There are two types of twins - fraternal twins and identical twins. Find out more about the difference between the two types and twin-twin transfusion syndrome.
Fraternal twins are formed from the fertilisation of two eggs by two different sperm. They are also known as 'dizygotic twins', or 'non-identical twins'.
With fraternal twins, the two foetuses (developing babies) each have a separate placenta, inner membrane (the amnion) and outer membrane (the chorion). They don't usually look identical and might or might not be the same sex.
Identical twins are formed from the splitting of one embryo. They are also known as 'monozygotic twins'.
There are different types of identical twins, depending on what they share in the womb.
- Almost one third of identical twins have their own placenta, inner membrane, and outer membrane. The medical term for these twins is ‘dichorionic diamniotic’ or DCDA twins.
- Almost two-thirds of identical twins share the same placenta and chorion, but have their own amnion. These are ‘monochorionic diamniotic’ or MCDA twins.
- The rest — only about 4% of identical twins — share everything, and are called ‘monochorionic monoamniotic’ (MCMA) twins.
Although identical twins are the same sex and are genetically identical, they can develop quite different personalities. You can find a good description of the different types of monozygotic twins, with pictures, at the Twins Research Australia.
If you have triplets or more, the principles are similar.
Twin-twin transfusion syndrome
Identical twins who share the same placenta and chorion can sometimes share a condition called twin–twin transfusion syndrome (TTTS). In this condition, blood flows from one twin to the other, resulting in one baby getting too much blood and the other baby not getting enough. This affects the health of both babies, sometimes severely.
Most identical twins don’t get TTTS. But if they do, it is more likely to happen to MCDA twins than to MCMA twins.
If your twins have TTTS, there are many different ways to treat it - ask your doctor for advice.
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Last reviewed: October 2019