Identical twins originate from one zygote, but their placental arrangement depends on the precise timing of the embryonic splitting event. This timing determines the type of membranes and placental structures that support their development throughout pregnancy.
The Nuance of Identical Twin Development
Identical twins, also known as monozygotic twins, form when a single fertilized egg splits into two embryos. The timing of this division is the determining factor for the configuration of fetal membranes and whether the twins will share a placenta. This splitting can occur at different stages of early development, leading to three distinct types of identical twin pregnancies.
If the division happens very early, typically within the first three days after fertilization, the twins will develop as dichorionic-diamniotic (DCDA). In this scenario, each twin forms its own placenta, chorionic sac, and amniotic sac. This arrangement results in twins having entirely separate support systems, mimicking the setup of fraternal twins, though they are genetically identical.
When the single egg splits between days four and eight after fertilization, the twins are monochorionic-diamniotic (MCDA). These twins share a single placenta and a common chorionic sac, but they each develop within their own separate amniotic sac. This is the most prevalent type of identical twin pregnancy, accounting for approximately 60-70% of all identical twin gestations.
Should the division occur after day eight, the twins will be monochorionic-monoamniotic (MCMA). In this rare instance, the twins share a single placenta, a single chorionic sac, and a single amniotic sac. This late splitting means the twins develop within the same fluid-filled environment, representing the least common type of identical twinning.
Detecting Placental Arrangements
Determining the specific placental and amniotic arrangement in a twin pregnancy is a crucial step in prenatal care. Early ultrasound examinations, ideally between 11 and 14 weeks of gestation, provide the most accurate assessment.
During the ultrasound, specific markers are observed to differentiate between the types of twin pregnancies. For dichorionic-diamniotic twins, the presence of a thick dividing membrane that forms a triangular projection at the base, known as the “lambda sign” or “twin peak sign,” indicates separate chorions and placentas.
In contrast, monochorionic-diamniotic twins exhibit a thinner dividing membrane where the chorionic layer is absent, appearing as a “T-sign” at its junction with the shared placenta. For monochorionic-monoamniotic twins, the ultrasound will reveal the absence of any dividing membrane between the two fetuses.
Implications of Shared Placental Blood Supply
When identical twins share a single placenta, as in monochorionic pregnancies, they also share a common blood supply, which can introduce specific risks. This shared circulation means that blood vessels within the placenta are interconnected, potentially leading to an unequal distribution of blood between the twins. Close monitoring is necessary throughout these pregnancies to detect and manage potential complications.
One significant complication is Twin-to-Twin Transfusion Syndrome (TTTS), which affects approximately 10-15% of monochorionic twin pregnancies. In TTTS, blood flow across the shared placenta becomes unbalanced, with one twin, the “donor,” transfusing blood to the other twin, the “recipient.” The donor twin may experience restricted growth and reduced amniotic fluid, while the recipient twin can develop an overload of blood, leading to excessive amniotic fluid and potential heart strain.
Another concern is selective Intrauterine Growth Restriction (sIUGR), where one twin receives a disproportionately smaller share of the placental blood supply and nutrients, resulting in a significant size disparity. The smaller twin experiences restricted growth, while the larger twin typically develops normally. This condition also necessitates careful monitoring to assess the health of both fetuses.
For the rarest type, monochorionic-monoamniotic (MCMA) twins, an additional risk is umbilical cord entanglement. Because these twins share the same amniotic sac, their umbilical cords can become intertwined as they move within the confined space. This entanglement can restrict blood flow through the cords, posing a serious threat to the well-being of both twins.