Twin Related Anomalies
Anne Kennedy, MD
DIFFERENTIAL DIAGNOSIS
Common
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Fetal Demise
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Twin-Twin Transfusion Syndrome
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Conjoined Twins
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Twin Reversed Arterial Perfusion
Rare but Important
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Fetus-in-Fetu
ESSENTIAL INFORMATION
Key Differential Diagnosis Issues
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Most important step in evaluation of multiple gestation is determination of chorionicity
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Certain problems only occur in monochorionic gestation
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Twin-twin transfusion syndrome (TTTS)
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Twin reversed arterial perfusion (TRAP)
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Conjoined twins
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Assess fluid distribution
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Show fluid on either side of inter-twin membrane
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Measure maximum vertical pocket for each twin
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If fluid is asymmetric, determine which side is abnormal
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Decreased in one twin: Oligohydramnios differential
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Increased in one twin: Polyhydramnios differential
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Increased in one twin & decreased in the other: TTTS most likely diagnosis
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Helpful Clues for Common Diagnoses
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Fetal Demise
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Not uncommon in early pregnancy
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Cardiac activity should always be seen in an embryo > 5 mm in crown rump length
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If early, sac may completely resorb → “disappearing twin”
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If later in monochorionic gestation, TRAP sequence may develop
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Cord entanglement is a specific cause of demise in monoamniotic twins
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Twin-Twin Transfusion Syndrome
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Monochorionic gestation
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Asymmetric or discordant growth
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Abnormal fluid distribution: One twin with oligohydramnios, one with polyhydramnios
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Look for “stuck” twin with shrink-wrapped membrane in severe cases
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TTTS staging
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Stage 1: Donor bladder visible, normal Doppler
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Stage 2: Donor bladder empty, normal Doppler
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Stage 3: Donor bladder empty, abnormal Doppler
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Stage 4: Hydrops in recipient
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Stage 5: Demise of one or both
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Conjoined Twins
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Monoamniotic gestation = no inter-twin membrane
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Must have contiguous skin covering between fetuses
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Nomenclature
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Site of attachment & “pagus” (e.g., cephalopagus)
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“Di” & site of duplication (e.g., dicephalus)
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Twin Reversed Arterial Perfusion
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Monochorionic gestation with one normal “pump” twin and one anomalous twin
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Anomalous twin
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Absent cardiac activity or rudimentary heart
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Often well-developed torso/lower extremities
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Upper extremities/cranium poorly developed or absent
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Marked skin edema ± effusions/ascites
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Hallmark of this condition is umbilical artery flow toward the abnormal fetus rather than toward the placenta
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Helpful Clues for Rare Diagnoses
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Fetus-in-Fetu
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Apparent singleton: “Twin” is incorporated into the “singleton” as a mass
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Always think about this with an unusual fetal mass
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Other Essential Information
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Anomaly rate higher in twin than singleton pregnancies
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Any anomaly/aneuploidy can occur regardless of chorionicity
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Specific entities arise from vascular connections in monochorionic placentas
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TTTS arises secondary to artery to vein shunt within monochorionic placenta
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Donor twin shunts arterial blood into recipient twin’s venous circulation → donor oligemia → effective “hypotension”
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↓ Renal perfusion → ↓ urine output → oligohydramnios
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↓ Blood to placenta → ↓ return of oxygenated blood → chronic hypoxia → poor growth
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Recipient twin receives all its own return from placenta & portion of co-twins → plethora/fluid overload
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↑ Volume status → ↑ cardiac output → eventual development of hydrops
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TRAP sequence arises secondary to artery to artery shunt within monochorionic placenta
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Pump twin deoxygenated blood → co-twin umbilical arteries → preferential perfusion of lower extremities → lack of upper body/cranial development
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Blood enters fetus via UA not UV → lack of normal circulation through heart → abnormal cardiac development
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Alternative Differential Approaches
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Twin with hydrops
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Anomalous twin in TRAP sequence: Massive skin edema/effusions common
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Pump twin in TRAP sequence at risk for hydrops from cardiac compromise
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Both twins in TTTS at risk for hydrops (greater in recipient due to volume overload)
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May have hydrops from other causes unrelated to placental sharing
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All cases need careful anatomic survey, infection work up ± chromosome evaluation
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Perform formal fetal echocardiography for structure, function, rate and rhythm
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Twin with encephalomalacia
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Any destructive process may end in encephalomalacia regardless of chorionicity
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May occur secondary to chronic hypoxia in donor twin in TTTS
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Monochorionic twin demise places surviving twin at risk for ischemic injury described as “twin embolization syndrome”
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Shared placental circulation → vascular communications between fetuses
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Death of one twin → sudden drop in peripheral resistance for survivor → effectively severe hypotensive episode
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Ischemic injury seen in brain/myocardium/kidneys
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Sonographic findings
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↑ Echogenicity in periventricular white matter, loss of grey-white matter differentiation
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Eventual cystic encephalomalacia, porencephaly, microcephaly
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Image Gallery
![]() Ultrasound shows an inter-twin membrane
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