Whether identical twins share a placenta depends on the earliest moments of development. Identical, or monozygotic, twins form when a single fertilized egg splits into two embryos, meaning they share the same genetic code. The timing of this spontaneous division determines the structures that nourish and protect each developing fetus throughout the pregnancy. The difference hinges entirely on how quickly the single zygote separates into two distinct entities after conception occurs.
Defining the Structures: Chorionicity and Amnionicity
Understanding twin structure involves two protective membranes surrounding the developing fetuses. The outer layer is the chorion, which ultimately fuses with the uterine wall to form the placenta, the organ responsible for nutrient and gas exchange. Inside the chorion is the amnion, a thinner membrane that encloses the amniotic fluid.
The number of chorions and amnions present in a twin gestation defines the arrangement, classified as chorionicity and amnionicity. For identical twins, there are three combinations. The safest arrangement is Dichorionic/Diamniotic (Di/Di), where each twin has its own placenta and its own inner sac.
Monochorionic/Diamniotic (Mono/Di) involves the twins sharing a single chorion and thus a single placenta, but each maintains a separate amniotic sac. The rarest type is Monochorionic/Monoamniotic (Mono/Mono), where the twins share both the placenta and the inner amniotic sac. Chorionicity, or the number of chorions, is the factor that determines whether the twins share a placenta.
The Critical Timing of Zygote Division
The structure of the twin pregnancy is a direct consequence of when the fertilized egg divides. If the split happens within the first three days after fertilization, the result is a Dichorionic/Diamniotic pregnancy. This early separation allows each embryo to develop its own complete set of membranes, including two separate placentas, although these placentas may sometimes fuse together on the uterine wall.
The majority of identical twin pregnancies are Monochorionic/Diamniotic, resulting from a split that occurs between four and eight days post-fertilization. Since the chorion has already begun to form, the twins share one placenta, but the inner amniotic membranes separate them. This arrangement accounts for about 60 to 70 percent of monozygotic twins.
A split that takes place after eight days, typically between nine and twelve days, is less common and leads to a Monochorionic/Monoamniotic pregnancy. Since both the outer and inner membranes have formed, the fetuses share a single placenta and a single amniotic sac. This situation occurs in about one percent of identical twin gestations. A division occurring after twelve days can result in conjoined twins.
Unique Medical Risks of Shared Placentas
The structures defined by the timing of the split carry significant implications for the health of the pregnancy, particularly in monochorionic cases where the placenta is shared. Shared placentas often contain vascular connections, or anastomoses, that link the blood circulation of the two fetuses. This can lead to an uneven distribution of blood flow between the twins.
One complication arising from these connections is Twin-to-Twin Transfusion Syndrome (TTTS). Blood is transfused disproportionately from one twin (the donor) to the other (the recipient). The donor twin can become anemic and small, while the recipient twin can develop fluid overload and heart strain. Unequal sharing of the placental mass can also lead to Selective Fetal Growth Restriction (sIUGR), where one twin receives a significantly smaller portion of the organ and fails to grow properly.
Monochorionic/Monoamniotic twins, who share a single amniotic sac, face the danger of umbilical cord entanglement. Because there is no membrane separating them, the twins can twist their two umbilical cords around each other. This entanglement can restrict or cut off the blood supply to one or both fetuses.
How Doctors Determine Twin Structure
Determining the structure of a twin pregnancy is a priority for medical providers, as it dictates the frequency of monitoring and management protocols. Ultrasound examination is the primary tool used for this determination, and it is most accurately performed during the first trimester of pregnancy. Early diagnosis allows doctors to establish the correct risk profile for the duration of the gestation.
Special attention is paid to the junction where the dividing membrane meets the placenta. In Dichorionic/Diamniotic pregnancies, the ultrasound often reveals a triangular projection of placental tissue extending into the base of the membrane. This visual cue is known as the “Lambda sign” or “Twin Peak sign,” confirming the presence of two separate chorions.
Monochorionic/Diamniotic pregnancies display a thin membrane that inserts abruptly into the placenta at a right angle, creating the “T-sign.” The absence of the thick placental projection confirms that the twins share a single chorion. Identifying these specific sonographic signs early is a fundamental step in ensuring specialized care for the twin pregnancy.