Monoamniotic twin pregnancy (including conjoined twins)

INTRODUCTION — Monoamniotic twin pregnancies (which are always monozygotic and monochorionic) are the least common type of twin pregnancy. They can have many of the same complications as monochorionic diamniotic twin pregnancies but carry a higher risk for congenital anomalies and fetal death. Conjoined twins and mirror image twins are unique to monoamniotic twin pregnancies.

This topic will discuss issues specific to monoamniotic twin pregnancies. General aspects of twin pregnancy and complications shared with monochorionic diamniotic twin pregnancies are reviewed separately.

●(See "Twin pregnancy: Overview".)

●(See "Twin pregnancy: Routine prenatal care".)

●(See "Twin pregnancy: Management of pregnancy complications".)

●(See "Twin-twin transfusion syndrome: Screening, prevalence, pathophysiology, and diagnosis" and "Twin-twin transfusion syndrome: Management and outcome".)

●(See "Twin anemia-polycythemia sequence (TAPS)".)

●(See "Selective fetal growth restriction in monochorionic twin pregnancies".)

●(See "Twin reversed arterial perfusion (TRAP) sequence".)

PLACENTA, MEMBRANES, AND CORDS — Monoamniotic twin gestations have a single placenta with one amnion and one chorion; thus, there is no dividing membrane between the twins (figure 1). The two separate umbilical cords typically insert within 6 cm of each other. The insertions are located centrally in two-thirds of cases and marginal/velamentous in the remainder [1]. Intertwin placental vascular anastomoses are always present [2].

PATHOGENESIS

●Zygote division at ≥8 days postfertilization – Chorionicity/amnionicity in twins is determined by the timing of postfertilization division of the zygote, as shown in the figure (figure 1). Monoamniotic twins occur when division occurs on days 8 to 12. Mirror image twins may occur when embryonic division occurs at the late end of this range, after the embryonic plate begins to lateralize [3,4]. These twins have mirror image features involving handedness and hair whorls, as examples; situs inversus of one twin has also been reported. If division is delayed to ≥13 days, the twins will be conjoined. (See 'Conjoined twins' below.)

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, iatrogenic manipulation of the zona pellucida (ie, during intracytoplasmic sperm injection and assisted hatching), and extension of the duration of culture five to six days before transfer [5-11]. Although monozygotic twinning is increased with in vitro fertilization, the incidence of monoamniotic twins is not consistently increased and is decreased in some studies.

●Membrane rupture – Monoamnionicity rarely results from spontaneous or iatrogenic rupture of intertwin membranes. (See 'Pseudo-monoamniotic twins' below.)

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 [6,12,13].

Sex ratio — Females predominate among monoamniotic twins. The male/(male + female) ratio in these pregnancies ranges from 0.28 to 0.35 [14-17]. By comparison, the ratio is 0.51 in singletons and dizygotic twins.

Two theories have been proposed to explain why the frequency of males decreases in the continuum from singletons to monoamniotic twins [18-20]:

●The process of X-inactivation overlaps with the timing of monozygotic twinning and thus may directly contribute to development of monozygotic twins

●The XX karyotype may confer a survival benefit

DIAGNOSIS

Clinicopathologic diagnosis — The clinicopathologic diagnosis of monoamnionicity is made in twins with all of the following features [21-26]:

●Absence of an intertwin dividing membrane

●A single nonfused placenta

●Same sex (a rare exception is monovular dispermic twinning [1])

Postnatally, 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 (same sex twins with a single nonfused placenta and no dividing membrane). Additional diagnostic findings (described below) may be present on prenatal ultrasound that may not be detectable postnatally. Serial ultrasound examinations improve diagnostic certainty as some findings are best identified early in the first trimester (yolk sac, intertwin membrane) and others in the late first trimester or early second trimester (cord entanglement).

●Cord entanglement – Sonographic observation of cord entanglement is pathognomonic for monoamniotic twins and may be seen as early as the late first trimester [12,27-33]. The diagnosis of cord entanglement is based on visualization of intertwined umbilical cords that have different fetal heart rates (FHRs) upon Doppler insonation of the various vessels (image 1). Three-dimensional (3D) ultrasound can also aid in the diagnosis of cord entanglement [34]. (See 'Cord entanglement' below.)

●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.

●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. Maternal anatomy (eg, retroverted uterus, fibroids) also can impede visualization of the membrane.

●Visualization of one yolk sac with two fetal poles – If an ultrasound examination is performed before eight weeks of gestation, visualization of one yolk sac with two fetal poles strongly suggests monoamniotic twins [35-38]. However, rare case reports have described visualization of two yolk sacs in monoamniotic twin pregnancies [39,40] and one yolk sac with two embryos in diamniotic twin pregnancies [41]. Therefore, the number of yolk sacs in twin pregnancies can support the suspected diagnosis but does not reliably make or exclude monoamniotic twins.

●Same-sex fetuses – In the second and third trimesters, sex concordance can be determined. Concordance 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 [42].

Differential diagnosis

Pseudo-monoamniotic twins — The term pseudo-monoamnionicity has been used to describe diamniotic twin pregnancies in which the intertwin membrane has ruptured. Pseudo-monoamnionicity should be suspected in twin pregnancies that have undergone an invasive intrauterine procedure [12,43-46], especially if a dividing membrane was seen on a previous ultrasound, but it may occur spontaneously [12,46].

The perinatal mortality rate is as high as for true monoamniotic twin gestations [46]. (See 'Fetal and neonatal outcomes' below.)

OUTCOMES

Maternal outcomes — 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 outcomes

Overview — Monoamniotic twin pregnancies are subject to the following fetal and neonatal 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:

•Twin-twin transfusion syndrome (TTTS) (see "Twin-twin transfusion syndrome: Screening, prevalence, pathophysiology, and diagnosis" and "Twin-twin transfusion syndrome: Management and outcome"). TTTS is less common in monoamniotic twins than in diamniotic twins (2 to 6 percent [16,47] versus 9 to 15 percent [48,49]). This may be due to a higher frequency of protective arterioarterial anastomoses in monoamniotic placentas [50]

•Twin anemia-polycythemia sequence (TAPS). (See "Twin anemia-polycythemia sequence (TAPS)".)

•Twin reversed arterial perfusion (TRAP) sequence. (See "Twin reversed arterial perfusion (TRAP) sequence".)

•Neurologic sequelae from fetal demise of the co-twin. (See "Twin pregnancy: Management of pregnancy complications", section on 'Death of one twin'.)

•Selective fetal growth restriction. (See "Selective fetal growth restriction in monochorionic twin pregnancies".)

●Complications that only occur in monoamniotic twins (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 [51,52]. Loose cord entanglements probably occur as early as the first trimester and have the potential to tighten and occlude the umbilical blood vessels at any time. Intermittent occlusion of the vessels may be associated with neurologic morbidity and severe prolonged occlusion can be lethal [51,53-55].

In a 2013 systematic review specifically evaluating 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 [56]. 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 of monoamniotic pregnancies, which have become routine in recent years, may have prevented perinatal death from cord entanglement and could not be assessed.

It has been hypothesized that medical amnioreduction with a nonsteroidal anti-inflammatory drug (NSAID) might diminish fetal movement and, in turn, the risk of tightening and occlusion of entangled cords [53,57,58]. The largest of three studies of this intervention treated 20 monoamniotic pregnancies with entangled cords with sulindac from 20 to 32 weeks of gestation and reported no fetal or neonatal deaths [57]. In the smallest study, the intervention was attempted in two pregnancies and only one neonate survived [53]. No guidelines recommend this approach because NSAIDs have well-documented harms (eg, premature closure of the ductus arteriosus, oligohydramnios, impaired neonatal kidney function).

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 twins than diamniotic monochorionic twins [52,59-62]. Major anomalies have been reported in 7 to 28 percent of monoamniotic twin pregnancies compared with 6 percent of diamniotic monochorionic twin pregnancies [15,52,62-65]. Monochorionic twins are at an increased risk of developing congenital heart anomalies (59.3 in 1000 live births, relative risk 6.3 compared with singletons and dichorionic twins) [66] and this risk appears to be highest in monoamniotic twins [16,67,68]. The increased risk of anomalies may be related to late cleavage and/or circulatory imbalances across anastomotic vessels in the placenta. This hypothesis may explain why anomalous monoamniotic twins are less likely to have an underlying chromosomal abnormality than other anomalous fetuses [63].

Multiple case reports have described discordance for anomalies (eg, one anomalous and one nonanomalous twin, both twins anomalous but with different anomalies) [69-73]. Concordant anomalies occur in less than 25 percent of cases [74].

Fetal and perinatal mortality — Perinatal mortality at ≥24 weeks of gestation was 8 percent (95% CI 6 to 10 percent) in a 2019 meta-analysis of studies of monoamniotic twin pregnancy (25 retrospective studies, 1628 nonanomalous twins) [75]. This is higher than in other twins at this gestational age because monoamniotic pregnancies are at risk for both the unique complications associated with their placentation (eg, cord entanglement and occlusion, conjoined twins, acute hemodynamic imbalances caused by the large placental anastomoses) and the complications associated with other types of twin placentation (eg, TTTS, TRAP, TAPS, congenital anomalies, preterm birth) [76].

The meta-analysis also reported the following fetal outcomes [75]:

●Fetal demise occurred in 5.8 percent of fetuses overall (95% CI 4.0-8.1). Single and double fetal 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:

•24 to 30 weeks: 4.3 percent (95% CI 2.8-6.2)

•31 to 32 weeks: 1.0 percent (95% CI 0.6-1.7)

•33 to 34 weeks: 2.2 percent (95% CI 0.9-3.9)

•≥35 weeks: No deaths among the 150 fetuses who reached 35 weeks. Although this analysis did not show a higher risk for fetal demise near term, this is likely because of obstetric management. (See 'Fetal heart rate monitoring' below and 'Delivery timing, route, and other issues' 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 were due to fetal anomalies, TRAP sequence, and conjoined twinning; and the remainder were due to TTTS, tight cord entanglement, and acute hemodynamic imbalances through the large placental vascular anastomoses [76].

●The incidence of fetal demise was lower with inpatient compared with outpatient management (3 versus 7.4 percent). (See 'Fetal heart rate monitoring' below.)

Approximately 38 percent of pregnancies were delivered before the scheduled date due to preterm labor or an abnormal fetal heart rate (FHR) pattern.

Studies that have reported higher fetal and perinatal death rates generally included pregnancies less than 24 weeks, thus including miscarriages, pregnancy terminations, and deaths before the lower limit of viability [77].

OBSTETRIC CARE

General principles — Management of monoamniotic twin pregnancy is similar to the general management of any twin pregnancy described separately (see "Twin pregnancy: Overview" and "Twin pregnancy: Labor and delivery"), with some exceptions. Given the complexity of managing these pregnancies and the complications that can result, we discuss the option of fetal reduction to a singleton gestation or termination of the entire pregnancy with patients diagnosed early in pregnancy. (See "Multifetal pregnancy reduction and selective termination", section on 'Monochorionic fetuses'.)

Issues specific to obstetric management of ongoing monoamniotic twin pregnancies are discussed below.

Screening for/diagnosis of 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 [78,79]. Both twins of a monozygotic pair are either affected or unaffected, with rare exceptions due to postzygotic and epigenetic mechanisms [80].

●Screening – Due to the complexities of aneuploidy screening in monoamniotic twins, we suggest referral to a genetic counselor, if feasible. Standard methods for Down syndrome screening can be used (eg, cell-free DNA [cfDNA], maternal serum biomarkers and ultrasound markers). cfDNA screening has the highest detection rate and the lowest false-positive rate compared with other screening methods [81]. (See "Twin pregnancy: Routine prenatal care", section on 'Screening for Down syndrome (trisomy 21)'.)

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 [82]. In the setting of monochorionic twins with normal karyotypes, an enlarged NT based on standard gestational age-based tables or a >20 percent discordance between NT measurements can be a marker for a pregnancy at risk for twin-twin transfusion syndrome (TTTS) [83]. (See 'Monitoring for/diagnosis of TTTS and TAPS' below.)

●Prenatal diagnosis – 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 a G-banded karyotype alone is sufficient. We order a chromosomal microarray when a diagnostic procedure is performed because of an enlarged NT or a structural anomaly in any of the fetuses since diagnostic performance is higher than 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/diagnosis of fetal anomalies — A detailed fetal anatomic survey of each twin is performed in all monoamniotic twin pregnancies at 18 to 20 weeks, given the increased incidence of serious anatomic malformations (including conjoined twins and twin reversed arterial perfusion [TRAP] sequence).

We obtain fetal echocardiography if cardiac views during the anatomy survey are suboptimal or suggest an abnormality, or the patient has another of the common indications for fetal echocardiography. We do not perform fetal echocardiography routinely, although some authorities do so for all monoamniotic or all monochorionic twin pregnancies, since congenital heart anomalies are more common in these pregnancies than in singletons [16,67,68,84]. (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 increased risk for preterm birth, demise, and other perinatal complications. An option to reduce these risks for the nonanomalous twin is a combination of selective fetal reduction of the anomalous twin by fetoscopic cord ligation (which protects the nonanomalous twin from morbidities that may occur upon demise of its co-twin) plus cord transection (which protects the nonanomalous twin from future complications from cord entanglement) [85,86]. The procedure is described in detail separately. (See "Multifetal pregnancy reduction and selective termination", section on 'Monochorionic fetuses'.)

Monitoring for/diagnosis of TTTS and TAPS — We monitor monoamniotic twins for TTTS and twin anemia-polycythemia sequence (TAPS) according to the same schedule as for monochorionic diamniotic twins (table 1). A comparison of international guidelines on management of twin pregnancy noted lack of agreement regarding the frequency of ultrasound monitoring of twins and no clear difference between monoamniotic and monochorionic diamniotic twins [87]. Sonography guidelines varied between every 1 to 2 weeks to every 2 to 4 weeks, with the greatest number (33 percent) recommending surveillance every 2 weeks. (See "Twin-twin transfusion syndrome: Screening, prevalence, pathophysiology, and diagnosis", section on 'Our approach to monitoring'.)

Monoamniotic twins have a lower rate of TTTS than diamniotic twins (2 to 6 percent [16,47] versus 9 to 15 percent [48,49]), 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 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 monoamniotic twins [88]. (See "Twin-twin transfusion syndrome: Screening, prevalence, pathophysiology, and diagnosis" and "Twin anemia-polycythemia sequence (TAPS)".)

TTTS can be challenging to diagnose in monoamniotic twins, particularly at an early stage, since the key diagnostic feature (oligohydramnios/polyhydramnios sequence) cannot be detected in a single sac. Therefore, in monoamniotic twins, TTTS should be suspected when one twin has a large bladder and the other has a nonvisualized bladder. Characteristic Doppler indices (absent or reversed end-diastolic velocity in the umbilical artery, reversed flow in a-wave of the ductus venosus, or pulsatile flow in the umbilical vein) support the diagnosis. (See "Twin-twin transfusion syndrome: Screening, prevalence, pathophysiology, and diagnosis", section on 'Diagnostic criteria'.)

Enlarged NT is more common in monochorionic twins and has been attributed to early TTTS, in addition to all of the causes of enlarged NT seen in singleton gestations [89]. In one study, fetal NT above the 95^th centile for gestational age had positive and negative predictive values for the development of TTTS of 38 and 91 percent, respectively [90]. 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 [83]. (See "Enlarged nuchal translucency and cystic hygroma".)

Management of TTTS and TAPS — Monoamniotic twin pregnancies with TTTS or TAPS are at high risk of perinatal mortality and preterm birth <32 weeks [91]. Management of TTTS and TAPS in monoamniotic twins is similar to that in diamniotic twins. However, in monoamniotic pregnancies undergoing laser therapy, visualizing the vascular equator between the twins is more challenging due to the absence of a dividing membrane (which is often proximate to the equator). The equator can still be located by following blood vessels distally from their origins at the two cord insertion sites. Management is reviewed in detail separately. (See "Twin-twin transfusion syndrome: Management and outcome" and "Twin anemia-polycythemia sequence (TAPS)".)

Other fetal monitoring

Fetal heart rate monitoring — Antepartum fetal heart rate (FHR) monitoring is indicated because monoamniotic twins are at increased risk for fetal demise, which has been attributed, at least in part, to cord entanglement and occlusion. Monitoring and timely intervention may prevent some of these deaths. Protocols for FHR monitoring vary among centers. The best approach is unclear as no high-certainty data are available to guide decision-making regarding the initiation, frequency, or duration of monitoring. No approach can prevent all fetal deaths.

●Our approach – When the pregnancy reaches a gestational age at which delivery because of a nonreassuring FHR tracing would be considered (24 weeks in our practice), we obtain FHR tracings two to three times weekly for one hour to check for occurrence of multiple deep variable decelerations, which suggests cord compression from entanglement. When the fetuses are sufficiently mature physiologically to exhibit reactivity, reactivity is also assessed but a one-hour tracing is still obtained since the goal is to look for signs of impending cord occlusion (variable decelerations) as well as reactivity. Other clinicians and patients may reasonably choose a different gestational age for beginning monitoring and a different frequency and duration of monitoring.

At 26 to 28 weeks of gestation, we admit patients with monoamniotic twins for more intensive fetal surveillance, but some clinicians offer admission as early as 24 weeks. The FHR is monitored three times daily until a reactive nonstress test is obtained if the fetuses are sufficiently mature physiologically to exhibit reactivity, although some clinicians monitor for a full 60 minutes to detect variable decelerations even if the tracing is reactive before 60 minutes have elapsed.

●Other approaches – After reviewing the literature, one group concluded that fetal death due to cord entanglement and occlusion 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 [74]. For inpatients, they suggested a reasonable monitoring regimen was a nonstress test or hour-long FHR 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.

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 [92]. The survey authors also noted that 75 percent of respondents recommended daily intermittent fetal monitoring, with 81 percent performing fetal testing two to three times daily. Reviews have generally reported better pregnancy outcomes with inpatient compared with outpatient care [74,75].

●Evidence – In a meta-analysis of monoamniotic twin pregnancy (25 studies, 1628 nonanomalous twins ≥24 weeks of gestation), 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 [75]. 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 [93]. 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. Until better data are available, most clinicians have taken the prudent approach of admitting these patients at 26 to 28 weeks of gestation for inpatient monitoring.

Ultrasound

●Fetal weight – Fetal weight is assessed every four weeks from approximately 22 weeks to evaluate 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 [93].

●Doppler velocimetry – We recommend not using Doppler velocimetry to detect altered flow suggestive of compression of cord vessels. Although high blood velocity in the umbilical vein [33,94], a notch in the umbilical artery waveform [95,96], or persistent absent end diastolic flow in the absence of growth restriction [97] have been reported as suggesting compromise of the fetoplacental circulation by cord compression, they have poor predictive value for occurrence of an acute adverse event and their use has no proven benefit in this context.

●Biophysical profile – There are no data on use of the biophysical profile for monitoring fetal well-being in monoamniotic twin pregnancies.

Antenatal corticosteroids — We administer a course of antenatal corticosteroids at the time of hospital admission at 26 to 28 weeks. For patients who remain undelivered after three weeks, a single rescue course of steroids is reasonable; the timing should be decided on a case-by-case basis. Use of rescue steroids is discussed separately. (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 monochorionic diamniotic 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 a cord entanglement that is lethal to the surviving twin is not known. Complications and management of single fetal demise is discussed in more detail separately. (See "Twin pregnancy: Management of pregnancy complications", section on 'Death of one twin'.)

Delivery timing, route, and other issues

●Timing – We plan delivery between 32+0 and 34+0 weeks of gestation because of the increasing risk of fetal demise in the third trimester, which we estimate to be at least 5 percent in pregnancies that continue beyond this gestational age range [17,52,98]. Although, as described above [75], several studies have observed that healthy fetuses who achieve 35 weeks of gestation have a low rate of 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 of nonanomalous singleton live births, the neonatal death rates after birth at 32 and 34 weeks were 0.2 percent (1 out of 451) and 0 (0 out of 1058), respectively [99], which supports the hypothesis that the risk of death associated with continuing a monoamniotic twin pregnancy beyond 32 to 34 weeks is higher than the risk of death from birth at this gestational age, 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 found that the median gestational age for planned delivery was 34 weeks [92]. A review of national and international guidelines found that all considered 32 weeks the earliest gestational for planned delivery of uncomplicated cases, but the latest gestational age varied from 34 to 36 weeks [101]. The American College of Obstetricians and Gynecologists suggests delivery at 32+0 to 34+0 weeks of gestation [102].

The timing of nonplanned delivery is dictated by the clinical scenario. In the absence of standard maternal or fetal indications for delivery, the FHR pattern that triggers intervention before 32 to 34 weeks because of concerns about impending cord occlusion is a clinical judgment.

●Route – Guidelines consistently suggest cesarean birth for monoamniotic twin pregnancies to avoid intrapartum complications from cord entanglement and this is our preferred route of birth [101]. This cautionary approach is based on expert opinion; no randomized trials or large cohort studies have been performed. Some providers, however, have offered a trial of labor and had good outcomes if the first twin is cephalic, the twins are concordant in size, and a well-informed patient chooses this approach [103].

●Caution – It is critical to remember that the umbilical cord of the second twin may loop around the neck of the first twin. When delivering a first twin with a nuchal cord, the cord should be eased over its head rather than transected. If transection is necessary, then the second twin should be delivered expeditiously.

CONJOINED TWINS

Definition and classification — Conjoined twins are a type of monoamniotic twins in which the body parts of one twin are fused with the same body parts of the co-twin; the degree of fusion varies from minor to substantial. They are classified as cephalopagus, thoracopagus, omphalopagus, ischiopagus, parapagus, craniopagus, rachipagus, and pygopagus (figure 2), based on the site of fusion.

Epidemiology — Incidence is estimated to be 1.5 per 100,000 births worldwide (versus 8 per 100,000 births for all monoamniotic twins [104]).

Female twins are affected more often than males.

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 on imaging (image 2) [105-107]. Other findings, which are not all specific to conjoined twins, but support the diagnosis include enlarged nuchal translucency (NT) 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 with respect to each other [108,109]. Polyhydramnios is present in up to 50 percent of cases in late pregnancy [108].

Fusion of fetal organs confirms the diagnosis (image 3). In early pregnancy, this may appear as juxtaposed embryos with a single midline cardiac motion. In the latter half of pregnancy, a detailed anatomy survey can aid in defining the location and extent of the conjoined area.

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 (MRI) [110-113] 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, and patient-specific simulations [114].

Management

●Multidisciplinary team – Patients should be cared for at a center with prior experience 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 – Delivery at 35 weeks following administration of a course of antenatal corticosteroids is a reasonable approach due to the increased risk of stillbirth or complications related to polyhydramnios and preterm birth. There is insufficient data to strongly support a specific gestational age for timing of delivery [109]. In contrast to other monoamniotic twin pregnancies, early delivery to avoid cord entanglement and occlusion is not a factor because conjoined twins do not move independently.

●Cesarean birth – Cesarean birth is recommended in most cases, especially third-trimester pregnancies, to avoid dystocia. The optimal uterine incision depends on patient-specific factors, including the gestational age, the type of fusion, and neonatal prognosis. The abdominal and uterine incisions need to be sufficiently large to extract the twins atraumatically; therefore, a vertical skin incision and classical uterine incision are often 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 [115-117]. 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.

Prognosis — In a review of 379 cases of conjoined twins, 45.6 percent were live born, 27.2 percent were stillborn, and 27.2 percent underwent pregnancy termination [104]. The prognosis for live births is poor because congenital anomalies are always present and often preclude survival of one or both twins, even if surgical separation is performed.

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

●Definition – Monoamniotic twins are an uncommon subtype of monozygotic monochorionic twins characterized by a single placenta with one amnion and one chorion (figure 1). (See 'Placenta, membranes, and cords' above and 'Epidemiology' above and 'Pathogenesis' above.)

●Prenatal diagnosis – 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. Sonographic visualization of cord entanglement with Doppler insonation of the individual umbilical cords revealing different heart rates is diagnostic of monoamniotic twins (image 1). (See 'Prenatal diagnosis' above.)

●Maternal outcomes – 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 outcomes – Monoamniotic twin pregnancies are subject to complications that may occur in any twin pregnancy (eg, preterm birth, growth restriction of one or both twins, congenital anomalies), complications that only occur in monochorionic twins (eg, twin-twin transfusion syndrome [TTTS], twin anemia-polycythemia sequence [TAPS], neurologic sequelae from fetal demise of co-twin, selective fetal growth restriction, twin reversed arterial perfusion [TRAP] sequence), and complications that only occur in monoamniotic twins (cord entanglement, conjoined twins). (See 'Fetal and neonatal outcomes' above.)

Perinatal mortality for pregnancies ≥24 weeks of gestation is higher than in singleton and dichorionic twin pregnancies and is approximately 8 percent. Cord entanglement and congenital anomalies are major causes of fetal death. (See 'Fetal and perinatal mortality' above and 'Cord entanglement' above and 'Congenital anomalies' above.)

●Prenatal care

•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, cell-free DNA [cfDNA; preferred], maternal serum biomarkers and sonographic measurement of nuchal translucency). (See 'Screening for/diagnosis of 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 other standard indications for fetal echocardiograph. (See 'Screening for/diagnosis of fetal anomalies' above.)

•Twin-twin transfusion syndrome and twin anemia-polycythemia sequence – Monoamniotic twins have a lower rate of TTTS than diamniotic twins (2 to 6 percent versus 9 to 15 percent), but the risk is sufficient to justify screening by the same protocol (table 1). TTTS can be challenging to diagnose in monoamniotic twins, particularly at an early stage, since oligohydramnios/polyhydramnios sequence cannot be detected in a single sac. It should be suspected if one twin has a large bladder and the other has a nonvisualized bladder. Characteristic Doppler indices support the diagnosis. (See 'Monitoring for/diagnosis of TTTS and TAPS' above.)

•Fetal heart rate (FHR) monitoring – Our approach to FHR monitoring is shown in the algorithm (algorithm 1). Perinatal mortality cannot be completely eliminated by close fetal surveillance. (See 'Fetal heart rate monitoring' above and 'Other fetal monitoring' above.)

•Antenatal corticosteroids – We administer antenatal corticosteroids at the time of hospital admission. For patients who remain undelivered after three weeks, a single rescue course of steroids is reasonable; the exact timing should be decided on a case-by-case basis. (See 'Antenatal corticosteroids' above.)

●Timing and route of delivery – Given the risk of fetal mortality from cord entanglement and occlusion in these pregnancies, we suggest delivery by cesarean (Grade 2C), consistent with many national and international guidelines. We suggest delivery between 32+0 and 34+0 weeks of gestation (Grade 2C). This timing considers both the risk of stillbirth with ongoing pregnancy and the risk of morbidity from preterm birth. (See 'Delivery timing, route, and other issues' above.)

●Conjoined twins (figure 2) are a type of monoamniotic twins in which the body parts of one twin are fused with the same body parts of the co-twin. The prognosis for live births is poor because congenital anomalies are always present and often preclude survival of one or both twins, even if surgical separation is performed. (See 'Definition and classification' above and 'Prognosis' 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 confirms the diagnosis (image 3). (See 'Prenatal diagnosis' above.)

•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 birth; however, vaginal birth may be appropriate in the second trimester if postnatal survival is unlikely. A vertical skin incision and classical hysterotomy are often needed for cesarean birth. (See 'Management' above.)

ACKNOWLEDGMENT — The UpToDate editorial staff acknowledges Henry Roque, MD, MS, who contributed to an earlier version of this topic review.