What percentage of twin pregnancies are monochorionic (mono-chorionic)?

Percentage of Monochorionic Twin Pregnancies

Monochorionic twins comprise 25% to 30% of all twin pregnancies, according to the most recent guidelines from the American College of Radiology. 1

Understanding Monochorionic Twin Pregnancies

Monochorionic twins represent a significant subset of twin pregnancies with important clinical implications:

• While monochorionic twins make up 25-30% of twin pregnancies, they account for approximately 30% of all pregnancy-related complications in twins 1, 2
• Almost all monochorionic twins are monozygotic (identical), sharing a single placenta 1
• Most monochorionic twins are diamniotic (separate amniotic sacs), but a small percentage (approximately 1% of all monozygotic twins) are monoamniotic (sharing the same amniotic cavity) 1

Clinical Significance of Monochorionic Twins

Understanding the prevalence of monochorionic twins is crucial because:

• Monochorionic twins have significantly higher mortality rates (approximately 10%) compared to dichorionic twins, primarily due to complications specific to monochorionic placentation 1
• These pregnancies require more intensive monitoring starting at 16 weeks' gestation with biweekly ultrasound scans 1
• Specific complications unique to monochorionic twins include:
  • Twin-to-twin transfusion syndrome (TTTS) - occurs in 10-20% of monochorionic diamniotic twins 1
  • Twin anemia-polycythemia sequence (TAPS) - occurs spontaneously in approximately 5% of monochorionic diamniotic twins 1
  • Twin reversed arterial perfusion (TRAP) sequence - a rare but serious complication 1
  • Unequal placental sharing leading to selective intrauterine growth restriction 1, 3

Monitoring Requirements

The higher risk profile of monochorionic twins necessitates specialized monitoring:

• All monochorionic twins should undergo fetal echocardiography due to a 2% risk of congenital heart disease in uncomplicated cases and 5% risk in cases with TTTS 1
• Monitoring typically begins at 16 weeks for monochorionic twins, with:
  • Fetal biometry every 2-3 weeks
  • Weekly assessment for potential TTTS or other monochorionic-specific complications 1
  • Evaluation of umbilical artery Doppler and middle cerebral artery peak systolic velocity from 20 weeks onward 1

Comparison to Dichorionic Twins

• Dichorionic twins account for approximately 70-75% of all twin pregnancies 1
• All dizygotic (fraternal) twins are dichorionic, while some monozygotic twins can also be dichorionic if splitting occurs very early (1-3 days after fertilization) 1
• Dichorionic twins have separate placentas or a fused placenta with separate circulations, eliminating the risk of vascular anastomoses that lead to complications in monochorionic twins 1

Understanding the proportion of monochorionic twins and their associated risks is essential for appropriate risk stratification, monitoring protocols, and management of twin pregnancies.