What are the different types of twins?

As a leading DNA testing laboratory, we often meet families that have experienced multiple births, with twins being the most common of all.

The upward ‘trend’ in multiple births has grown significantly over the last 20 years, with factors such as fertility treatments and women starting their families later thought to be contributing factors.

But how much do people really know about twin births?

How many different ‘types’ of twins are there, and how are they formed?

In this blog, we take a closer look at the unique nature of twins, how they develop in the womb, and how you can tell whether twins are identical or not.

What are twins?

A person can be considered a twin if they were formed alongside their sibling in the womb during a single pregnancy.

Many people will have heard about the two most common types of twins – identical and fraternal (non-identical).

Fraternal or non-identical (dizygotic) twins are the most common twin type worldwide and are formed when two separate eggs are fertilised by separate sperm. The fertilised eggs then implant in the uterus, resulting in two babies from the same pregnancy.

Non-identical twins can be the same sex, or different sexes (one boy and one girl).

However, identical (monozygotic) twins will always be the same sex, as they are formed when a single fertilised egg splits, creating two embryos that share near-identical genes. 

Twin types

Many people may not be aware that twins come in many forms other than simply identical or non-identical (fraternal).

To understand the different types of twins, it’s important to understand how they develop in the womb.

The sac of fluid that surrounds and protects a foetus is commonly referred to as the ‘amniotic sac’. However, this sac is actually made up of two membranes: an inner membrane called the amnion, and an outer membrane called the chorion.

The type of twin that is formed in the womb depends on several factors including whether the babies share the same sac/membrane and placenta or have separate sacs/membranes and placentas.  

Here are the three main types of twin pregnancy that you may not have heard of…

Dichorionic Diamniotic (DCDA) twins

DCDA make up the highest percentage of twin pregnancies and occur when each twin has a separate amnion (inner membrane), chorion (outer membrane) and placenta.

Because DCDA twins have separate sacs and placentas, it is a common misconception that these twins cannot be identical.

However, modern advances in scientific techniques including twin DNA testing have shown that around a third of identical twins are DCDA twins.

Whether or not these babies are identical depends on how soon the egg splits after conception.

If the fertilised egg splits into two embryos within at least three days of conception, this is early enough for both embryos to develop separate membranes and placentas.

Monochorionic Diamniotic (MCDA) twins

This is the most common type of identical twins, accounting for around 70 per cent of all identical twin births.

MCDA twins occur when both babies share a placenta and a chorion (outer membrane). However, they each have their own separate amnion (inner membrane).

These types of twins are usually formed when a single fertilised egg splits 4-8 days after fertilisation (after the sperm meets the egg).

Monochorionic Monoamniotic (MCMA) twins

MCMA twins are extremely rare, accounting for only 1 per cent of identical twin births.

These types of twins share an amnion (inner membrane) and a chorion (outer membrane) and are usually formed when a single fertilised egg splits nine days after fertilisation.

It is often easy to identify MCMA twins using an ultrasound scan, and pregnant women carrying MCMA twins are usually more closely monitored throughout their pregnancy.

This is because there is an increased risk of twin-to-twin transfusion syndrome (TTTS) in MCMA twins.

How can you tell if twins are identical?

For pregnant women expecting twins, medical professionals will attempt to identify whether twins are identical using ultrasound scans.

However, this is not always possible, as it is dependent on the sonographer being able to determine whether the babies share sacs and placentas.

A DNA test is the only 100% accurate and reliable way to find out whether twins are identical or not.

Because identical twins share near-identical genetic profiles, expert geneticists can confirm whether twins are identical using a simple cheek swab DNA sample.

At AlphaBiolabs, for example, our in-house scientists analyse and compare up to 42 DNA markers (loci) in the two samples.

If the twins share enough matching DNA markers, this indicates that the twins are identical.

Where can I buy a Twin DNA test?

Thanks to advances in modern scientific techniques, it’s never been easier to find out for sure whether twins are identical or not.

Available for just €139, our home Twin DNA test can be ordered quickly and easily online.

With only cheek swabs required, this non-invasive test can be used on siblings of any age – from newborn babies to grown adults.

The DNA (zygosity) certificate provided on completion of the test can also be used as a unique keepsake for multiple birth parents.

Need to know more about our Twin DNA test? Call our friendly and knowledgeable Customer Services team on 01 402 9466 or email info@alphabiolabs.ie.   

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