INTRODUCTION — Monoamniotic twin pregnancies are the least common type of twin pregnancy. They have many of the same complications as monochorionic diamniotic twin pregnancies (eg, twin-twin transfusion syndrome); however, they are characterized by higher risks for congenital anomalies and fetal death.
This topic will discuss issues specific to monoamniotic twin pregnancies, including conjoined twins. General aspects of twin pregnancy and complications of monochorionic diamniotic twin pregnancies are reviewed separately.
●(See "Twin pregnancy: Overview".)
●(See "Twin pregnancy: Routine prenatal care".)
●(See "Twin pregnancy: Management of pregnancy complications".)
PLACENTA — Monoamniotic twin gestations have a single placenta with one amnion and one chorion (figure 1). The two separate umbilical cords typically insert within 6 cm of each other. The cord insertions are located centrally in two-thirds of cases and either marginal or velamentous in the remainder [1]. Intertwin placental vascular anastomoses are always present [2].
PATHOGENESIS — Timing of postfertilization division of the zygote determines placentation in twins (figure 1). Monoamniotic monochorionic placentation occurs with division on days 8 to 12; conjoined twins result from division at or after day 13. By comparison, diamniotic twins result from division earlier in gestation (day 4 to 8 for diamniotic monochorionic placentation and day 1 to 3 for diamniotic dichorionic placentation).
The factors responsible for timing of embryo division are not known. Use of assisted reproductive techniques appears to play a role as in vitro fertilization increases the frequency of monozygotic twinning. The increase has been attributed to the in vitro culture environment and to extending the duration of culture five to six days before transfer. In some studies, manipulation of the zona pellucida, which is performed with intracytoplasmic sperm injection and assisted hatching, increased the frequency of monoamniotic twins [3-8].
Reports about atypical monoamniotic twins suggest a role for factors other than timing of division. Mirror image twins are a subset of monoamniotic twins in which embryonic division occurs approximately on postfertilization day 9. These twins have mirror image features involving handedness and hair whorls, as examples; situs inversus of one twin has also been reported. Mirror image twinning has been attributed to division after the embryonic plate begins to lateralize [9,10].
EPIDEMIOLOGY — Monoamniotic twins account for approximately 0.01 percent of spontaneously conceived pregnancies, 1 percent of twin pregnancies, and 5 percent of monochorionic twin pregnancies [4,11,12].
Sex ratio — The frequency of males decreases in the continuum from singletons to monoamniotic twins [13-15]. Among singleton births, the male/(male + female) ratio is 0.51. By comparison, among monoamniotic twins, females predominate and this ratio varies from 0.28 to 0.35 [16-18]. Two theories have been proposed to explain this observation: (1) the process of X-inactivation overlaps with the timing of monozygotic twinning and thus may directly contribute to development of monozygotic twins [15,19] and (2) the XX karyotype may confer a survival benefit [19].
DIAGNOSIS
Clinicopathologic diagnosis — The clinicopathologic diagnosis of monoamnionicity is made when twins have all of the following characteristics [20-25]:
●No intertwin dividing membrane
●A single placenta that is not fused
●Same sex (a rare exception is monovular dispermic twinning [1])
The pathologist should confirm that the amnion is continuous between the placental cord insertions. A discontinuous amnion may be a remnant of a disrupted diamniotic membrane. If the amnion is completely separated from the chorion, histologic differentiation between monoamniotic and diamniotic monochorionic twins is not possible.
Prenatal diagnosis — Most monoamniotic twins are diagnosed prenatally based on the same three features described above for clinicopathologic diagnosis. Confirmation of these findings on serial ultrasound examinations improves diagnostic certainty as some findings are best identified early in pregnancy and others later in pregnancy. Additional diagnostic findings may be present on prenatal ultrasound that may not be detectable postnatally.
●Cord entanglement – Sonographic observation of cord entanglement is pathognomonic for monoamniotic twins and may be seen as early as the late first trimester [11,26-32]. The diagnosis of cord entanglement is based on visualization of intertwined umbilical cords that have different fetal heart rates upon Doppler insonation of the various vessels (image 1). Three-dimensional (3D) ultrasound can also aid in the diagnosis of cord entanglement [33]. (See 'Cord entanglement' below.)
●Visualization of one yolk sac with two fetal poles – If an ultrasound examination is performed before 8 weeks of gestation, visualization of one yolk sac with two fetal poles strongly suggests monoamniotic twins [34-37]. However, rare case reports have described visualization of two yolk sacs in monoamniotic twin pregnancies [38,39] and one yolk sac with two embryos in diamniotic twin pregnancies [40]. Therefore, the number of yolk sacs in twin pregnancies does not reliably make or exclude the diagnosis of monoamniotic twins.
●Lack of visualization of an intertwin membrane – The amnion first becomes sonographically apparent at approximately 7.5 to 8 weeks when it separates from the fetal body. Transvaginal ultrasound examination at 9 to 10 weeks of gestation is the optimum method and time for evaluating the presence/absence of an intertwin membrane and its number of layers (if an intertwin membrane is present). Evaluation for the presence/absence and number of layers of membranes between 10 and 14 weeks of gestation is almost as informative. Later in gestation, however, a thin intertwin membrane (ie, monochorionic diamniotic placentation) can be missed.
●Presence of one placental disk – A single placenta on ultrasound examination is necessary for diagnosis but is not diagnostic of monoamnionicity as a dichorionic pregnancy may have a single fused placenta. Clearly separate placentas definitively exclude the diagnosis of monoamnionicity and may be visualized on ultrasound in the first trimester.
●Same-sex fetuses – In the second and third trimesters, sex concordance can be determined. It is necessary for diagnosis but is not diagnostic of monoamnionicity, whereas sex discordancy excludes the possibility of monoamniotic twins, except in rare cases involving postzygotic events [41].
Differential diagnosis
Pseudo-monoamniotic twins — The term pseudo-monoamnionicity has been used to describe diamniotic twin pregnancies in which the intertwin membrane has ruptured. When this occurs, it is usually a complication of amniocentesis or other invasive fetal procedures [11,42-45], although it may occur spontaneously [11,45]. The perinatal mortality rate is as high as for true monoamniotic twin gestations [45]. (See 'Outcomes' below.)
OUTCOMES
Maternal — Maternal complications and outcomes of twin pregnancy are generally similar for monoamniotic and diamniotic placentation, and are reviewed separately. (See "Twin pregnancy: Overview", section on 'Maternal complications'.)
Fetal and neonatal — Monoamniotic twin pregnancies are subject to the following complications:
●Complications that may occur in any twin pregnancy (eg, preterm birth, growth restriction of one or both twins, congenital anomalies). (See "Twin pregnancy: Overview", section on 'Types of complications'.)
●Complications that only occur in monochorionic twins (eg, twin-twin transfusion syndrome [TTTS], twin anemia polycythemia sequence [TAPS], twin reversed arterial perfusion [TRAP] sequence, neurologic sequelae from fetal demise of co-twin, selective fetal growth restriction). (See "Twin-twin transfusion syndrome: Screening, prevalence, pathophysiology, and diagnosis" and "Twin-twin transfusion syndrome: Management and outcome" and "Twin reversed arterial perfusion (TRAP) sequence" and "Selective fetal growth restriction in monochorionic twin pregnancies" and "Twin pregnancy: Management of pregnancy complications", section on 'Death of one twin' and "Twin anemia-polycythemia sequence (TAPS)".)
In contrast to other complications, TTTS is less common in monoamniotic twins than in diamniotic twins (2 to 6 percent [17,46] versus 9 to 15 percent [47,48]). This may be due to a higher frequency of protective arterioarterial anastomoses in monoamniotic placentas [49]. (See 'Screening for twin-twin transfusion syndrome' below.)
●Complications that only occur in monoamniotic twins (eg, cord entanglement, conjoined twins). (See 'Cord entanglement' below and 'Conjoined twins' below.)
Cord entanglement — Cord entanglement occurs in most, if not all, monoamniotic twin pregnancies [50,51]. Loose cord entanglements probably begin in the first trimester and have the potential to tighten at any time. Intermittent occlusion of umbilical blood vessels may be associated with neurologic morbidity; severe prolonged occlusion can be lethal [50,52-54].
In a systematic review of the impact of cord entanglement on perinatal outcome, overall survival among fetuses with cord entanglement at birth was 89 percent (202 out of 228 fetuses), and subanalysis found no significant difference in perinatal mortality between fetuses with versus without cord entanglement [55]. However, these findings should be interpreted with caution as the total number of deaths was small and information about prenatal diagnosis and management of these pregnancies was limited. Fetal monitoring and early delivery, which has become routine in monoamniotic pregnancies, may have prevented perinatal death from cord entanglement.
Congenital anomalies — The incidence of congenital anomalies is higher in twins than in singletons, higher in monozygotic twins than in dizygotic twins, and higher in monoamniotic monochorionic twins than diamniotic monochorionic twins [51,56-59]. Major anomalies have been reported in 7 to 28 percent of monoamniotic monochorionic twin pregnancies compared with 6 percent of diamniotic monochorionic twin pregnancies [16,51,59-62]. Congenital cardiac anomalies are more common in monoamniotic twins than singletons and possibly more common than in diamniotic twins [17,63,64]. The increased risk of anomalies may be related to late cleavage and/or circulatory imbalances across anastomotic vessels in the placenta.
Multiple case reports have described discordant anomalies in monoamniotic twins [65-69]; anomalies are concordant in less than 25 percent of cases [70].
Perinatal mortality — Perinatal mortality is higher in monoamniotic twins (range 12 to 23 percent [16,18]) than in other twins because these pregnancies are subject to the unique complications of their twin placentation (eg, cord entanglement, conjoined twins) plus the complications associated with other types of twin placentation (eg, TTTS, TRAP, TAPS, congenital anomalies, preterm birth).
In a meta-analysis of monoamniotic twin pregnancy (25 studies, 1628 nonanomalous twins ≥24 weeks of gestation), the following outcomes were described [71]:
●Fetal demise occurred in 5.8 percent of fetuses overall (95% CI 4.0-8.1). Single and double intrauterine deaths occurred in 2.5 percent (95% CI 1.8-3.3) and 3.8 percent (95% CI 2.5-5.3) of fetuses, respectively.
●The frequency of fetal demise by gestational age was 4.3 percent (95% CI 2.8-6.2) of fetuses at 24 to 30 weeks, 1.0 percent (95% CI 0.6-1.7) at 31 to 32 weeks, and 2.2 percent (95% CI 0.9-3.9) at 33 to 34 weeks. No deaths occurred among the 150 fetuses who reached 35 to 36 weeks. Although this analysis did not show a higher risk for fetal demise near term, this is likely because of obstetric management. (See 'Admission for close fetal monitoring versus ambulatory care' below and 'Delivery' below.)
●The suspected etiologies of the deaths were TTTS or growth restriction (30 percent) and unexpected (54 percent). Another review concluded that about half of fetal deaths in these monoamniotic twins are due to fetal anomalies, twin reversed arterial perfusion sequence, and conjoined twinning, and the remainder are due to TTTS, tight cord entanglement, and acute hemodynamic imbalances through the large placental vascular anastomoses [72].
●The incidence of fetal demise was lower with inpatient compared with outpatient management (3 versus 7.4 percent). (See 'Admission for close fetal monitoring versus ambulatory care' below.)
●Approximately 38 percent of pregnancies were delivered before the scheduled date due to preterm labor or an abnormal fetal heart rate pattern.
OBSTETRIC CARE — Management of monoamniotic twin pregnancy is similar to the general management of twin pregnancy described elsewhere, with some exceptions. (See "Twin pregnancy: Overview" and "Twin pregnancy: Labor and delivery".)
Issues specific to obstetric management of monoamniotic twin pregnancies are discussed below.
Screening for aneuploidy — The risk of trisomy 21 (Down syndrome) in each fetus of a monozygotic twin pair has been reported to be the same or lower than the risk in a singleton pregnancy of a mother of similar age [73,74]. Both twins of a monozygotic pair are either affected or unaffected, with rare exceptions due to postzygotic and epigenetic mechanisms [75].
Due to the complexities of aneuploidy screening in monoamniotic twins, referral to a genetic counselor is recommended, if feasible. Standard methods for Down syndrome screening can be used (eg, assessment of cell-free DNA, maternal serum biomarkers and ultrasound markers). For screening tests involving nuchal translucency (NT; first-trimester combined test), the formula for determining the pregnancy-specific aneuploidy risk uses the mean NT measurement at 11+0 to 13+6 weeks of gestation for the two fetuses [76]. In the setting of monochorionic twins with normal karyotypes, an enlarged (also called increased) NT (based on standard gestational age-based tables) or >20 percent discordance between NT measurements can be a marker for a pregnancy at risk for twin-twin transfusion syndrome (TTTS) [77]. (See 'Screening for twin-twin transfusion syndrome' below.)
A single invasive procedure (amniocentesis, chorionic villus sampling) is performed for prenatal diagnosis, when indicated. When invasive testing is performed for a high-risk serum or blood screening test result or because of advanced maternal age, we believe conventional G-banded karyotyping alone is sufficient. We order a chromosomal microarray when a diagnostic procedure is performed because of an enlarged NT or a structural defect in any of the fetuses since diagnostic performance is higher than conventional karyotype in these settings. (See "Prenatal diagnosis of chromosomal imbalance: Chromosomal microarray", section on 'Higher diagnostic yield' and "Enlarged nuchal translucency and cystic hygroma".)
Screening for fetal abnormalities — Given the increased incidence of serious anatomic malformations, which are discordant in over 75 percent of cases, a detailed fetal anatomic survey of both twins is performed in all monoamniotic twin pregnancies at 18 to 20 weeks [70].
We do not perform fetal echocardiography routinely, although some authorities do so for all monoamniotic twin pregnancies, since congenital cardiac anomalies are more common in monoamniotic twins than singletons, and more common than in diamniotic twins [17,63,64]. We obtain fetal echocardiography if cardiac views during the anatomy survey are suboptimal or suggest an abnormality, or the patient has standard indications for fetal echocardiography. (See "Congenital heart disease: Prenatal screening, diagnosis, and management", section on 'Indications for echocardiography'.)
Management of discordant anomalies — An anomalous twin can place the nonanomalous twin at risk for preterm birth, demise, and other perinatal complications. Selective fetal reduction by cord ligation to prevent complications to the co-twin from single twin demise and cord transection to prevent future complications from cord entanglement is an option. This intervention may benefit the nonanomalous twin by increasing its chances of survival and lowering its risk for morbidity [78,79]. The procedure is described in detail separately. (See "Multifetal pregnancy reduction and selective termination", section on 'Monochorionic fetuses'.)
Screening for twin-twin transfusion syndrome — Enlarged NT is more common in monochorionic twins and has been attributed to early twin-twin transfusion, in addition to all of the causes of enlarged NT seen in singleton gestations [80]. In one study, fetal NT above the 95th centile for gestational age had positive and negative predictive values for the development of TTTS of 38 and 91 percent, respectively [81]. In another report from the same institution, ≥20 percent discordance in NT was associated with a greater than 30 percent risk of early fetal death or development of severe TTTS [77]. (See "Enlarged nuchal translucency and cystic hygroma", section on 'Twin-twin transfusion'.)
Monoamniotic twins have a lower rate of TTTS than diamniotic twins (2 to 6 percent [17,46] versus 9 to 15 percent [47,48]), but the risk is sufficient to justify screening, beginning in the second trimester (table 1). While TAPS can be seen in up to 6 percent of monochorionic diamniotic twins, the rate in monochorionic monoamniotic twins is expected to be lower due to larger vascular anastomoses in these pregnancies. To our knowledge, there has been only one case report of TAPS in monochorionic monoamniotic twins [82]. (See "Twin-twin transfusion syndrome: Screening, prevalence, pathophysiology, and diagnosis" and "Twin anemia-polycythemia sequence (TAPS)".)
Management of TTTS and TAPS in monoamniotic twins is similar to that in diamniotic twins. Although visualizing the equator between the twins is more challenging due to absent membranes, blood vessels can be followed distally from their origins at the two cord insertion sites. These pregnancies are at high risk for perinatal mortality and preterm birth <32 weeks [83]. (See "Twin-twin transfusion syndrome: Management and outcome" and "Twin anemia-polycythemia sequence (TAPS)".)
Admission for close fetal monitoring versus ambulatory care — It is common practice to hospitalize patients with monoamniotic twin pregnancies at the beginning of the third trimester for intensive fetal heart rate monitoring, but standards vary across institutions (see 'Fetal monitoring' below). We admit these patients at 26 to 28 weeks of gestation.
A survey of maternal-fetal medicine specialists noted that 84 percent of respondents recommended inpatient fetal monitoring, with 54 percent beginning inpatient monitoring at 26 to 28 weeks [84]. The authors also noted that 75 percent of respondents recommended daily intermittent fetal monitoring, with 81 percent performing fetal testing two to three times per day.
Reviews have generally reported better pregnancy outcomes with inpatient compared with outpatient care [70,71]. In the meta-analysis of monoamniotic twin pregnancy (25 studies, 1628 nonanomalous twins ≥24 weeks of gestation) discussed above, fetal demise occurred in 3 percent (95% CI 1-5) of monoamniotic twin pregnancies managed as inpatients versus 7 percent (95% CI 4-11) of those managed as outpatients [71]. However, a review of data from three cohort studies found that deaths among fetuses with various degrees of birth weight discordance were similar in those managed as inpatients or as outpatients [85]. All of the studies were small, and none were randomized trials. Importantly, in some studies, inpatients had more intensive fetal monitoring than outpatients, which may have accounted for the observed benefit rather than other characteristics of hospitalization. On the other hand, inpatients sometimes had more pregnancy complications than outpatients and thus were at higher baseline risk for an adverse outcome. Given these limitations, it is unclear whether the risk of fetal death is clearly increased with outpatient management. A prudent approach until better data are available is to admit these patients at 26 to 28 weeks of gestation for inpatient monitoring.
Fetal monitoring
●Fetal heart rate monitoring – Antepartum fetal monitoring is indicated because monoamniotic twins are at increased risk for fetal demise, which has been attributed, at least in part, to cord entanglement. Monitoring and timely intervention may prevent some of these deaths. The best approach to monitoring is unclear as no high-quality data are available to guide decision making. No approach can prevent all fetal deaths.
When the pregnancy reaches a gestational age at which delivery because of a nonreassuring fetal heart rate tracing would be considered (generally 24 to 28 weeks), we obtain extended fetal heart rate tracings twice weekly for approximately an hour to check for multiple deep variable decelerations, which suggests cord compression from entanglement. When the fetuses are sufficiently mature physiologically to exhibit reactivity, a nonstress test can be performed. Other clinicians may reasonably choose a different gestational age for beginning monitoring and a different frequency and duration of monitoring.
For inpatients ≥28 weeks of gestation, we monitor the fetal heart rate three times per day until a reassuring nonstress test is obtained (see 'Admission for close fetal monitoring versus ambulatory care' above). After reviewing the literature, one group concluded that fetal death due to cord entanglement was not a sudden acute event in the majority of cases, but rather secondary to a sub-acute event that can be prevented with sufficiently frequent monitoring in the vast majority of cases [70]. For inpatients, they suggested a reasonable monitoring regimen was a nonstress test or hour-long fetal heart rate strip at least three times daily, based on available literature. For outpatients, they suggested a daily nonstress tests three to four times per week, with the understanding that outpatient monitoring is not ideal.
●Doppler – Doppler velocimetry to detect altered flow has been proposed as suggestive of compression of cord vessels. High blood velocity in the umbilical vein [32,86], a notch in the umbilical artery waveform [87], or persistent absent end diastolic flow [88] were reported as suggesting the fetoplacental circulation is compromised. However, given the poor specificity of these findings without associated adverse effects, we recommend not using Doppler sonography for routine monitoring of monoamniotic twins [89].
●Fetal weight – Fetal weight is monitored for growth restriction and discordancy, although weight discordance >20 percent had low predictive accuracy for mortality in a study of 242 monoamniotic twin pregnancies from three research collaboratives on twin pregnancy [85]. There is, therefore, insufficient evidence to recommend a different frequency of growth monitoring compared with that in monochorionic diamniotic twins (table 1). (See "Twin-twin transfusion syndrome: Screening, prevalence, pathophysiology, and diagnosis" and "Twin anemia-polycythemia sequence (TAPS)".)
Antenatal corticosteroids — In the absence of a threatened preterm birth before 28 weeks of gestation, we administer a course of antenatal corticosteroids at the time of admission, which is typically at 28 weeks. For patients who remain undelivered after three weeks, a single rescue course of steroids is reasonable; the exact timing between 31 weeks and delivery should be decided on a case-by-case basis. The American College of Obstetricians and Gynecologists recommends a single rescue course of antenatal corticosteroids in pregnancies <34 weeks that are at imminent risk of preterm birth within the next seven days and had a prior course of antenatal corticosteroids at least seven days previously [90]. (See "Antenatal corticosteroid therapy for reduction of neonatal respiratory morbidity and mortality from preterm delivery", section on 'Use of rescue (salvage, booster) ACS'.)
Death of one twin — The optimum management of monoamniotic twin pregnancies with a single fetal demise is unclear as few data are available to help guide decision-making. The factors involved in management of diamniotic monochorionic twins with a single fetal death also apply to monoamniotic twins, but monoamniotic twins have the additional risk of cord entanglement. Whether death of the co-twin increases or decreases the likelihood of cord entanglement lethal to the surviving twin is not known. (See "Twin pregnancy: Management of pregnancy complications", section on 'Death of one twin'.)
Delivery
●Route – Monoamniotic twin pregnancies are delivered by cesarean to avoid complications during labor from cord entanglement [91,92].
●Timing – We, and others [60,91,93-97], plan the delivery of these pregnancies between 32+0 and 34+0 weeks of gestation because of the increasing risk of perinatal mortality in the third trimester, which we estimate to be at least 5 percent in pregnancies that continue beyond this gestational age range [18,51,91]. If only one demise occurs, this imperils the co-twin because of the risk of neurologic injury (see 'Death of one twin' above). Although, as described above [71], several studies have observed that healthy fetuses who achieve 35 weeks of gestation have a low rate of fetal demise, there were only 150 cases in this age range in the meta-analysis, and they were spread out among different institutions with different surveillance protocols.
Neonatal outcomes are reasonably good at 32 weeks for newborns cared for in well-equipped neonatal intensive care units. In one large study, the neonatal death rate after birth at 32 and 34 weeks was 0.2 percent (1 out of 451) and 0 (0 out of 1058), respectively [98,99]; thus, the risk of death associated with preterm birth appears to be lower than that with continuing pregnancy, although delivery places the fetus at risk for major and minor morbidity. Available evidence is insufficient to allow a strong recommendation about the optimal gestational age for planned delivery of these pregnancies; no randomized trials have been performed [100]. Results of a questionnaire sent to maternal-fetal medicine specialists showed the median gestational age for planned delivery was 34 weeks [84].
The timing of nonplanned delivery is dictated by the clinical scenario (eg, nonreassuring results on antepartum fetal monitoring).
CONJOINED TWINS
Incidence — Conjoined twins are a rare type of monoamniotic twins, estimated to occur in 1.5 per 100,000 births worldwide [101]. Female twins are affected more often than males.
Types — When twins are conjoined, the fusion occurs between same body parts. Conjoined twins are classified as cephalopagus, thoracopagus, omphalopagus, ischiopagus, parapagus, craniopagus, rachipagus, and pygopagus (figure 2), based on the site of fusion.
Prenatal diagnosis — The diagnosis should be suspected in first-trimester monoamniotic twin pregnancies when the embryonic/fetal poles are closely associated and do not change in position with respect to each other (image 2) [102-104]. Fusion of fetal organs may be obvious (image 3).
Other findings, which are not all specific to conjoined twins, include juxtaposed embryos with a single midline cardiac motion, increased nuchal translucency or cystic hygroma, inseparable fetal parts, no sign of separate movement of the twins, fewer limbs than expected, a single umbilical cord with more than three vessels, hyperextension of the cervical spines of fetuses who face each other, or both heads or breeches consistently at the same level to each other [105,106]. In the latter half of pregnancy, a detailed anatomy survey can aid in defining the location and extent of the conjoined area.
Polyhydramnios is present in up to 50 percent of conjoined twins in late pregnancy [105].
Congenital anomalies are always present in conjoined twins. The prognosis is poor because these anomalies often preclude survival of one or both twins, even if surgical separation is performed. (See "Neonatal complications, outcome, and management of multiple births", section on 'Conjoined twins'.)
Additional imaging studies — Color Doppler, fetal echocardiography, and 3D ultrasound examination can confirm the diagnosis and clarify anatomy, which is critical for assessing prognosis and pre- and postnatal decision making. Fetal magnetic resonance imaging [107-110] may also aid in defining anatomy and surgical preplanning. A surgical group at Texas Children's Hospital uses a combination of volumetric computed tomography, 3D modeling and 3D printing for surgical planning, patient-specific simulations, and education [111].
Management
●Multidisciplinary team – Patients should be cared for at a center experienced in the management of conjoined twins. At a minimum, the management team involves specialists in maternal fetal medicine, pediatric surgery, neonatology, and radiology. Management is largely based on clinical experience in such centers and data from case reports, small series, and expert opinion.
●Delivery timing – There is insufficient literature to guide specific timing of delivery; however, delivery at 35 weeks following administration of antenatal corticosteroids is a reasonable approach due to the increased risk of stillbirth or complications related to polyhydramnios and preterm birth [106]. Early delivery to avoid cord entanglement is not a factor because the twins do not move independently.
●Cesarean birth – The optimal uterine incision depends on patient-specific factors, including the gestational age, the type of attachment, and neonatal prognosis. The abdominal and uterine incisions should be large enough to deliver the twins atraumatically; therefore, a classical uterine incision will usually be required.
●Approach to vaginal birth – Successful vaginal birth of undiagnosed conjoined twins has been reported, but there is a high risk of dystocia and maternal and/or fetal trauma, including uterine rupture and fetal death [112-114]. Vaginal birth may be attempted in the second trimester since the twin mass is much smaller than at term, and is reasonable for nonviable twins or for pregnancy termination.
SOCIETY GUIDELINE LINKS — Links to society and government-sponsored guidelines from selected countries and regions around the world are provided separately. (See "Society guideline links: Multiple gestation".)
SUMMARY AND RECOMMENDATIONS
●Monoamniotic twins
•Pathogenesis – Monoamniotic twin pregnancies result from division of a single fertilized oocyte between days 8 and 12 postfertilization. The fetal membranes consist of one amnion and one chorion (figure 1). (See 'Introduction' above and 'Pathogenesis' above.)
•Incidence – The incidence of monoamniotic twins is approximately 1 in 10,000 pregnancies. (See 'Epidemiology' above.)
•Fetal mortality – Fetal mortality, which occurs in roughly 20 percent of all monoamniotic twin pregnancies, is the most serious complication of monoamniotic placentation. Cord entanglement is a major cause of fetal death in these pregnancies. (See 'Outcomes' above.)
•Diagnosis – Sonographic visualization of cord entanglement with Doppler insonation of the individual umbilical cords revealing different heart rates is diagnostic of monoamniotic twins. Early prenatal sonographic findings strongly suggestive of monoamniotic twins include one yolk sac with two fetal poles and no evidence of an intertwin membrane. In the second and third trimesters, the diagnosis is excluded if two placentas, an intertwin membrane, or discordant fetal sexes are noted. (See 'Diagnosis' above.)
•Trisomy 21 screening – The risk for aneuploidy in each fetus of monozygotic twin pregnancies is similar to or lower than the risk in maternal age-matched singleton pregnancies. Both twins of a monozygotic pair are either affected or unaffected, with rare exceptions. Standard methods for Down syndrome screening can be used (eg, assessment of cell-free DNA, maternal serum markers). (See 'Screening for aneuploidy' above.)
•Anomalies -- Major anomalies have been reported in 7 to 28 percent of monoamniotic twin pregnancies. Congenital cardiac anomalies are more common in monoamniotic twins than singletons. A fetal anatomic survey is performed at 18 to 22 weeks. We do not obtain fetal echocardiography routinely, but perform this evaluation if cardiac views during a detailed anatomy survey are suboptimal or suggest an abnormality, or the patient has standard indications for fetal echocardiograph. (See 'Screening for fetal abnormalities' above.)
•Twin-twin transfusion syndrome – Monoamniotic twins have a lower rate of twin-twin transfusion syndrome than diamniotic twins (2 to 6 percent versus 9 to 15 percent), but the risk is sufficient to justify screening. We begin screening for discordant bladder volume, discordant growth, and polyhydramnios at 16 weeks of gestation and repeat ultrasound examinations every two weeks until delivery. We also measure middle cerebral artery peak systolic velocity at each visit; >1.5 multiples of the median (MoM) in one twin and <1.0 MoM in the other are suggestive of twin-anemia polycythemia sequence. (See 'Screening for twin-twin transfusion syndrome' above.)
•Prenatal care
-Antepartum fetal surveillance – Beginning at 24 to 28 weeks of gestation, we obtain twice-weekly fetal heart rate tracings for approximately an hour to check for multiple deep variable decelerations, which suggests cord compression from entanglement. When the fetuses are sufficiently mature physiologically to exhibit reactivity, a nonstress test can be performed.
-Antepartum hospital admission – At 26 to 28 weeks of gestation, we admit these patients to facilitate more intense fetal surveillance (nonstress tests three times per day). However, perinatal mortality cannot be completely eliminated by close fetal surveillance. (See 'Admission for close fetal monitoring versus ambulatory care' above and 'Fetal monitoring' above.)
-Antenatal corticosteroids – We administer antenatal corticosteroids at the time of hospital admission. For patients who remain undelivered after three weeks, a rescue course of steroids is reasonable but should be decided on a case-by-case basis. (See 'Antenatal corticosteroids' above.)
•Timing and route of delivery – Given the high rate of fetal mortality in these pregnancies, we suggest cesarean delivery between 32+0 and 34+0 weeks of gestation (Grade 2B). (See 'Obstetric care' above.)
●Conjoined twins
•Classification – The fusion in conjoined twins occurs between same body parts. They are classified as cephalopagus, thoracopagus, omphalopagus, ischiopagus, parapagus, craniopagus, rachipagus, and pygopagus (figure 2), based on the site of fusion. (See 'Types' above.)
•Diagnosis – The diagnosis of conjoined twins should be suspected in first-trimester monoamniotic twin pregnancies when the embryonic/fetal poles are closely associated and do not change in position with respect to each other (image 2). Fusion of fetal organs may be obvious (image 3). Other findings, which are not all specific to conjoined twins, include fetal hyperextension, increased nuchal translucency or cystic hygroma, no sign of separate movement of the twins, juxtaposed embryos with a single midline cardiac motion, fewer limbs than expected, a single umbilical cord with more than three vessels, or both heads or breeches consistently at the same level to each other.
Color Doppler, fetal echocardiography, 3D ultrasound examination, magnetic resonance imaging, and 3D printing can clarify anatomy, which is critical for assessing prognosis and pre- and postnatal decision making.
•Timing and route of delivery – Delivery at 35 weeks following administration of antenatal corticosteroids is a reasonable approach due to the increased risk of stillbirth or complications related to polyhydramnios and preterm birth. Conjoined twins are usually delivered by cesarean because of the risk of dystocia with vaginal delivery; however, vaginal delivery may be appropriate in the second trimester if postnatal survival is unlikely. A classical hysterotomy is usually needed. (See 'Management' above.)
ACKNOWLEDGMENT — The UpToDate editorial staff acknowledges Henry Roque, MD, MS, who contributed to an earlier version of this topic review.
