Obstetrics and Gynecology

Twin twin gestations - mono/di

Monochorionic diamniotic twin gestations

1. What every clinician should know

Monochorionic diamniotic (MCDA) twins are the product of a single fertilized ovum, resulting in genetically identical fetuses. If the ovum divides between 3 and 8 days after fertilization, MCDA twins are the result. MCDA twins are characterized by a single shared placental mass (monochorionicity) and separate amniotic sacs (diamnionicity).

Although the overall incidence of twinning in the developed world has dramatically increased over recent years, largely as the result of increased use of assisted reproductive technologies, the rate of monozygotic (single-ovum) twins has experienced a more modest increase over time. The incidence of MCDA twins is relatively constant across world populations at 3-4 per 1,000 live births. MCDA pregnancies account for roughly 70% of monozygotic pregnancies.

2. Diagnosis and differential diagnosis

A. Diagnostic criteria

When managing a twin pregnancy, it is critically important to sonographically establish chorionicity at the earliest possible gestational age. In the first trimester, chorionicity can be established with nearly 100% accuracy. For chorionicity determinations in the second trimester prior to 24 weeks, accuracy decreases but remains high (95% or greater). At more advanced gestational ages, it can become increasingly difficult to determine chorionicity using ultrasound alone.

Prior to 10 weeks of gestation, the number of gestational sacs can be used to determine chorionicity. Also in the first trimester, the "twin peak" (or "lambda") sign can be used to demonstrate dichorionicity, with 97% sensitivity and 100% specificity. This sign refers to a wedge-shaped projection of chorionic tissue within the inter-twin membrane that can only occur in the setting of two closely apposed but distinct placental masses.

Monochorionicity is better appreciated by the presence of a "T" sign, or a thin inter-twin membrane that appears to perpendicularly insert into a shared placental mass without any evidence of a "twin peak." The "T" sign has performance characteristics similar to the "twin peak" sign when assigning chorionicity in the first trimester. At more advanced gestational ages, in addition to inspecting placental masses, measuring inter-twin membrane thickness and observing for "twin peak" or "T" signs, fetal sex should be considered when assigning chorionicity.

When determining amnionicity, although it can be difficult to visualize a dividing membrane prior to 9 weeks, beyond this gestational age there should be no such challenge. The number of yolk sacs visualized can also assist with early diagnosis of amnionicity, with two yolk sacs usually - but not always - indicating diamnionicity. In the second and third trimesters, a dividing membrane occasionally can be overlooked in the presence of a "stuck" twin due to twin-twin transfusion syndrome (TTTS), preterm premature rupture of membranes (PPROM) or another complication leading to selective anhydramnios. It is therefore important to observe free movements of both twins within amniotic space before determining amnionicity.

In cases presenting at advanced gestational ages where chorionicity cannot be confidently established, zygosity testing can be performed via amniocentesis. If DNA testing demonstrates dizygosity, then the pregnancy is certainly dichorionic.

B. Differential diagnosis

The differential diagnosis for a MCDA twin pregnancy includes a dichorionic twin pregnancy and a monoamniotic twin pregnancy.

3. Management


As noted above, antepartum management of a twin pregnancy begins in the first trimester, with confirmation of chorionicity and amnionicity. With diagnosis of a MCDA twin pregnancy, referral to a perinatologist for ongoing care or co-management is recommended. Should MCDA twin-specific complications arise, a tertiary care referral is recommended.

While first trimester aneuploidy screening may be pursued with twin pregnancies, data are limited and suggest reduced detection rates in twin screening when compared to singleton gestations. Aneuploidy screening in MCDA twins using nuchal translucency (NT) measurements is also potentially confounded by the association of discordant inter-twin NT measurements with TTTS risk and also cardiovascular malformations, for which MCDA twins are at increased risk. As detection may be reduced by at least 15%, patients presenting with twin pregnancies should be offered the option for invasive genetic testing for definitive karyotype determination.

Loss rates following chorionic villus sampling (CVS) or amniocentesis for MCDA twins are incompletely studied, but limited data do not support markedly increased risks. As there are rare reports of discordant karyotypes within monochorionic twin gestations, sampling both fetuses is ideal when testing MCDA twins. Lastly, an argument could be made for CVS over amniocentesis in MCDA twins, as an amniocentesis-related complication such as an amnion-chorion separation, inadvertent septostomy or bleeding may preclude fetoscopic laser therapy in a case that subsequently develops TTTS.

Comprehensive fetal anatomical surveys are recommended beginning at 16-18 weeks gestational age for MCDA twins. In addition to visualizing the fetal anatomy, amniotic fluid volume assessments are used as surveillance for TTTS and other MCDA-specific complications. However, the optimal frequency of antenatal surveillance thereafter remains controversial. In many centers, every-other-week assessments are performed, with more frequent observation with evidence of concern. Doppler velocimetry studies may also play a role in the assessment of complicated MCDA twin pregnancies, but their role in the setting of otherwise uncomplicated MCDA twins is unclear.

The specific components of sonographic testing are also controversial: Whereas amniotic fluid volume and fetal weight assessments are integral parts of surveillance, the role of Doppler velocimetry studies is unproven. In some centers, Doppler studies are performed at each assessment, while in other institutions Doppler studies are reserved for evidence of fetal growth disorder or amniotic fluid volume discordance.

Fetal echocardiography is also recommended at approximately 18-22 weeks due to increased rates of cardiac malformations in MCDA twins and also due to acquired cardiac structural malformations and dysfunction observed in TTTS.

Due to the known association of twin pregnancies with spontaneous preterm delivery, transvaginal cervical length assessments are recommended, beginning with the fetal anatomical survey and continued every other week in the midtrimester. With measurements less than 2.5 cm closed length, weekly assessments may be performed.

Otherwise uncomplicated MCDA twins remain at risk of unexpected demise of one or both fetuses, which can have severe consequences for a surviving co-twin. "Otherwise uncomplicated" refers to an absence of polyhydramnios or oligohydramnios, abnormal Doppler findings, severe inter-twin growth discordance (greater than 20%), or fetal growth restriction. There are limited data supporting the optimal gestational age for delivery of such twins. Delivery timing decisions ultimately weigh risks of iatrogenic prematurity for both twins against the rare risk of an unexpected twin death and the consequences of this demise for a co-twin.

Although delivery timing should involve shared decision-making between the physician and her informed patient, reasonable timing appears to be between 36 and 38 weeks of gestation. At our institution, we perform weekly sonograms beginning at 32 weeks through delivery.


Intrapartum considerations for MCDA twins are no different than those for delivery of dichorionic twins. Especially at premature gestational ages, delivery should be coordinated with neonatologists. Neonatal CBC tests are recommended. Placental pathology also is recommended.

4. Complications

MCDA-specific complications

In addition to the more general complications that all twin pregnancies are at risk to experience (including increased risk of preterm labor, PPROM, premature delivery, gestational diabetes mellitus, preeclampsia and other hypertensive disorders of pregnancy, as well as considerations related to vaginal delivery of a second twin), MCDA twins are at risk of developing specific complications, largely -- but not entirely -- related to their shared placental circulation. These complications include twin-twin transfusion syndrome (TTTS) and other manifestations of fetofetal transfusion, unequal placental sharing (UPS), twin reversed arterial perfusion (TRAP) sequence, discordant fetal anomalies and an unexpected co-twin in utero demise.

Approximately 15% of MCDA will develop TTTS, a complication owing in part to unbalanced shunting of blood from one twin (the donor) to the other (the recipient), placing both at risk for death or survival with severe neurological and other morbidities. While donor morbidity results from severe anemia and hypovolemia, recipient disease manifests with hypervolemic, hypertensive overload that eventually results in cardiac failure. In early-onset, advanced-stage TTTS, death of at least one twin occurs in approximately 95% of cases and - among survivors - there is a rate of profound neurological injury that may be as high as 30-50%.

To establish a sonographic diagnosis of TTTS, it is necessary to demonstrate presence of the polyhydramnios-oligohydramnios sequence (POS), which involves donor twin oligohydramnios (maximum vertical pocket of fluid of 2.0 cm or less) and recipient twin polyhydramnios (maximum vertical pocket of fluid of 8.0 cm or greater). Other important findings that can be used to stage disease severity include: persistent absence of a donor bladder (Quintero stage II), abnormal Doppler findings (Quintero stage III), recipient twin hydrops (Quintero stage IV) and death of one or both twins (Quintero stage V).

With regard to Quintero stage III Doppler velocimetry findings, donor abnormalities are usually observed within the umbilical artery (absent or reversed end-diastolic velocity) whereas abnormal recipient findings are generally observed within the ductus venosus (absent or reversed a-wave). Elevated peak systolic flow (greater than 1.5 MoM) within the donor twin middle cerebral artery (MCA) is consistent with severe anemia and supports a TTTS diagnosis although it does not influence disease staging. Donor twin MCA abnormalities may also be observed in the absence of POS in other fetofetal transfusion pathology, such as the Twin Anemia Polycythemia Sequence (TAPS).

UPS refers to a MCDA twin pregnancy where there is pathological discordance in territorial share of the common placenta, resulting in selective growth restriction of one twin. Although this finding is often associated with oligohydramnios, UPS can be distinguished from TTTS sonographically by an absence of polyhydramnios within the amniotic sac of the normally-grown co-twin. In cases where donor twin growth restriction is associated with POS, it can difficult to determine whether this is solely a case of TTTS or whether TTTS is further complicated by UPS. There is no pathophysiological reason why these two disease entities should be mutually exclusive. Correlation with Doppler studies, including MCA studies and fetal echocardiography, can help when attempting to make this distinction.

MCDA twins are at a 2.5-fold increased risk of congenital malformations, most notably cardiac malformations. For this reason, fetal echoardiography is recommended in addition to routine fetal anatomical surveys.

TRAP sequence is a complication specific to monochorionic twins in which an early vascular disruption occurring during embryogenesis causes a dysmorphic acardiac twin to receive circulatory support from a pump twin via aberrant placental arterio-arterial anastamoses. The acardiac twin has either an absent heart or rudimentary cardiac structure along with a severely deformed or absent torso, head and extremities. As a result, the acardiac fetus is completely dependent upon the pump twin and incapable of ex utero survival. While usually morphologically normal, the pump twin is at risk for hemodynamic compromise due to the excessive circulatory demand of supporting an additional fetal mass.

Even in the absence of the complications listed above, MCDA twin pregnancies are at risk to experience an unexpected fetal demise of one or both twins. At 32 weeks of gestation, the prospective risk of an unexpected twin demise is approximately 1-2%. Current theory is that due to hemodynamic instability within a shared placental circulation around the time of this death, the co-twin is at risk to transiently exsanguinate into the reduced-pressure placental circulation and demised fetus, thereby hypoperfusing its own brain and other vital organs. This places the co-twin at a roughly 10% risk of also dying (a double demise) or, if it should survive, a 15-20% risk of profound neurological injury.

This insult is believed to be assumed instantaneously around the time of a twin death such that there is no robust evidence to support protective benefit to emergency cesarean delivery on behalf of the co-twin as a risk-reducing strategy. However, despite a lack of data to support this practice, some perinatologists recommend immediate delivery following a twin death in those pregnancies at less premature gestational ages (for example, late-preterm pregnancies). When expectant management is instead pursued, case reports exist of severe anemia detected within a surviving co-twin that has been successfully corrected with in utero transfusion. While data are limited to conclusively demonstrate neurological benefit to this practice, middle cerebral artery Doppler studies are reasonable as part of the post-demise assessment of a surviving co-twin.

Management of complications

Management of complications of monochorionic diamniotic twins must be tailored to the particular abnormality, underscoring the importance of correct determination of chorionicity but also timely and accurate identification of pathology. For example, whereas an appropriate therapeutic option for early-onset, advanced stage TTTS might be fetoscopic laser therapy, with a TRAP presentation radiofrequency ablation reduction of the acardiac fetus would be considered optimal therapy. A detailed explanation of the counseling and range of options specific to each complication is outside the scope of this chapter, and is best managed by referral to an experienced fetal therapy center. For further details on these conditions and their therapies, including their risks and benefits, please see their respective chapters.

5. Prognosis and outcome

While the majority of monochorionic diamniotic twins are delivered free of significant complications, these pregnancies are at substantially elevated risk over the baseline risk for singleton or even dichorionic twin pregnancies for complications resulting in fetal death, adverse neurological outcomes and other neonatal morbidities, and premature delivery.

6. What is the evidence for specific management and treatment recommendations?

"Multiple gestation: complicated twin, triplet, and higher-order multifetal pregnancy. ACOG Practice Bulletin No. 56". Obstet Gynecol. vol. 104. 2004. pp. 869-83.

Summary of evidence basis for certain management practices and considers selected complications.

Bajoria, R, Kingdom, J.. "The case for routine determination of chorionicity and zygosity in multiple pregnancy". Prenat Diagn. vol. 17. 1997. pp. 1207-25.

Bishop, DK.. "Yolk-sac number in monoamniotic twins". Obstet Gynecol. vol. 116. 2010. pp. 504-7.

Fuchs, KM, D'Alton, ME.. "Monochorionic diamniotic twin gestations, Chapter 167". Copel's Obstetric Imaging. 2012. pp. 714-8.

(Comprehensive summary of the sonographic evaluation of a monochorionic diamniotic twin pregnancy.)

Lee, YM, Cleary-Goldman, J, Thaker, HM, Simpson, LL.. "Antenatal sonographic prediction of twin chorionicity". Am J Obstet Gynecol. vol. 195. 2006. pp. 863-7.

(Investigated accuracy of antenatal chorionicity determination when compared to placental histopathology determinations, proved highly accurate under 24 weeks.)

Lee, YM, Wylie, BJ, Simpson, LL, D'Alton, ME.. "Twin chorionicity and the risk of stillbirth". Obstet Gynecol. vol. 111. 2008. pp. 301-8.

(One of several similar experiences considering prospective risk of fetal demise in monochorionic diamniotic twin pregnancies.)

Robinson, BK, Miller, RS, D'Alton, ME, Grobman, WA.. "Effectiveness of timing strategies for delivery of monochorionic diamniotic twins". Am J Obstet Gynecol. vol. 207. 2012. pp. 53.e1-53.e7.

(Decision analysis considering optimal timing for the delivery of otherwise uncomplicated monochorionic diamniotic twins.)
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