What is the management plan for discordant twins?

Management of Discordant Twins

Discordant twins require intensified ultrasound surveillance with frequency determined by chorionicity, and closer follow-up is warranted when significant discordance in fetal size or amniotic fluid develops, regardless of chorionicity. 1

Defining Discordance

• Estimated fetal weight (EFW) discordance >25% between twins is the most widely accepted threshold for diagnosis 1
• Alternative criteria include EFW <10th percentile in one twin or discordant fetal abdominal circumference >10% 1
• First-trimester crown-rump length differences ≥5 days carry high risk for major congenital anomalies in the smaller twin and warrant meticulous anatomic surveillance 2

Surveillance Protocol by Chorionicity

Dichorionic Twins with Discordance

• Perform ultrasound every 3-4 weeks for routine dichorionic twins, but increase frequency when discordance is identified 1
• At each scan from 20 weeks onward, assess: fetal biometry, amniotic fluid volume, umbilical artery Doppler studies, and calculate/document EFW discordance 1
• The prognosis is generally better than monochorionic twins because dichorionic twins lack shared placental circulation, reducing neurological injury risk to the co-twin 3

Monochorionic Twins with Discordance

• Begin surveillance at 16 weeks with ultrasound every 2-3 weeks, with some experts recommending every 2 weeks or more frequently 1
• Monochorionic twins with selective intrauterine growth restriction (sIUGR) occur in up to 25% of cases and carry significantly higher risk due to placental vascular anastomoses 1
• Umbilical artery Doppler classification is critical for risk stratification:
  • Type 1 (constant end-diastolic flow): best prognosis 1
  • Type 2 (constant absent/reversed end-diastolic flow): high risk for neonatal complications including sepsis, CNS abnormalities, respiratory distress, and death 1
  • Type 3 (intermittent absent/reversed end-diastolic flow): variable outcomes 1
• Severe oligohydramnios or "stuck twin" phenomenon with abnormal Doppler is a significant predictor of mortality in the growth-restricted twin 1

Additional Assessments

• Perform comprehensive fetal anatomic assessment focusing on cardiovascular and renal systems, as these are most commonly associated with discordance 4
• Evaluate for velamentous cord insertion (present in 22% of monochorionic twins), which increases risk of small for gestational age, sIUGR, lower gestational age at birth, and intrauterine fetal demise 1
• Screen for vasa previa, especially in monochorionic pregnancies with velamentous insertion, as this can cause acute fetal hemorrhage and death at delivery 1
• Perform transvaginal cervical length assessment to stratify preterm delivery risk 1

Common Pitfalls

• Discordant growth alone is not an indication for preterm delivery; timing should be based on Doppler findings, gestational age, and development of complications 5
• Biophysical profile testing has insufficient evidence to support routine use in twins with reactive non-stress tests and no other risk factors 1
• First-trimester discordance (≥5 days difference in crown-rump length) is an ominous finding requiring heightened surveillance for major anomalies including diaphragmatic hernia, ventriculomegaly, cardiac defects, and sacral agenesis 2
• Risk of fetal demise decreases after 32 weeks even in monochorionic pregnancies, but surveillance should continue 1