as monozygotic twins, and their incidence is influenced by a number of factors (see Table 40-2). It has been reported that the common factor linking race, age, weight, and fertility to multiple gestation may be follicle-stimulating hormone levels.
TABLE 40-2. Factors Influencing the Rate of Dizygotic Twinning
DETERMINATION OF ZYGOSITY AND CHORIONICITY
The main reason to determine chorionicity antenatally is that it is beneficial in assessing obstetrical risks (see Table 40-3). Determination of zygosity frequently requires sophisticated genetic tests because dizygotic twins can look alike, while monozygotic twins are not always identical.
TABLE 40-3. Overview of the Incidence of Twin Pregnancy Zygosity and Corresponding Twin-Specific Complications
The number of chorions can be detected sonographically as early as the first trimester (Figure 40-3). Finding one chorion indicates a monozygotic twin pregnancy whereas two chorions may indicate either monozygotic or dizygotic twinning (see Figure 40-4). The presence of two separate placental sites and a thick dividing membrane supports dichorionicity. If there is a triangular projection of placental tissue extending beyond the chorionic surface between the layers of the dividing membrane—termed the “twin peak” sign—the pregnancy is dichorionic (Figure 40-5). Fetuses of opposite gender are almost always dichorionic as well. A rare exception occurs if one twin is phenotypically female due to Turner syndrome (45,X) and her sibling is 46,XY. A combination of placental location, dividing membrane thickness, presence or absence of the twin peak sign, and fetal gender may determine chorionicity with 96 percent accuracy.
FIGURE 40-3 Sonograms of first-trimester twins. A. Dichorionic diamnionic twin pregnancy at 6 weeks’ gestation. Note the thick dividing chorion (yellow arrow). One of the yolk sacs is indicated (blue arrow). B. Monochorionic diamnionic twin pregnancy at 8 weeks’ gestation. Note the thin amnion encircling each embryo, resulting in a thin dividing membrane (blue arrow). (Reproduced, with permission, from Cunningham FG, Leveno KJ, Bloom SL, et al (eds). Williams Obstetrics. 23rd ed. New York, NY: McGraw-Hill; 2010.)
FIGURE 40-4 Placenta and membranes in twin pregnancies. A. Two placentas, two amnions, two chorions (from either dizygotic twins or monozygotic twins with cleavage of zygote during first 3 days after fertilization). B. Single placenta, two amnions, and two chorions (from either dizygotic twins or monozygotic twins with cleavage of zygote during first 3 days). C. One placenta, one chorion, two amnions (monozygotic twins with cleavage of zygote from the fourth to the eighth day after fertilization).
FIGURE 40-5 A. Sonographic image of the “twin peak’ sign, also termed the “lambda sign,” in a 24-week gestation. At the top of this sonogram, tissue from the anterior placenta is seen extending downward between the amnion layers. This sign confirms dichorionic twinning. B. Schematic diagram of the “twin peak” sign. A triangular portion of placenta is seen insinuating between the amniochorion layers. (Reproduced, with permission, from Cunningham FG, Leveno KJ, Bloom SL, et al (eds). Williams Obstetrics. 23rd ed. New York, NY: McGraw-Hill; 2010.)
Examination of the placenta and membranes serves to establish zygosity at delivery in approximately two-thirds of cases. As the first neonate is delivered, one clamp is placed on a portion of its cord. Cord blood is not collected until after delivery of the other twin. As the second neonate is delivered, two clamps are placed on that cord. It is important that each segment cord remain clamped until the delivery of the second infant, to prevent hemorrhage through placental anastomoses. With one common amnionic sac, or with juxtaposed amnions not separated by chorion arising between the fetuses, the infants are monozygotic (see Figure 40-4). If adjacent amnions are separated by chorion, the fetuses could be either dizygotic or monozygotic, but dizygosity is more common.
MATERNAL ADAPTATION TO TWIN PREGNANCY
The degree of maternal physiological change is greater with multiple fetuses than with a singleton. Beginning in the first trimester, and temporally associated with higher serum beta-human chorionic gonadotropin (β-hCG) levels, women with twins often have nausea and vomiting in excess of women with a singleton pregnancy. The normal maternal blood volume expansion is 500 mL greater in twin pregnancies, and the average blood loss at delivery is nearly 500 mL more than with a single fetus. The increased iron and folate requirements imposed by a second fetus predispose the mother to anemia. As compared with a woman carrying a singleton, cardiac output is increased by 20%, as a result of both increased heart rate and stroke volume. The larger size of the uterus with twin fetuses intensifies the anatomical changes that occur during normal pregnancy.
Indeed, the uterus and its contents may achieve a volume of 10 L or more and weigh in excess of 20 lb!
Twin pregnancies are at increased risk for a number of maternal and fetal complications, including preterm birth and the development of pregnancy hypertension. Complications unique to twin pregnancy are presented in Chapter 41. Those that are also found in singleton pregnancies are discussed below.
Approximately 60 percent of twins deliver at 36 weeks or less. Attempts at prolonging gestation have focused on bed rest, prophylaxis with oral beta-mimetic tocolytic drugs, and cervical cerclage. Routine elective hospitalization has not been found beneficial in prolonging twin pregnancy or improving infant survival. However, as many as 50 percent of twin pregnancies may develop a specific indication for hospitalization, such as hypertension or preterm labor. Most randomized trials of tocolytic drugs in twin pregnancies have not shown significant reductions in preterm delivery rates, and beta-mimetic therapy entails a higher risk in twin gestations, in part because increased plasma volume and cardiovascular demands make these women especially susceptible to pulmonary edema. Similarly, neither prophylactic cervical cerclage nor supplemental progesterone therapy has been demonstrated to significantly reduce preterm delivery.
Hypertensive disorders due to pregnancy are much more frequent with multiple fetuses. At Parkland Hospital, the incidence of pregnancy-related hypertension in women with twins is 20 percent. Not only does hypertension develop more often, but it also tends to develop earlier and be more severe. Management of hypertension in women with twins is identical to that for singletons and is detailed in Chapter 23.
Spontaneous abortion is more likely with multiple fetuses. There are three times more twins among aborted than among term pregnancies, and monochorionic abortuses greatly outnumber dichorionic abortuses. In some cases the entire pregnancy is lost, but, in many cases, only one fetus is lost and the pregnancy delivers as a singleton. One twin is lost or “vanishes” before the second trimester in 20 to 60 percent of spontaneous twin conceptions. Usually, there is no evidence of the lost fetus at birth, and patients can be reassured that losing a fetus in this manner does not increase the risk of pregnancy complications.
Duration of Twin Pregnancy
Delivery before term is the major reason for the increased risk of neonatal death and morbidity in twins. In addition, maternal hypertension, fetal-growth restriction, and placental abruption are common indications for preterm delivery of twins.
Is there a safe upper limit of twin gestation? It has been reported that from 39 weeks’ gestation onward, the risk of stillbirth in twins exceeds the risk of neonatal death. At Parkland Hospital, twin gestations have empirically been considered to be prolonged at about 40 weeks.
For further reading in Williams Obstetrics, 23rd ed.,
see Chapter 39, “Multifetal Gestation.”