Key Terms
Monozygotic twins: a pregnancy of 2 fetuses derived from the same zygote that divided between the day of fertilization and the 14th day. They can have phenotypic and genotypic differences and can have either a dichorionic or a monochorionic placentation.
Dizygotic twins: a pregnancy of 2 fetuses derived from 2 different zygotes, resulting from the fertilization of 2 oocytes from the same cycle. They always have a dichorionic placentation.
Chorionicity: type of placentation in cases of multiple pregnancy defined by the number of chorions.
Lambda sign: sonographic sign resulting from the juxtaposition of 2 layers of amnion and 2 layers of chorion in a dichorionic twin pregnancy.
Twin pregnancies represent about 3% to 4% of all pregnancies.1 Large regional and racial differences with up to 15-fold variation in the prevalence of twinning at birth have been noted for many years (Figure 15-1). Environmental and dietary factors, seasonality, and family clustering considerably influence the twinning rate.
In Western countries about 35% of these pregnancies are iatrogenic.1 Increased use of ovulation induction and assisted reproduction techniques (ART), coupled with delay in the reproductive age of pregnant women, have contributed to an increase in multiple pregnancies.1-3 These iatrogenic pregnancies contributed not only to the increase in the rate of dizygotic (DZ) twins but also of monozygotic (MZ) twin pregnancies.2,3 In iatrogenic pregnancies the ratio is altered and MZ twin pregnancies are more prevalent (6-fold increase).2-5
The growing concern with multiple pregnancies is their higher mortality and greater incidence of adverse perinatal outcome compared to singleton pregnancies, mainly from increased incidences of prematurity, fetal growth lag, structural malformations and chromosomal abnormalities.6-9
Although multiple pregnancies represent 3% to 4% of the population, they contribute to 10% to 14% of the overall perinatal mortality, a rate 5 to 10 times higher than that of singletons (Figures 15-2A and B). The perinatal mortality rate in monochorionic (MC) is twice that seen in dichorionic (DC) pregnancies, 12% versus 5%, (P < 0.001).9 The increase is primarily due to the fetal loss rates prior to 24 weeks, which are significantly higher in MC twins (60 per 1000) compared to DC twins (7 per 1000).6 Aside from the increase of preterm delivery and low birthweight when compared to DC twins and singleton pregnancies, MC twin pregnancies have higher rates of fetal malformations, discordance in fetal growth, and long-term neurologic morbidity.7-9
The aim of early diagnosis of twin gestations and their associated complications is the reduction of perinatal morbidity and mortality. Sonography allows determination of the number of fetuses, amnionicity, chorionicity, placental location, fetal presentation, and the detection of complications such as growth discrepancy, abnormal vascular anastomoses, amniotic fluid volume imbalance, fetal malformations, and cord entanglement.
Since twin pregnancies represent over 98% of multiple pregnancies and the majority of the published literature in the areas covered by this chapter are of twins and not higher order multiples, this chapter will focus on the different aspects of ultrasonographic evaluation, the most common complications, and the role of invasive procedures in the management of twin pregnancies. Selective sections of the chapter will be applicable to higher order multiples, such as determination of chorionicity and amnionicity.
The typical human cycle produces only one egg. Therefore, in about 99% of spontaneous pregnancies, a unique fetus derives from a single zygote. In 0.8% of spontaneous conceptions, more than one oocyte is produced and fertilized in each cycle, resulting in a polyzygotic twin pregnancy. This type of twinning results in genetically different individuals (also known as polyzygotic, nonidentical, or fraternal twins) and has a hereditary tendency. It is associated with a recurrence risk 3 times higher than that of the general population. Each zygote develops its fetal–placental–amniotic compartment, and there are no (or very rare) vascular communications between them.
In another 0.4% of spontaneous pregnancies, a single ovum normally destined to produce a single embryo splits to form MZ multiples. The MZ twinning is a form of reproduction whereby more than one individual results from a single zygote.
Presently, 4 theories are available to explain the mechanism of zygotic splitting: the repulsion hypothesis, the existence of codominant axes, depressed calcium levels in the early embryo, and the blastomere herniation hypothesis through a gap in the zona pellucida. All these theories are, however, not convincingly clear to explain the origin of MZ triplets and quadruplets and do not clarify the higher rate of MZ twins related to ART techniques. More recently the hypothesis is that human fertilized oocytes more splitting-prone are able to undergo 1 or 2 successive binary fissions,10 or various combinations offered by subsequent secondary fissions, just as in the case for the 9-banded armadillo, and thus are able to give rise to a variety of combinations of MZ pregnancies (Figure 15-3). This latter theory would be able to explain MZ triplets and quadruplets and “mirror-image” characteristics, as well as some midline asymmetries in MZ twins.
Dizygotic twins, fraternal or nonidentical, result from the fertilization of 2 ova by different spermatozoids, resulting in different genetic contents. The type of placenta resulting from this process of fertilization, in which the trophoblast formation precedes the egg implantation, has 2 chorions and 2 amniotic sacs, defining a dichorionic diamniotic pregnancy. Hence, DZ twins always yield dichorionic placentas.
Monozygotic twins comprise one-third of all twin pregnancies. These identical or uniovular twins result from the early division of a single egg (originating from the fertilization of an oocyte and a single spermatozoid) into 2 more or less identical cell masses that contain the same genotype (exception: heterokaryotypic monozygotes). This cleavage will occur between the fertilization and the gastrulation period (Figure 15-4). Individual variations are the result of postzygotic mutations and arbitrary division of mitochondrial DNA. The placentation process inherent to these twins is more complex, depending on the very moment in which the separation occurs. Less frequently, MZ and DZ twinning may occur simultaneously in a pregnancy with 3 or more embryos (Figure 15-5).
Figure 15-4.
Schematic drawing demonstrating the outcome of twinning at different stages of early embryonic life. (Top) Fission before the formation of the inner cell mass and any differentiation will produce 2 embryos with 2 separate chorions, amnions, and placentas. (Middle) Twinning at the early blastocyst stage, after formation of the inner cell mass, will cause the development of 2 embryos with 1 placenta and 1 chorion but 2 separate amnions. (Bottom) If separation occurs after the formation of the embryonic disc, the amnion has already formed and will lead to a monoamniotic-monochorionic pregnancy. Incomplete fission at this stage or later will result in conjoined twins.
In about one-third of naturally conceived MZ twins, the division will occur during the first 72 hours postconception (during the 2- to 8-cell stage), resulting in a dichorionic diamniotic (DCDA) twin pregnancy (Figure 15-6). Each twin receives trophoblastic and somatic stem cells into their cell masses. Therefore, twinning must have happened before the differentiation and physical separation of somatic and trophoblastic cells into the outer and inner cell masses, respectively, in the cavitated blastocyst at about 3 days postconception. DCDA placentas are derived from the splitting of about 8 blastomeres.
Splitting at any time thereafter results in a single MC placenta that continues as such even in the face of subsequent MZ twinning in the inner cell mass. About two-thirds of MZ twins will develop an MC diamniotic (MCDA) placentation as the result of the division of the embryonic cell mass between the third and the eighth day postconception (see Figure 15-6). Two fetuses will develop with the same genome, a single placenta, and 2 amniotic cavities. Therefore, MZ twins may have either a dichorionic or MC placentation.
The sharing of a single placenta is in itself an anomaly, creating the possibility of unequal vascular territories for each twin and the eccentric location of the umbilical cords. Placental vascular anastomoses connecting the two fetal circulations are present in essentially all MC placentas. Unequal placental share or imbalance in flow through the vascular communication will develop in 25% to 30% of MC pregnancies and is directly related to the higher risk of perinatal complications when compared to DC twins and singleton pregnancies. The placental aberrations account for disorders that are unique to MC twinning, including twin-twin transfusion syndrome (TTTS), twin reverse arterial perfusion (TRAP) sequence, twin anemia polycythemia sequence (TAPS), and selective fetal growth restriction (sFGR).
If the division occurs between the 9th and 12th day, a single placenta and a unique amniotic cavity will be formed, that is, all placental structures will be shared, resulting in a monochorionic monoamniotic (MCMA) twin pregnancy (1%) (see Figure 15-6). The highest level of sharing occurs rarely when the division happens around the 13th day, producing conjoined twins (estimated prevalence of 1 in 100,000). Although commonly accepted as a term, “conjoined twins” is a misnomer. The anomaly results from an incomplete division, not fusion. After that time the undivided egg will maintain the expected course to produce a singleton. The different types of twins are listed in Table 15-1 (further details are in the text). The table is organized from the most dissimilar on top to the most similar at the bottom (Figures 15-7 and 15-8).
| Dizygotic twins (1/90): | ||
| Superfecondation | Not the same father | Many case reports in the literature of the 19th century |
| Superfetation | Not the same cycle | Historically these were misinterpretations of growth discordance, but recent DNA studies have demonstrated that the condition is occasionally possible, in particular with assisted reproductive techniques |
| Fraternal twins | Same father, same cycle | The usual twins |
| Monozygotic twins (1/250): | ||
| Diamniotic dichorionic | Same zygote, 2 separate sacs | Early separation |
| Diamniotic monochorionic | Same zygote, 2 separate amnions | |
| Monoamniotic monochorionic | Same zygote, same sac | Late separation |
| Conjoint | Equally but incompletely divided | Incomplete separation |
| Duplicata incompleta | Incompletely duplicated | |
| Ectoparasitic twin (Figure 15-7) | Partial fetus attached to sib | Partial division |
| Fetus in fetu | Embedded | |
If MZ twinning events would take place at a constant rate during the first 12 days postconception, proportions of DCDA, MCDA, and MCMA twins would vary from those observed. The under-representation at birth of MCMA and conjoined twins is explained by a higher intrauterine lethality of the later stages of division. The relative excess of DC-MZ twins may be the result of their having fewer placental complications. The level of placental sharing is inversely proportional to the incidence of twins that result from this sharing—the incidence of MZ twins is about 1:250; the incidence of MC twins is two-thirds of the MZ pregnancies (1:350 to 400 births); the incidence of MCMA twins is 1:2500 live births; and the one from conjoined twins is fewer than 1:40,000 live births. On the other hand, perinatal mortality and morbidity are related directly with the level of sharing, that is, the greater the level of sharing the greater the risk of adverse pregnancy outcome (Figure 15-9). Whereas MZ with DC placentation and DZ twins present pregnancies with similar risks, those with MC placentation have an important increase in risk of adverse outcome, mainly if the shared circulation is unbalanced or there is unequal placenta share in MCDA twins or delayed separation resulting in MCMA or conjoined twins.
A heterotopic pregnancy is the occurrence of a twin pregnancy in which one gestational sac is implanted outside the uterine cavity (ectopic pregnancy) along with a single intrauterine pregnancy (eutopic pregnancy). This is a rare event in spontaneous conception, occurring in <1/30,000 pregnancies; however, with ART the incidence increases to between 1% and 5%, with the highest incidence in pregnancies achieved after in vitro fertilization (IVF).11
During the ultrasound, besides an intrauterine gestational sac being visualized, there is also an ectopic pregnancy, which is classified depending on the site of implantation: tubal, cervical, cornual, abdominal, or ovarian.
The diagnosis can be made by transvaginal ultrasound during the first trimester. This diagnosis should be considered in cases of abdominal pain (vaginal bleeding may be absent) and in those with a failed pregnancy with rising human chorionic gonadotropin (β-hCG).
The presence of a small pelvic mass associated with an early intrauterine pregnancy is suggestive of a heterotopic pregnancy. The differential diagnosis can be made by the identification of the heartbeat. Patients with diagnosis of ectopic pregnancy, in particular the ones who have undergone infertility treatment, must have the uterine cavity investigated to exclude heterotopic pregnancy. The presence of an intrauterine gestation sac in a patient without symptoms should not exclude the diagnosis of a concomitant extrauterine pregnancy until the pelvis is carefully visualized. Heterotopic pregnancies can occur even in patients without risk factors.
Twin pregnancies that include a complete hydatidiform mole (CHM) and coexisting fetus with a normal karyotype, anatomy, and placentation are also rare occurrences, with incidences of 1/22,000 to 1/100,000 pregnancies (Figure 15-10). These pregnancies are at increased risk for fetal loss (60% to 80%) and maternal complications including vaginal bleeding, hyperemesis, thyrotoxicosis, and early-onset severe preeclampsia. The risk of persistent gestational trophoblastic disease is independent of whether the pregnancy continues or is terminated, with rates reported to vary from 20% to 50%.12
A major advantage of routine ultrasound is the early detection of twin pregnancies and establishment of chorionicity and amnionicity. It is chorionicity, not zygosity, that impacts perinatal outcome and determines several aspects of ongoing antenatal management. Accurate determination of chorionicity is critical for: fetal aneuploidy screening; evaluation and management of twin pregnancies discordant for structural malformations; growth and amniotic fluid volumes; management of surviving co-twin following intrauterine demise; and the early detection of disorders unique to MC twins including TTTS and TAPs. Ideally the distinction between high-risk MC and lower-risk DC twin pregnancies will be made in the first trimester.
Prior to 10 weeks the number of gestational sacs, yolk sacs, and amniotic sacs will assist in determining chorionicity and amnionicity.13 Shortly after the sixth postmenstrual week, heartbeats are visible, and one can confidently count the number of embryos by the number of beating hearts. The relationship between the number of gestational sacs and embryonic heartbeats gives strong evidence for chorionicity. Two gestational sacs are consistent with DC twinning, while two embryonic heartbeats in a single gestational sac are indicative of an MC twin pregnancy. The number of yolk sacs tend to equal the number of amnions; however, this not a reliable diagnostic criterion. It is wise to wait until the eighth postmenstrual week to determine amnionicity. The amniotic membrane grows outward from the embryo. Early on the two separate amniotic sacs in a DCDA pregnancy will not have contacted each other to create an intertwin septum. The amniotic membranes are so thin that they may remain inconspicuous until 8 to 9 weeks, leading to the incorrect diagnosis of an MCMA twin gestation. In case of doubt, scanning the patient in lateral decubitus will often demonstrate the membrane: 1 of the 2 embryos appears “suspended” in the midst of the sac, resting on the membrane that is not yet visible. If a separating membrane is not identified, it is important to look for sonographic signs of cord entanglement with the help of color Doppler. Alternatively, transvaginal ultrasound is often successful in differentiating separate amnions early in gestation.
At 10 to 14 weeks, the gold standard “window” for chorionicity definition, the chorion frondosum is sufficiently thick to be identified between the 2 layers of amnion as a wedge-shaped structure in a DC twin pregnancy (not necessarily in a DZ twin pregnancy). This is the pathognomonic “lambda” sign (also occasionally called “twin peak”) (Figure 15-11).14,15 The finding of a paper-thin membrane (less than 2 mm), T-shaped septum without chorion interposed between the 2 amnions is diagnostic of an MC twin pregnancy (Figure 15-12). Prospective studies have indicated that chorionicity can be determined with 96% accuracy in the first trimester.16
After 16 weeks, the chorion frondosum physiologically regresses, and chorionicity assignment becomes less reliable (in the second trimester a false characterization of chorionicity can occur in 10% to 12% of cases).15,17 The lambda sign may be harder to identify.18 If sexes are alike, monchorionicity can be wrongly inferred since the absence of the lambda sign in the second and third trimester cannot exclude DC placentation.
The use of a combination of sonographic parameters, including number of placentas, fetal phenotype, membrane thickness, and lambda sign, has shown to accurately determine chorionicity in the second trimester with a sensitivity and specificity of 92% and 97%, respectively.19 If 2 separate placentas of different location, 2 fetuses of a different sex, or a thick interfetal membrane (>2 mm) with more than 2 layers are found, dichorionicity should be suspected (see Figure 15-11).20 The confirmation of an MC placentation is only achievable by the detailed examination of the placenta. This exam includes a careful macroscopic examination of the placenta that considers the number of placental masses, fetal and maternal surfaces of each placental disk, fusion of the chorion and amnion, thickness and translucency of the septum, and the vascular pattern of the fetal surface (Figures 15-13A and B).
Figure 15-13.
A: Macroscopic specimen of a dichorionic diamniotic placenta with apparent thick intertwin membrane. (Used with permission from O. Brandão.) Corresponding histologic specimen and drawing showing the 4 layers that constitute the intertwin membrane. B: Macroscopic specimen of an MC-diamniotic placenta with a translucid intertwin membrane. (Used with permission from Sousa Barros.) Corresponding histologic specimen and drawing showing the 2 layers that constitute the intertwin membrane.
Establishing chorionicity allows stratification of risk and as such is standard of care in the ultrasound evaluation of twin pregnancies. If chorionicity cannot be determined, a second opinion should be sought from a referral center. If uncertainty remains about chorionicity, it is safest to manage the pregnancy as MC twin gestation. As such, the ultrasound diagnosis should no longer be reported as “twin pregnancies;” rather, the diagnosis should be DCDA, MCMA, or MCMA twin gestation.21
Traditionally, twins are labeled according to each twin’s location relative to the birth canal, although we have observed that the presenting twin is not always the same one from examination to examination. Correct labeling of twin fetuses is needed for consistency in assigning and interpreting serial ultrasound, as well as screening and prenatal diagnostic test results (Figure 15-14). A much better terminology is to describe the relative positions of the twins where the fetus in the gestational sac closest to the maternal cervix labeled Twin 1 (or A). The twins are then described based on their relative orientation to each other as either lateral (left/right) or vertical (lower/upper). Using this approach, over 90% of laterally oriented and essentially all vertical oriented twins will maintain their orientation.22 Early in gestation, mapping of the twins’ placental umbilical cord insertion sites relative to the placenta edges and membrane insertion can be considered as well. Pregnancies with discordance labeling should include other descriptive parameters, such as in an evolving TTTS case, “Twin B, suspected donor, right upper.” Orientation in utero may not necessarily reflect delivery order, such as in cases of discordant structural malformations that are not obvious by external examination ultrasound prior to delivery.
Since chorionicity needs to be established for risk stratification, DC and MC twin pregnancies should be viewed and managed as separate conditions. In uncomplicated DC twins, pregnancy ultrasound surveillance should include first trimester imaging for nuchal translucency and chorionicity and second trimester anatomic survey (18-22 weeks, maternal BMI dependent). In the absence of maternal complications, fetal malformations or growth disorders, serial growth ultrasounds every 4 weeks should be obtained thereafter. In addition to the studies performed for DC twins, MC twins should have ultrasound performed every other week beginning at 16 weeks, with the addition of Doppler velocimetry studies for early detection of TTTS, TAPS, and selective fetal growth restriction (sFGR). The addition of weekly fetal monitoring with biophysical physical profiles has been suggested to be a reasonable strategy for all uncomplicated twins.23 Figures 15-15 and 15-16 illustrate the recommended pathways for ultrasound monitoring of uncomplicated DC and MC twin pregnancies, respectively. The management schemes have been adapted from the 2016 International Society of Ultrasound in Obstetrics and Gynecology Practice Guidelines: Role of Ultrasound in Twin Pregnancy.24,25
Monochorionic monoamniotic (MCMA) twins share not only the chorion (the outer membrane) but also the amnion (the inner membrane) and thus are in the same gestational sac (Figure 15-17). They result from zygotic splitting between 7 and 13 days after fertilization and represent 1% of twin pregnancies. They have a shared chorion and a single amniotic cavity. Interestingly, MCMA placentas have significantly greater numbers of both superficial and deep anastomoses than do uncomplicated MCDA pregnancies. This observation suggests a vascular basis for the extreme rarity of TTTS in monoamniotic pregnancies.26
Figure 15-17.
A: Sonographic images of MC monoamniotic twins: absence of visible intertwin membrane with fetuses abnormally close together, evidenced by a sweep of the sonographic field. Close approximation of the cord insertions. B: Three-dimensional scan of MC monoamniotic twins: absence of visible intertwin with 2 fetuses that are intimately in contact.
Differentiating MCDA from MCMA twins is not easy, but it is very important because it identifies patients at higher risk for cord accidents.
Monoamniotic twins can be suspected if the following features are observed (Figures 15-18, 15-19, 15-20).
Single-placenta and same-sex twins
Absence of a dividing membrane, namely in the 3 planes of three-dimensional ultrasound
Insertion of both cords very close to each other
Entanglement of the cords,27 demonstrated by color or power Doppler
Normal amniotic fluid volume around both fetuses
Visualization of a single yolk sac early in pregnancy (not always reliable)
Observation of the passive motion of the embryos when the patient is rolled to the side
MCMA twins can easily be confused with MCDA twins, especially when there is TTTS and one of the twins is stuck (see elsewhere in this chapter). A careful search for a membrane is the only way to ascertain the diagnosis. The absence or reduction of amniotic fluid around the stuck twin should raise the suspicion of a diamniotic gestation.
Monoamniotic twins may be affected by multiple pathologic conditions including TTTS (although much less commonly than in MCDA twins), entangled umbilical cords,27,28 and increased risk of congenital anomalies (15% to 20%).
Cord entanglement occurs in 40% to 70% of monoamniotic twins because of their increased mobility in the second trimester and can be seen as early as the first trimester. During the third trimester, the reduced space is usually no longer sufficient to allow the twins to move around. In cases of cord entanglement, despite apparent cord compression with absent end-diastolic velocities (AEDVs), some fetuses may grow appropriately. The significance of AEDV in monoamniotic twins may thus be less predictive than in singletons. In the absence of other signs of fetal deterioration, presence of a notch in the umbilical artery velocity waveform seen with cord entanglement is not indicative of an adverse outcome.29
Recent reports have demonstrated a reduction in perinatal mortality rates, from 30% to 50% to 10% to 15% with the use of serial ultrasounds and fetal monitoring in the late second and third trimester. However, despite early ultrasound diagnosis, over half of MCMA twin pregnancies will not survive beyond 16 weeks due to fetal malformations and idiopathic spontaneous abortion.30
Conjoined twins are MCMA twins fused at any portion of their bodies as a result of an incomplete division of the embryonic disk after the 13th day of conception (Figure 15-21).31 The term conjoined is actually a misnomer because most researchers consider the pathogenesis of the condition to result from failure of complete separation rather than from fusion of the twins.
This is a rare condition, and the reported frequency differs from 0.1 to 0.35 per 10,000 births.32 If stillborns are excluded, the estimate is 0.05 per 10,000 births. Females are more commonly affected, with a male-to-female ratio of 1.6–3 to 1. No association with maternal age, race, parity, or heredity has been observed. The recurrence risk is negligible.
Several sonographic signs can be observed in this condition, as follows:
Absence of an interamniotic membrane between the twins
Inability to separate fetal bodies
Fixed position of the fetuses relative one to another, even after external stimulation or maternal movement
Bifid appearance of the first-trimester fetal pole (V- or Y-shaped twin pregnancy) and continuous skin contours at the same anatomic level
The heads and bodies of both twins are seen at the same level (Figures 15-22 and 15-23)
Unusual extension of the spines
Unusual proximity of the extremities
Presence of a single heart (Figure 15-24)
Abnormal number of vessels (more than 3) in the umbilical cord
The presence of these signs changes according to the different types of conjoined twins. These must be considered whenever an MCMA pregnancy is suspected. Discordant presentation does not exclude conjoined twins. Although the diagnosis of conjoined twins is easier during the first trimester, the type and severity of the condition is better evaluated during the second trimester, when a more precise examination of the shared organs can be done. Diagnosis with three-dimensional transvaginal sonography during the first trimester has also been described. If diagnosis is made before viability, termination of pregnancy can be offered.
Conjoined twins are classified according to the area of the bodies where the fusion takes place. The symmetrical and equal forms, in which the twins have equal or nearly equal duplication of structures, are called duplicata completa. Whenever there is an unequal duplication of structures, the twins are called duplicata incompleta, and this category includes the most severe types of conjoined twins in which just a few organ systems are duplicated. The most frequent types of conjoined twins are thoracopagus (40% to 74%), omphalopagus (10% to 33%), pygopagus (18%), ischiopagus (6%), and craniopagus (1% to 6%). The classification of conjoined twins is described in Table 15-2.
Duplicata Incompleta. Duplication occurs in only one part or region of the body (Figure 15-25). Examples are diprosopus (1 body, 1 head, and 2 faces), dicephalus (1 body and 2 heads), and dipygus (1 head, thorax and abdomen with 2 pelvises and/or external genitalia).
Duplicata Completa. Two complete conjoined twins.
Terata Catadidyma. Conjunction in the lower part of the body (Figure 15-26). Examples are ischiopagus (joined by the inferior portion of the coccyx and sacrum) and pygopagus (joined by the lateral and posterior portions of the coccyx and sacrum).
Terata Anadidyma. Conjunction in the upper part of the body (Figure 15-27). Examples are syncephalus (joined by the face) and craniopagus (joined at the homologous portion of the cranial vault).
Terata Anacatadidyma. Conjunction at the midpart of the body (Figure 15-28). Examples are thoracopagus (joined at the thoracic wall), xiphopagus (joined at the xiphoid process), omphalopagus (joined at the area between the xiphoid cartilage and the umbilicus), and rachipagus (joined at the level of the spine above the sacrum).
Meanwhile, a new classification has been proposed based on the theoretical site of union.
Ventral Union. Twins united along the ventral aspect.
Cephalopagus. The twins are fused from the top of the head down to the umbilicus. There are 2 rudimentary (fused) faces, 4 arms, and 4 legs. The lower abdomen and pelvis are separated. The cephalothoracopagus janiceps type is a rare variety of conjoined twins in which the fetuses are joined face to face, with the face of each fetus being split at the midline and in half turned outward, so that each observed face is made up of the right face of one fetus and the left face of the other. The name originates from Janus in Roman mythology, the god of gates and doorways; his statue has 2 faces, facing east and west, representing the beginning and ending of the day, and 1 head.
Thoracopagus. The twins are united face to face from the upper thorax down to the umbilicus; the heart is always involved. There are 4 arms, 4 legs, and 2 pelvises.
Omphalopagus. The twins are joined face to face primarily in the area of the umbilicus and sometimes involving the lower thorax, but 2 distinct hearts are always preserved. There are 2 pelvises, 4 arms, and 4 legs.
Ischiopagus. The twins are united ventrally from the umbilicus to a large conjoined pelvis with 2 sacrums and 2 symphysis pubises. They appear more frequently joined end to end, with the spine in a straight line, but they can also present face to face with a joined abdomen. There are 4 arms, 4 legs and, in general, a common external genitalia and a common anus.
Lateral Union. Twins are joined side by side, with shared umbilicus, abdomen, and pelvis.
Parapagus. These twins share a conjoined pelvis, 1 symphysis pubis, and 1 or 2 sacrums. When the union is limited to the abdomen and pelvis (does not involve the thorax), it is called dithoracic parapagus. If there is 1 trunk with 2 heads, it is called dicephalic parapagus. If there is a single trunk and a single head with 2 faces, it is called diprosopic parapagus. There are 2, 3, or 4 arms and 2 or 3 legs.
Dorsal Union. Twins are joined at the dorsal aspect of the primitive embryonic disk. There is no involvement of the thorax and abdomen.
Craniopagus. Twins are united at any portion of the skull, except the face or foramen magnum. They share bones of the cranium, meninges, and occasionally brain surface. There are 2 trunks, 4 arms, and 4 legs.
Pygopagus. Twins share dorsally the sacrococcygeal, perineal regions and occasionally the spinal cord. There are 1 anus, 2 rectums, 4 arms, and 4 legs.
Rachipagus. Twins are fused dorsally above the sacrum, involving different segments of the column. This type is extremely rare.
Conjoined twins have a unique presentation, and the few differential diagnoses could include lymphangioma, teratoma, or cystic hygroma.
Congenital anomalies of organs other than the shared ones are present in 50% of cases of conjoined twins. Cardiac defects are the most common association (20% to 30%); thus, echocardiography is recommended in all cases. Neural tube defects and midline fusion defects, orofacial clefts, imperforate anus, and diaphragmatic hernia are also frequently seen. Polyhydramnios is observed in 50% to 75% of cases.
Most conjoined twins are born prematurely, 40% are stillborn, and 35% die within 24 hours.32 Among the survivors, the prognosis and attempts in surgical separation will depend on the type of conjunction, degree of involvement of the shared organs, and the presence of associated anomalies. The most ominous prognosis was found among those twins who shared liver and/or heart. Attempts of separation in cases of a common liver can be done as long as 2 biliary tracts are seen. In the presence of a shared heart, separation is only attempted if 2 normal hearts coexist in a single pericardium.
The method of choice for delivery is cesarean section to maximize survival and prevent maternal and fetal trauma.
MC twinning is a type of gestation in which the fetuses share a single chorion (the outer membrane) and may or may not share the amnion (the inner membrane). Vascular anastomoses are found in almost all MC placentas. Thus, interfetal transfusion is a normal event in MC twin pregnancies. When intertwin transfusion in MC twins is balanced, clinical manifestations of TTTS syndrome do not occur.
Bajoria and coworkers (1995)33 suggested that TTTS syndrome resulted from an unbalanced intertwin transfusion due to uncompensated arteriovenous anastomoses (Figure 15-29). Pregnancies affected by TTTS had fewer arterio-arterial anastomoses (AAA) (Figure 15-30) present (24% vs 84% of MC twins without TTTS).34 Seventy-eight percent of MC pregnancies in this series with 1 or more arterio-venous anastomosis (AVA) and no AAA developed TTTS.34 When an AAA is found, the risk of developing TTTS is reduced 9-fold.
Figure 15-29.
A: The arrow points to an arteriovenous anastomosis in a monochorionic diamniotic placenta. Note the darker color of the artery crossing over the vein. (Used with permission from O. Brandão.) B: Digested cast representing an arteriovenous anastomosis (arrow) in a monochorionic diamniotic placenta. (Reproduced with permission from Wee LY, Taylor M, Watkins N, et al. Characterisation of deep arterio-venous anastomoses within MC placentae by vascular casting. Placenta. 2005 Jan;26(1):19-24.)
Monochorionic twins have a continuous spectrum of severity in the imbalance between their fetoplacental circulations, depending on an angioarchitectural basis and hemodynamic and hormonal factors. Blood is transfused from the donor to the recipient via vascular connections (Figure 15-31). The donor becomes anemic, hypovolemic, and growth restricted and also develops high-output cardiac insufficiency and oligohydramnios as a consequence of reduced urinary production. In contrast, the recipient develops circulatory overload with congestive heart failure and polyhydramnios. The elevated urinary production from the recipient twin leads to polyhydramnios and an overdistention of the amniotic cavity, which compresses the donor and its vascular supply against the uterine wall, further decreasing perfusion to the donor fetus. The reduction in amniotic fluid on the donor side results in a close apposition of the intertwin membrane that fixes the donor fetus to the uterus, a condition known as “stuck twin” (Figure 15-32). While the process usually takes several weeks, beginning early in the second trimester an observation is the “folding membrane” sign, in which a redundant membrane progressively collapses over the donor. During this stage, folds can be seen in the membrane. Because there is no loss of protein or cellular components from its circulation, colloid osmotic pressure draws water from the maternal compartment across the placenta, establishing a vicious cycle of hypervolemia, polyuria, and hyperosmolarity, leading to high-output cardiac failure, hydrops, and polyhydramnios.
Figure 15-31.
In twin-twin transfusion syndrome (top drawing; note the artery-to-vein connection), the donor twin (left) becomes anemic, hypovolemic, and growth restricted, and as a consequence has reduced urinary production. Because swallowing of the fluid is not impaired, the amniotic fluid volume progressively decreases (yellow lines represent the interamniotic membrane). Conversely, the recipient twin (right) becomes hypervolemic. The elevated urinary production from the recipient twin leads to polyhydramnios and an overdistention of the amniotic cavity, which compresses the donor and its vascular supply against the uterine wall, further decreasing perfusion to the donor fetus. The end condition is the “stuck twin” (lower drawing).
Therefore, TTTS reflects primarily a pathological form of circulatory imbalance that develops chronically between hemodynamically connected MC twin fetuses. The true incidence of TTTS is unknown due to the high perinatal loss rate of undetected pregnancies in the second trimester. Published estimates suggest that 9% to 15% in MCDA twin pregnancies will be affected with TTTS.25 Assuming 30% of twin pregnancies are MZ, 70% of which are MC, with a twin pregnancy rate of 3% in the USA 2780 to 4200 twin pairs are affected annually with TTTS. TTTS accounts for 17% of perinatal mortality, nearly 12% of neonatal deaths, and 8.4% of infant deaths in twins. This is 3 to 10 times higher than that of singletons.
Though dramatically devastating if left unchecked, this clinically identifiable and treatable condition often goes undetected due to a lack of systematic imaging in MC twin pregnancies along with a failure to educate patients on the need to notify healthcare providers early on if signs and symptom of preterm labor develop.35 Classically, it is described as follows:
Affecting two babies, not one.
Affecting structurally normal babies, although with advanced disease upwards of 10% of recipient twins will be found to have right ventricular outflow tract obstruction due to valvular or subvalvular pulmonary stenosis or atresia.
The pathological epicenter being in the placenta, not in the babies.
Being associated with perinatal morbidity and mortality.
Being amenable to curative therapy.
The net result of transfusion between twins depends on the following:
Vascular anastomoses: Combination of type of connections (number, type, and diameter) and direction of connections.
Placental sharing: Unequal placental sharing, either by discrepant size of placental territory “distributed” to each fetus or by velamentous insertion of umbilical cord, may further impair growth in TTTS fetuses.36
Asymmetry in the progressive reduction of an initially large number of bi-directional AV connections formed during the embryonic unification of placental and fetal vessels.
Unbalanced RAS: Upregulation of RAS (donor) and downregulation of RAS (recipient) with transfer of angiotensin II may cause or contribute to the development of TTTS.
In the past, diagnosis of the syndrome was made only after delivery, and the standard neonatal criteria included the following:
A difference in cord hemoglobin concentrations of 5 g/dL or more.
A difference in birth weights of 20% or more.
Danskin and Neilson (1989),37 revisiting the neonatal criteria for diagnosis of TTTS, concluded that these findings were recorded both in MC and DC twins at similar rates.
Considering the increased risk of poor perinatal outcome when left untreated, the emphasis of screening, diagnosis, and treatment for TTTS has become an antenatal endeavor.
Early detection allows for closer surveillance of the “at-risk” MC pregnancy so that interventions can occur when critical thresholds are met. This is optimal since advanced, severe stages of TTTS are associated with a worse prognosis. Various ultrasound parameters have been proposed as potential screening markers for TTTS (Table 15-3).
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