What are the mechanisms of twin formation?

Mechanisms of Twin Formation

Twin formation occurs through two primary mechanisms: monozygotic twinning (identical twins) from a single fertilized egg that splits, and dizygotic twinning (fraternal twins) from two separate fertilized eggs. 1

Dizygotic (Fraternal) Twins

• Result from the fertilization of 2 separate oocytes by 2 different sperm cells
• Almost always have dichorionic-diamniotic placentation (two separate placentas and amniotic sacs)
• Account for approximately two-thirds of all twin pregnancies 1
• Influenced by maternal factors:
  • Family history of twinning, particularly on the maternal side 2, 3
  • Fertility treatments and assisted reproductive techniques 2

Monozygotic (Identical) Twins

Monozygotic twins result from a single fertilized egg (zygote) that splits into two genetically identical embryos. The timing of this split determines the placentation pattern:

1. Early splitting (1-4 days after fertilization):

   • Results in dichorionic-diamniotic twins (separate placentas and amniotic sacs)
   • Occurs in approximately one-third of monozygotic twins 1

2. Intermediate splitting (4-8 days after fertilization):

   • Results in monochorionic-diamniotic twins (shared placenta, separate amniotic sacs)
   • Most common type, occurring in about two-thirds of monozygotic twins 1
   • These twins share a single placenta with intertwin vascular anastomoses connecting their circulations 1, 4

3. Late splitting (8-12 days after fertilization):

   • Results in monochorionic-monoamniotic twins (shared placenta and amniotic sac)
   • Occurs in less than 1% of monozygotic twins 1
   • Higher risk due to potential cord entanglement 1

4. Very late splitting (>12 days after fertilization):

   • Results in conjoined twins (incomplete separation)
   • Extremely rare 1

Cellular Mechanisms

Recent research suggests that monozygotic twinning may involve specific cellular mechanisms:

• For monochorionic twins, a looser inner cell mass (ICM) in blastocysts increases the chance of ICM separation
• Physical forces from blastocoel formation or "8-shaped hatching" can exert pressure on the ICM, resulting in splitting 5
• The architecture of the zona pellucida (the protective layer around the embryo) may influence monozygotic twinning, particularly in IVF cases 6

Clinical Implications of Chorionicity

Understanding chorionicity is critical as it determines risk profiles:

• Monochorionic twins share a placenta with vascular anastomoses in over 95% of cases 1
• Monochorionic twins have more than twice the perinatal mortality rate compared to dichorionic twins (7.6% vs 1.6% fetal demise) 1
• Monochorionic placentation can lead to unique complications such as:
  • Twin-twin transfusion syndrome (TTTS)
  • Twin anemia-polycythemia sequence (TAPS)
  • Selective fetal growth restriction 1, 4

Unusual Findings

• Rare cases of monochorionic-dizygotic twins have been reported, challenging the traditional understanding that monochorionicity always indicates monozygosity 1
• Some research suggests that monozygotic and dizygotic twinning might share more embryogenic mechanisms than previously thought 7

Accurate determination of chorionicity by ultrasound is most reliable between 10-14 weeks of gestation, with the presence of a "lambda" or "twin peak" sign indicating dichorionic placentation 1.