Understanding DiDi Twins
DiDi twins, scientifically known as dichorionic-diamniotic twins, are a type of twin pregnancy where each baby has its own separate chorion (outermost membrane) and amnion (innermost membrane forming the amniotic sac). Each twin grows within its own individual sac and has its own distinct placenta, even if the placentas appear fused. DiDi pregnancies are the most common type, accounting for over 70% of all twin gestations.
All fraternal (non-identical) twins are by definition DiDi, as they originate from two separate fertilization events. Identical twins can also be DiDi if the single fertilized egg splits very early in development.
The Formation of DiDi Twins
DiDi twins can arise through two distinct biological pathways. The most frequent scenario involves the fertilization of two separate eggs by two different sperm. This process leads to fraternal, or dizygotic, twins, each with its own unique genetic makeup, independent placenta, and amniotic sac. These twins are no more genetically alike than any other siblings born at different times.
In another pathway, identical, or monozygotic, twins can also be DiDi. This occurs when a single fertilized egg splits very early in its development, typically within the first three days after fertilization, before the formation of the chorion and amnion. This allows each resulting embryo to develop its own complete set of membranes and a separate placenta.
Distinguishing DiDi Twins
Medical professionals primarily distinguish DiDi twins from other twin types through ultrasound examination, ideally performed in the first trimester. Early ultrasound scans can reveal two separate gestational sacs, each surrounded by a thick, echogenic chorion. A clear dividing membrane between the two fetuses is also a key indicator.
A significant ultrasound finding that confirms a DiDi pregnancy is the “lambda sign,” also known as the “twin peak sign.” This sign appears as a triangular projection of placental tissue that extends into the base of the inter-twin membrane, signifying the presence of two separate placentas, even if they are fused. The presence of the lambda sign is a highly accurate predictor of dichorionicity, especially when observed before 14 weeks of gestation. Its absence often suggests a monochorionic pregnancy.
DiDi twins are contrasted with monochorionic-diamniotic (MoDi) and monochorionic-monoamniotic (MoMo) twins, which involve shared placental structures. MoDi twins share one placenta but have separate amniotic sacs, while MoMo twins share both a single placenta and a single amniotic sac. The distinction in chorionicity and amnionicity is crucial because shared placental structures in MoDi and MoMo pregnancies carry higher risks of specific complications, such as twin-to-twin transfusion syndrome (TTTS), which are not typically associated with DiDi pregnancies.
Managing a DiDi Twin Pregnancy
Managing a DiDi twin pregnancy involves diligent prenatal care, though it is generally considered the lowest-risk type among twin pregnancies. Despite this relatively lower risk, DiDi twin pregnancies still require more frequent monitoring than singleton pregnancies. Regular ultrasound examinations are essential to track the growth and development of each fetus and to detect any potential complications early. The American College of Obstetricians and Gynecologists (ACOG) recommends serial ultrasound examinations every 4-6 weeks to monitor fetal growth.
Expectant parents carrying DiDi twins should also be aware of general considerations common to all twin pregnancies. These include increased maternal nutrient needs to support the growth of two babies, and a higher likelihood of preterm birth or lower birth weight compared to singleton pregnancies. Adherence to regular prenatal appointments and following medical advice regarding nutrition and activity are important for promoting a healthy outcome for both mother and babies.