Dichorionic-Diamniotic (Di/Di) twins represent the most frequently occurring type of multiple gestation, accounting for approximately 70% of all twin pregnancies. This configuration is associated with the lowest risk profile compared to other twin types. The acronym Di/Di stands for Dichorionic-Diamniotic, a medical term describing the specific arrangement of the membranes and placentas supporting the developing fetuses. Understanding this structural setup is fundamental to comprehending the management and potential outcomes of the pregnancy.
Defining Dichorionic-Diamniotic Twins
A Dichorionic-Diamniotic pregnancy is characterized by two completely separate biological support systems for the twins inside the uterus. The term “dichorionic” refers to the chorion, the outermost membrane that ultimately develops into the placenta. In a Di/Di setup, each twin has its own chorion, meaning they have two distinct placentas, or at least two separate chorionic layers, even if the placentas appear fused.
The second part of the term, “diamniotic,” refers to the amnion, the innermost membrane that forms the fluid-filled sac surrounding the fetus. In this twin type, each baby is contained within its own separate amniotic sac. This structural independence results in a thick dividing membrane between the two fetuses, composed of four layers: two amnions and two chorions.
The separation of the placentas and sacs means each twin has an independent blood supply and nourishment source. This minimizes the risk of vascular connections, which are a common complication in shared-placenta pregnancies. This thick dividing membrane and two separate placental sites can often be clearly identified through ultrasound, typically by the presence of the “lambda” or “twin peak” sign in the first trimester.
The Role of Fertilization and Zygosity
The formation of Dichorionic-Diamniotic twins is linked to the process of fertilization and the timing of cell division. Di/Di twins can arise from two distinct biological mechanisms, determining whether the twins are fraternal (dizygotic) or identical (monozygotic).
The vast majority of Di/Di twins are dizygotic, resulting from two separate eggs fertilized by two separate sperm. Dizygotic twins are genetically no more alike than any other pair of siblings. They always develop with their own complete set of membranes and placentas, making them inherently Di/Di. This configuration is the most common outcome because the two fertilized eggs implant independently. If the twins are of different sexes, they are definitively dizygotic.
Di/Di twins can also be monozygotic, or identical. This occurs when a single fertilized egg splits into two embryos very early in development, specifically within the first three days following fertilization. This early separation allows the cell masses to develop their own chorion and amnion before the membranes differentiate. This results in a Di/Di setup that is structurally indistinguishable from dizygotic twins. Approximately 20% to 30% of identical twins are Di/Di, making it impossible to determine zygosity solely based on the ultrasound structure when the babies are the same sex.
Pregnancy Management and Surveillance
The management of a Dichorionic-Diamniotic twin pregnancy reflects its lower complication profile compared to other twin types. Since they have independent placental systems, these pregnancies avoid serious complications like Twin-to-Twin Transfusion Syndrome (TTTS), which is exclusive to twins sharing a placenta. Despite the lower relative risk, a Di/Di pregnancy still requires increased surveillance compared to a singleton pregnancy due to the inherent risks of carrying multiples.
Management involves more frequent prenatal appointments and ultrasounds, often performed every four weeks starting after the anatomy survey around 20 weeks. These regular scans primarily monitor the growth of both fetuses to detect growth discordance, which is a significant difference in size between the two babies. Monitoring helps ensure both twins are receiving adequate nourishment, as growth restriction is a concern in twin pregnancies.
General twin risks, such as preterm birth and preeclampsia, remain elevated in Di/Di pregnancies. Therefore, monitoring for signs of preterm labor, including cervical length assessments, is a standard part of care. For uncomplicated Di/Di twins, delivery is planned earlier than a full 40-week gestation, often recommended between 38 weeks and 38 weeks and six days to mitigate the increased risk of complications that can occur late in the third trimester.