Monochorionic-diamniotic (mono/di) twins represent a specific type of twin pregnancy. This article explores their formation, identification, and the considerations involved in their care.
Understanding Twin Classifications
Twins are broadly categorized based on their genetic origin and the structures supporting their development in the womb. Zygosity refers to whether twins originate from one or two fertilized eggs. Monozygotic twins, often called identical twins, develop from a single fertilized egg that splits early in development. This means they share nearly identical genetic material and are always of the same sex. In contrast, dizygotic twins, known as fraternal twins, result from two separate eggs fertilized by two distinct sperm cells. They are genetically no more alike than any other siblings.
Beyond genetic origin, twins are also classified by chorionicity and amnionicity. Chorionicity describes the number of placentas, with monochorionic meaning one shared placenta and dichorionic meaning two separate placentas. Amnionicity refers to the number of amniotic sacs, where diamniotic indicates two separate sacs and monoamniotic indicates a single shared sac. Mono/di twins are monochorionic-diamniotic, meaning they share a single placenta but develop within their own individual amniotic sacs.
How Mono/Di Twins Form
Mono/di twins originate from a single fertilized egg, making them monozygotic, or identical. Their formation begins with a single zygote that undergoes division. For mono/di twins to develop, this division of the embryo occurs at a specific time: between four and eight days after fertilization. At this stage, the blastocyst has already formed its outer layer, the trophoblast, which will later develop into the placenta.
Because the split happens after the formation of the trophoblast, both developing embryos will share a single chorion, which gives rise to a shared placenta. However, the inner membrane, the amnion, has not yet fully developed or fused. This allows each embryo to form its own separate amniotic sac. This precise timing is the reason mono/di twins share a placenta while each retaining their own amniotic environment.
Identifying Mono/Di Twins
Identifying mono/di twins relies on detailed ultrasound examinations, especially in the first trimester (before 14 weeks). Sonographers look for visual cues to determine chorionicity and amnionicity. A single shared placenta is a primary indicator of monochorionic twins.
A key sign is the appearance of the dividing membrane between the two fetuses. In mono/di pregnancies, this membrane is typically thin and appears to insert perpendicularly into the shared placenta, forming a “T-sign” at the junction. This T-sign differentiates them from dichorionic pregnancies, which exhibit a thicker dividing membrane and a “lambda sign” or “twin peak sign” at the placental insertion. Accurate identification guides prenatal care.
Considerations for Mono/Di Pregnancies
Mono/di twin pregnancies require specific considerations due to their shared placenta and common blood supply. They are monitored more frequently than other twin pregnancies, often through bi-weekly ultrasounds starting around 16 weeks, to detect potential complications.
One specific condition associated with a shared placenta is Twin-to-Twin Transfusion Syndrome (TTTS). This occurs when there is an uneven sharing of blood flow through connections in the placenta. In TTTS, one twin (the donor) may receive insufficient blood, while the other (the recipient) receives too much, leading to imbalances in amniotic fluid and potential strain on the recipient’s heart. Another consideration is selective fetal growth restriction (sFGR), where one twin grows more slowly than the other due to unequal placental sharing. Specialized medical care manages these pregnancies to optimize outcomes for both twins.