1. Classification: Zygosity & Chorionicity
• Monozygotic (identical): single zygote splits → same genetic material. Dizygotic (fraternal): two ova fertilized.
• Chorionicity determined by timing of splitting: Days 1-3 → dichorionic diamniotic (Di-Di); Days 3-8 → monochorionic diamniotic (Mo-Di); Days 8-13 → monochorionic monoamniotic (Mo-Mo).
• Monochorionic twins share placenta → vascular anastomoses (arterio-venous, arterio-arterial, veno-venous).
2. TTTS (Twin-Twin Transfusion Syndrome)
• Occurs in 10-15% of monochorionic twins. Unbalanced deep AV anastomoses without compensatory superficial vessels.
• Donor twin: oligohydramnios, IUGR, anemia, hypovolemia, collapsed bladder.
• Recipient twin: polyhydramnios, hydrops, polycythemia, hypervolemia, cardiac hypertrophy.
• Quintero staging I-V: from abnormal fluid (I) to demise (V).
• Treatment: Fetoscopic laser photocoagulation of anastomoses (Solomon technique) – improves survival (≈60%) and neurologic outcomes vs amnioreduction. Amnioreduction used after 26 weeks or for palliation.
3. Atypical twinning
• Conjoined twins: 1:50,000 pregnancies, female predominance. Types: thoracopagus (28%), omphalopagus, craniopagus. Parasitic twin (exoparasitic, fetus-in-fetu).
• TRAP sequence (twin reversed arterial perfusion): acardiac twin perfused by pump twin via reverse flow → high-output failure in pump twin. Treatment: radiofrequency ablation or cord occlusion.
• Superfecundation (two separate acts of intercourse) & superfetation (second conception after pregnancy established).
4. Complications of multiple gestations
• Preterm birth (most twins deliver <37 weeks). Perinatal mortality 4× singletons, monochorionic higher risk.
• Spontaneous single intrauterine demise (~6% twin pregnancies); surviving twin at risk for cerebral palsy, DIC, renal cortical necrosis.
• Monoamniotic twins: cord entanglement → asphyxia risk, requires inpatient monitoring after viability.
• Congenital anomalies: 25% of monozygotic twins (higher risk for structural defects).
5. Management & Delivery
• First-trimester ultrasound to determine chorionicity.
• Serial growth scans, Doppler for TTTS surveillance (MCA-PSV for anemia, ductus venosus for cardiac function).
• Elective delivery: dichorionic twins at 37 weeks, monochorionic diamniotic at 36 weeks, monoamniotic at 34 weeks (often cesarean).
• Vaginal delivery recommended if first twin cephalic, with immediate availability of neonatal resuscitation team.
• Cerclage for incompetent cervix may prevent delivery before 28 weeks.
• Single-embryo transfer in ART reduces multiple gestation risk.
💡 Pearls: TTTS laser ablation is standard of care for Quintero stage II and above. Monoamniotic twins require hospital admission for monitoring after viability. First-trimester CRL discordance ≥12 mm predicts worse outcomes.