Many parents expecting twins with separate placentas and sacs often wonder if their babies could still be identical. While most twins with this arrangement are fraternal, a notable percentage can indeed be identical.
What Di/Di Twin Status Means
The term “di/di” is an abbreviation for dichorionic-diamniotic, which describes the presence of two separate chorions (outer membranes that develop into placentas) and two separate amnions (inner membranes forming amniotic sacs). This particular placental and sac arrangement is the most common form of twin pregnancy, accounting for over 70% of all twin gestations.
While all fraternal twins are dichorionic-diamniotic, a significant portion of identical twins can also develop in this manner. If a single fertilized egg splits very early, typically within the first three to four days after fertilization, the developing embryos can each form their own chorion and amnion. This early division results in identical twins who are also di/di, meaning the presence of two separate placentas does not rule out identical status.
Initial Clues and Common Misconceptions
Many people attempt to determine twin zygosity based on observable characteristics, but these are often misleading. Similarities in appearance, such as hair color, eye color, or facial features, are not definitive indicators. Identical twins can exhibit subtle differences due to environmental factors, while fraternal twins can sometimes look remarkably alike. For instance, sharing a blood type does not confirm identical status, as fraternal siblings can also have the same blood type.
Other non-genetic traits like temperament or personality are similarly unreliable for determining zygosity. Even differences in fingerprints, which are unique to each individual, do not rule out identical twin status, as fingerprints are influenced by environmental factors during development.
The Definitive Test
Genetic, or DNA, testing is the only accurate method to definitively determine if di/di twins are identical or fraternal. This testing typically involves collecting a DNA sample, often through a simple and painless cheek swab from each twin.
The laboratory compares multiple specific genetic markers, sometimes as many as 21 or 25, from each twin’s DNA. If the twins share 100% of these genetic markers, they are confirmed as identical (monozygotic). If they share approximately 50% of their DNA, similar to other siblings, they are determined to be fraternal (dizygotic). This method provides a highly accurate result, typically over 99.99% certainty.
Implications of Zygosity
Knowing the zygosity of di/di twins can be important for several practical reasons for parents and the twins themselves. From a medical perspective, understanding if twins are identical can provide insights into potential shared genetic predispositions for certain health conditions. Identical twins, sharing nearly identical DNA, are more likely to have similar susceptibilities to hereditary diseases, which can inform healthcare monitoring and management.
Beyond health, knowing zygosity can impact understanding the twins’ unique developmental and psychological paths. It can help parents appreciate the interplay of genetics and environment in shaping their children’s personalities and behaviors. For the twins as they grow, having clarity about their genetic relationship can contribute to their sense of identity and provide peace of mind.