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Types of twins

2-minute read

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

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

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.

More information

Find more information about twins at the Twins Research Australia and the Australian Multiple Birth Association.

Learn more here about the development and quality assurance of healthdirect content.

Last reviewed: October 2019

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Need more information?

Fraternal twins & identical twins | Raising Children Network

Whether you’re having fraternal twins or identical twins, it’s good to find out during pregnancy. Get the facts you need about different types of twins.

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Types of twins

There are two different types of twins: monozygotic or identical (MZ) and dizygotic, fraternal or non-identical (DZ). Learn more about each type of twin.

Read more on Twins Research Australia website

How can I tell if my twins are identical or not?

Parents of twins and twins themselves are often confused about their zygosity. It is human nature to look for differences, hence twins and/or their parents may think they are not identical when they very well could be. The only way to really know for sure is via a DNA test.

Read more on Twins Research Australia website

Do identical twins always look alike?

Even though identical twins are from the same sperm and egg and therefore have exactly the same set of chromosomes and therefore genes, researchers are realising that there are other forces at play the science of epigenetics.

Read more on Twins Research Australia website

Zygosity testing

Twins Research Australia receives many queries from twins and their families interested in zygosity testing to determine whether a particular twin pair is identical (monozygotic or MZ) or non-identical (dizygotic or DZ).

Read more on Twins Research Australia website

About twins

Learn about twin characteristics and twin types

Read more on Twins Research Australia website

Raising twins

Find out more on how to get your twins to sleep, what the 'twin bond' is and dealing with sibling rivalry.

Read more on Pregnancy, Birth & Baby website

Zygosity testing doubly important for twins

The official website of Twins Research Australia.

Read more on Twins Research Australia website

Expecting twins?

Congratulations from Twins Research Australia on your happy news. No doubt you are keen to find out as much as possible about twin pregnancy and the early days of parenting.

Read more on Twins Research Australia website

Feeding twins

Find practical tips on breastfeeding or bottle feeding your twins.

Read more on Pregnancy, Birth & Baby website

Call us and speak to a Maternal Child Health Nurse for personal advice and guidance.

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The information is not a substitute for independent professional advice and should not be used as an alternative to professional health care. If you have a particular medical problem, please consult a healthcare professional.

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