A diagnosis of “Di/Di twins,” short for dichorionic diamniotic, refers to a specific arrangement of the placenta and surrounding membranes. This classification is fundamental for medical management because it determines the level of risk and monitoring required throughout the pregnancy. Di/Di represents the most common type of twin pregnancy, making up approximately 76% of all twin gestations. It is also generally associated with the lowest risk of complications for the developing babies.
Defining Dichorionic Diamniotic Twins
The term dichorionic diamniotic precisely describes the physical setup of the twins within the uterus, broken down into two components that define the babies’ environment. The prefix “Di-” in both words means “two,” indicating that each twin has its own separate structures.
The first part, “dichorionic,” refers to the chorion, the outermost membrane surrounding the embryo that ultimately develops into the placenta. In a dichorionic pregnancy, each twin has its own chorion, resulting in two separate placentas. Even if the two placentas fuse into a single mass, they maintain separate, distinct blood supplies. This separation of placental tissue is often visible on an early ultrasound scan as a thick triangular projection of tissue called the “twin peak” or “lambda” sign.
The second part, “diamniotic,” refers to the amnion, the innermost membrane that forms the fluid-filled sac surrounding the baby. Diamniotic means there are two separate amniotic sacs. The presence of two chorions always means there will be two amnions. This complete separation provides a physical barrier between the two fetuses.
Zygosity: Are Di/Di Twins Always Fraternal?
A common point of confusion is whether Di/Di twins are always fraternal, or dizygotic, meaning they developed from two separate eggs fertilized by two separate sperm. All fraternal twins are, by definition, Di/Di because they start as two distinct fertilized eggs, each establishing its own complete set of membranes and placenta immediately. Fraternal twins share the same genetic relationship as any other pair of siblings, sharing about 50% of their DNA. Since they originate from two different eggs and sperm, they can be different genders or the same gender.
Di/Di twins can also be identical, or monozygotic, which means they originated from a single fertilized egg that split into two embryos. This occurs when the initial single zygote splits very early in development, specifically within the first one to four days after fertilization. This early separation allows enough time for each developing embryo to form its own chorion and amnion before implantation.
While the vast majority of Di/Di twins are fraternal, approximately one-third of all identical twin pregnancies result in a Di/Di arrangement. Therefore, the Di/Di designation describes the structure of the pregnancy membranes, not the genetic relationship between the twins. The only way to definitively determine if same-sex Di/Di twins are identical or fraternal is through post-birth DNA testing.
Why the Di/Di Designation Matters for Pregnancy
The Di/Di classification is the primary factor used by healthcare providers to assess the relative risk of a twin pregnancy. Because each twin has its own separate placenta, the risk of a shared blood supply is significantly minimized. Shared placental circulation is the mechanism behind severe complications like Twin-to-Twin Transfusion Syndrome (TTTS), which is not a concern for Di/Di pregnancies.
The separate amniotic sacs are equally beneficial, as they prevent the risk of the babies’ umbilical cords becoming tangled. This cord entanglement risk is a serious potential complication when babies share a single amniotic sac. While Di/Di twin pregnancies still carry a higher overall risk than a singleton pregnancy, they are considered the safest type of twin gestation.
The lower risk profile means that Di/Di twins require less intensive monitoring. Prenatal care for Di/Di twins typically involves regular ultrasound assessments every four weeks to track fetal growth and development. If no complications arise, delivery for Di/Di twins is often recommended between 37 and 38 weeks of gestation.