Discordant Twin Growth
Anne Kennedy, MD
DIFFERENTIAL DIAGNOSIS
Common
Idiopathic and Normal
Intrauterine Growth Restriction of One Twin
Twin-Twin Transfusion Syndrome
Unequal Placental Sharing
Less Common
Anomaly of One Twin
Rare but Important
Twin Reversed Arterial Perfusion
ESSENTIAL INFORMATION
Key Differential Diagnosis Issues
Must differentiate small but normal twin from abnormally grown twin
By convention term “discordance” is used when one twin meets criteria for intrauterine growth restriction (IUGR)
Estimated fetal weight (EFW) < 10th percentile
Size difference in twins not clinically significant if both show appropriate interval growth and normal fluid
Determination of chorionicity essential for management
Disparate size not uncommon in dichorionic gestation
Exclude anomaly/aneuploidy
Check placental cord insertion sites
Track growth/deepest fluid pockets at 3-4 week intervals
Doppler studies if either twin meets criteria for IUGR
Certain conditions only occur in monochorionic (MC) gestations
Twin-twin transfusion syndrome (TTTS)
Twin reversed arterial perfusion sequence (TRAP)
Unequal placental sharing
In MC twins, demise of one twin has profound consequences for the survivor
Demise of one MC twin may → “twin embolization syndrome” (better thought of in terms of exsanguination or hypotension rather than embolization)
Results in ischemic brain/cardiac injury in survivor
Preterm delivery confers risks of prematurity to both but may be a better option than continuing the pregnancy with risk of demise of one twin
In dichorionic twins with one at-risk fetus, goal is to maximize outcome for healthy fetus
Helpful Clues for Common Diagnoses
Idiopathic and Normal
Appropriate interval growth even if at low end of normal range
Normal amniotic fluid volume, normal Doppler studies, no anomalies
Intrauterine Growth Restriction of One Twin
Less than expected interval growth
Associated with unequal placental sharing
Look for velamentous or marginal cord insertion
Look at placental location
Implantation on septum or over large fibroids
Look for evidence of abruption
Early onset concerning for anomaly, syndrome or aneuploidy
Consider amniocentesis
Late onset more suggestive of placental insufficiency
Use Doppler to evaluate fetoplacental circulation
Twin-Twin Transfusion Syndrome
Monochorionic twins with artery-to-vein anastomoses in the placenta
Asymmetric size with true discordance in many cases
Recipient: Larger twin with polyhydramnios
Donor: Smaller twin with oligohydramnios
Staging of TTTS
Stage 1: Donor bladder visible, Doppler normal
Stage 2: Donor bladder empty, Doppler normal
Stage 3: Donor bladder empty, Doppler abnormal
Stage 4: Hydrops in recipient
Stage 5: Demise of one or both twins
Unequal Placental Sharing
Velamentous cord insertion
Cord inserts onto membranes away from placental margin
Strong association with TTTS; monitor carefully for fluid volume/growth
Look for associated vasa previa; if present mandates delivery by cesarean section before onset of labor
Marginal cord insertion
Cord inserts at edge of placenta rather than onto placental disc
Eccentric cord insertion on placenta
One twin has less than half of placenta
Helpful Clues for Less Common Diagnoses
Anomaly of One Twin
Anomalies more common in multiples than in singletons
Severe structural malformations seen in 2.6% of monochorionic twins in one series
Careful anatomic survey mandatory
Multiple anomalies increase suspicion for aneuploidy or syndrome
Amniocentesis may be offered
Fetal reduction may be offered
Helpful Clues for Rare Diagnoses
Twin Reversed Arterial Perfusion
TRAP twin often looks very abnormal
Absent cranial structures/upper extremities strongly suggests this diagnosis
Heart maybe “absent” or rudimentary
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