Monozygotic (Identical) Twins in IVF

Monozygotic (identical) twins occur when a single fertilized egg (zygote) splits into two separate embryos, each sharing the exact same genetic material. In spontaneous natural conception, monozygotic twinning is rare, occurring at a constant rate of approximately 0.4% (1 in 250 births) worldwide.

In assisted reproductive technologies (ART) and in vitro fertilization (IVF), the incidence of monozygotic twinning is slightly elevated—averaging between 1.5% and 3% of clinical pregnancies. While elective single embryo transfer (eSET) virtually eliminates dizygotic (fraternal) twins, identical twinning represents an intriguing biological phenomenon that fertility specialists carefully monitor.

Extensive reproductive research has explored the cellular and mechanical factors that may contribute to zygotic splitting during in vitro culture:

Dr. Fady Sharara has authored foundational clinical literature evaluating monozygotic twinning rates, zona architecture, and neonatal outcomes. His research confirms that high-precision embryology laboratory protocols minimize unnecessary zona trauma while optimizing embryo viability.

When monozygotic twins are identified on early high-resolution ultrasound at VCRM (usually at 6 to 8 weeks gestation), Dr. Sharara determines chorionicity and amnionicity—whether the twins share a placenta (monochorionic) or amniotic sac (monoamniotic).

• Early High-Resolution Sonography: Identification of the lambda sign or T-sign to establish monochorionic vs. dichorionic placentation.
• Co-management with MFM Specialists: Close coordination with top Maternal-Fetal Medicine perinatologists to monitor for twin-to-twin transfusion syndrome (TTTS) in monochorionic gestations.
• Advocacy for eSET: By transferring single blastocysts, VCRM eliminates high-order triplets and quadruplets that can occur when multiple embryos are transferred and one subsequently splits.