Identical twins originate from a single fertilized egg that divides into two separate embryos. This singular origin means they share nearly identical genetic material. Understanding how these twins develop, particularly the structures supporting them in the womb, is key.
How Identical Twins Develop with Separate Structures
Identical (monozygotic) twins can develop with separate sacs and placentas, depending on the precise timing of the single fertilized egg’s division.
If division occurs within the first three days, each twin forms its own chorion (outer membrane) and amnion (inner sac membrane). This results in dichorionic-diamniotic (DCDA) twins, where each baby has its own placenta and amniotic sac, structurally resembling fraternal twins. Approximately 20% to 30% of identical twin pregnancies are DCDA.
If division happens between four and eight days, twins share a single chorion and placenta, but each develops within its own amniotic sac. This is monochorionic-diamniotic (MCDA), the most common type, accounting for 60% to 70% of cases.
Between nine and twelve days, twins share both a single chorion (placenta) and a single amniotic sac. These are monochorionic-monoamniotic (MCMA) twins, comprising 1% to 5% of identical twin pregnancies. Division after twelve days increases the possibility of conjoined twins, where separation is incomplete.
Differentiating Identical and Fraternal Twins
The arrangement of the sacs and placentas provides important clues for distinguishing between identical and fraternal (dizygotic) twins.
Fraternal (dizygotic) twins originate from two separate fertilized eggs. They always develop with their own placentas and amniotic sacs, making them dichorionic-diamniotic. Therefore, DCDA twins could be either fraternal or identical, requiring further assessment to determine zygosity.
Conversely, if twins share a single placenta (monochorionic-diamniotic or monochorionic-monoamniotic), they are definitively identical. This is because a shared placental structure can only arise from the division of a single fertilized egg. Ultrasound imaging plays a primary role in determining chorionicity and amnionicity, typically assessed during the first trimester, ideally before 14 weeks of gestation.
Why This Distinction Matters for Pregnancy
Understanding whether twins share a placenta and amniotic sac is important for managing the pregnancy. Monochorionic pregnancies carry higher complication risks than dichorionic pregnancies. The shared placental blood supply can lead to various issues requiring close medical monitoring.
Complications include Twin-to-Twin Transfusion Syndrome (TTTS), where unequal blood flow causes one twin to receive too much blood and the other too little. Selective intrauterine growth restriction (sIUGR) involves one twin experiencing restricted growth due to an unequal share of the placenta. Twin Anemia-Polycythemia Sequence (TAPS) is a chronic, slow transfer of red blood cells, leading to anemia in one twin and polycythemia in the other.
Monochorionic-monoamniotic twins face the risk of umbilical cord entanglement, as they share a single amniotic sac with no separating membrane. Due to these complications, monochorionic twin pregnancies require more frequent monitoring, often with ultrasounds every two weeks starting from 16 weeks gestation. The sac and placenta arrangement also influences delivery planning. While these risks exist, medical advancements allow for careful monitoring and timely interventions, leading to positive outcomes for most twin pregnancies.