Dizygotic – Fraternal Twins Explained
Dizygotic twins develop from two separately fertilized eggs and are genetically distinct. They are commonly known as fraternal twins.
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Dizygotic twins develop from two separately fertilized eggs and are genetically distinct. They are commonly known as fraternal twins.
What does dizygotic mean?
Dizygotic (from Greek di = two and zygon = yoke/cell) refers to twins who develop from two separate eggs, each fertilized by a different sperm cell. These are commonly known as fraternal twins. Dizygotic twins are not genetically identical and share on average approximately 50% of their genes, just like ordinary siblings.
How dizygotic twins develop
In dizygotic twinning, two eggs are released and fertilized during the same menstrual cycle. This process is called superovulation. Each fertilized egg (zygote) develops into an independent embryo. Because each embryo forms its own placenta and amniotic sac, dizygotic twins always result in a dichorionic-diamniotic pregnancy (two placentas, two amniotic cavities).
Frequency and influencing factors
Dizygotic twins are more common worldwide than identical (monozygotic) twins. The following factors increase the likelihood of a dizygotic twin pregnancy:
- Genetic predisposition: Twins tend to run in certain families.
- Maternal age: Women between the ages of 35 and 39 have a higher rate of multiple pregnancies.
- Assisted reproduction: In vitro fertilization (IVF) and hormonal stimulation treatments significantly increase the chance of multiple births.
- Ethnicity: Women of West African descent have particularly high rates of dizygotic twinning.
- Parity: Women who have already had several children have an increased likelihood.
Difference from monozygotic twins
Unlike monozygotic (identical) twins, who develop from a single fertilized egg and share the same genetic material, dizygotic twins have different genetic profiles. They can be of different sexes and may look as different from each other as any other pair of siblings. Monozygotic twins, on the other hand, are always the same sex and closely resemble each other.
Medical relevance
Distinguishing between monozygotic and dizygotic twins is medically important, especially in prenatal diagnostics and genetic research. Dizygotic twins are not at risk for complications exclusive to monochorionic pregnancies, such as twin-to-twin transfusion syndrome (TTTS). Nevertheless, all twin pregnancies are considered higher-risk pregnancies and require close obstetric monitoring.
Diagnosing zygosity
Zygosity can be determined through several methods:
- Ultrasound: The number of placentas and amniotic sacs can be assessed in the first trimester. Two separate placentas are indicative of dizygosity.
- Genetic testing: A DNA comparison after birth provides definitive confirmation of zygosity.
- Sex of the children: Different sexes conclusively confirm dizygosity.
References
- Benirschke, K. et al. - Pathology of the Human Placenta. Springer, 6th edition, 2012.
- Blickstein, I. & Keith, L.G. - Multifetal Pregnancies: Epidemiology, Gestation & Perinatal Outcome. Taylor & Francis, 2005.
- World Health Organization (WHO) - Multiple Births: Global and Regional Estimates. WHO Press, Geneva, 2021.
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