Zygosity and chorionicity explained
The terminology associated with a multiple pregnancy can be confusing and overwhelming, it is often a challenge enough coming to terms with expecting more than one!
For twins, zygosity refers to the degree of genetic similarity within each pair:
- dizygotic (di = two, zygote = egg) are commonly referred to as fraternal twins
- monozygotic (mono = one, zygote = egg) are commonly referred to as identical twins.
Like twins, triplets and other higher order multiples can be categorised by their zygosity.
Chorionicity refers to the number of placentas in a multiple gestation and does not directly reflect zygosity. Whereas dizygotic twins are always dichorionic, monozygotic twins may be monochorionic or dichorionic depending on when the zygote divides.
Understanding zygosity and chorionicity is of value for a range of reasons including management of a multiple pregnancy as well as medical, personal, financial, scientific, legal and ethical considerations.
Read ICOMBO’s position statement on the zygosity and chorionicity here.
More information on specific topics can be found below.
What types of twins are possible and how can I determine between them?
Zygosity for triplets and higher order multiples
A short explanation of the various types of triplets, quadruplets and higher order multiples, and how they are formed.
What is chorionicity and why is important to determine it during pregnancy?
What are your chances of having twins? Do twins have different finger prints? These and lots of other questions about twins and multiples will be answered on this page.
Not sure of your zygosity, or the zygosity of your twin children? Download this quick quiz to see if you can find the answer.