Can identical twins look different?

Casey Randall Alphabiolabs

By Casey Randall, Head of Genetics at AlphaBiolabs
Last reviewed: 03/07/2024

As a provider of twin DNA testing, we are often asked about the science behind twin DNA, especially when it comes to identical twins.

One of the most common questions is whether identical twins can look different from one another. And if so, how can you ever know whether twins are truly genetically identical?

In this article, we examine how twins can look different while still being genetically identical. And conversely, twins who look the same might not be genetically identical!

What are identical twins?

Identical twins – also known as monozygotic twins – are formed when a single fertilized egg splits into two after conception.

They share the same genetic profile, including the same sex and blood type, and will usually have similar physical characteristics.

However, there will always be some physical differences between identical twins. For example, each twin will have a different bite pattern, and different fingerprints; they could also have different handedness (left or right-handed).

How are identical twins formed?

Identical twins are formed when a single sperm fertilizes an egg to create a single zygote which then goes on to split into two.

When the egg splits will determine whether the twins will share a placenta: the earlier the egg splits, the more independent an embryo will become by having its own amniotic sac, chorion (outer membrane) and placenta.

There are three medical definitions for the way in which identical twins are formed:

Dichorionic diamniotic (DCDA) twins

Each twin has their own separate placenta with its own separate inner membrane (amnion) and outer membrane (chorion); this occurs when the zygote splits within 3 days. One-third of identical twins are DCDA twins. DCDA twins may be incorrectly diagnosed as non-identical from an ultrasound scan.

Monochorionic diamniotic (MCDA) twins

Each twin shares a single placenta with a single outer membrane and two inner membranes. This usually occurs when a single fertilized egg splits after 4–8 days and accounts for 70 per cent of all identical twin babies.

Monochorionic monoamniotic (MCMA) twins

MCMA twins are easy to identify on an ultrasound scan. Both share the inner and outer membranes. This usually occurs when a single fertilized egg splits after 9 days of fertilization.

For multiple pregnancies, the mother will usually be monitored more closely by healthcare services. This is due to the increased risk of complications associated with these types of pregnancies.

For example, mothers of monochorionic twins (MCDA or MCMA) who share a placenta will be more closely monitored due to the risk of twin-twin transfusion syndrome (TTTS) – a condition in which the blood flows unequally between twins that share a placenta.

Both MCDA and MCMA twins are at risk for TTTS, although there is a higher risk with MCDA twins.

Can identical twins look different?

Surprisingly, yes. Identical twins can look different from each other. 

While they might start life with nearly identical genetic blueprints, a variety of factors can lead to differences in appearance, health, and personality over time.

Identical twins are the product of a single fertilized egg splitting into two, giving them the same genetic material. This shared genetic starting point sets the stage for remarkable similarities in appearance and physiology that continue to fascinate scientists and the public. However, the idea that identical twins are perfect carbon copies of each other is a myth.

The differences between identical twins, sometimes subtle and sometimes more pronounced, stem from a combination of genetic, environmental, and lifestyle factors. For instance, small mutations can occur after the egg splits, leading to differences that might affect everything from susceptibility to certain diseases to minor variations in appearance.

Environment also plays a role in shaping how identical twins grow and develop, both inside and outside of the womb. Factors such as nutrition, physical activity, and even the unique experiences each twin encounters can influence their physical and psychological development.

At AlphaBiolabs, we have seen many instances where twins or parents of twins thought they were non-identical, only to find out that they were in fact genetically identical upon taking a DNA test.

A DNA test is the only way to know for sure whether twins are genetically identical or not.

One customer, Karen Johnson, said: “After 50 years of thinking my sister and I looked a lot alike but were fraternal twins, we now know we are identical. The doctor in 1972 thought that because we were in two sacs we must be fraternal – but two sacs are common with identical twins too!”

Another customer, Kelly Coupland, said: “After receiving a lot of comments that our twin girls look very similar, we ordered the Twin DNA Test from AlphaBiolabs. They are DCDA twins (separate sacs and placentas), so as much as I really wanted identical twins, I thought they’d be non-identical.

“I will never forget the day I saw the email in my inbox. We were so excited to open it and receive the results – they are in fact identical!”

Order a Twin Zygosity DNA Test Online

What are the differences between identical and fraternal (non-identical) twins?

The fundamental difference between identical and fraternal (non-identical) twins is how they are formed in the womb.

Unlike identical twins, that are formed from a single fertilized egg which goes on to split into two separate embryos, non-identical (or fraternal) twins are the result of two separate eggs, fertilized by two separate sperms.

As such, these siblings are no more alike on a genetic level than siblings from different births. Additionally, non-identical twins can be both male, both female, or one male and one female.

Fraternal twins may also grow at different rates, while identical twins usually follow the same pattern of growth.

Related: Identical vs Fraternal twins

Can non-identical twins look the same?

Yes, in the same way that siblings from different births can look similar due to their shared DNA, non-identical twins from the same pregnancy can certainly look like one another.

In fact, there have been instances where parents thought their twins were identical, only to discover they were non-identical (fraternal) twins when a DNA test was performed.

As mentioned, the only way to truly know whether twins or multiple birth siblings are identical on a genetic level is with a DNA test.

How can I find out if twins are genetically identical?

A DNA test can be used to determine whether twins are genetically identical.

This is especially useful for birth parents who have previously been told that their children are not identical, as many children who do not look alike have in fact been found to be genetically identical when tested!

The same can also be true of twins who are thought to be identical due to looking alike, who are then found to be non-identical.

A twin DNA test – also known as a zygosity test – can be performed as soon as twins are born. The test is performed by collecting a sample of cheek cell DNA from each twin, using a cheek (buccal) swab.

This method of sample collection is simple, non-invasive, and completely pain free, meaning that the test can be performed on a person of any age. The twin DNA test is also not exclusive to twin births – we can DNA test up to five siblings from the same pregnancy. Simply tell us how many siblings need to be tested when placing your order*.

You will also receive a DNA certificate detailing the test results and each individual’s DNA profile – a unique keepsake for multiple birth parents.

Simply place your order online and we will send your test kit out to you immediately with FREE shipping.

As soon as we receive your samples at our laboratory, testing can begin. Your secure, password-protected results will be emailed to you in 3 business days.

Gail Evans, who purchased a twin DNA test for her adult sons, said: “For 26 years we believed that our twin boys were non-identical, as the consultant told me during the scan that the babies were in two separate sacs, and that there were two placentas present.

“Everyone struggled to tell the boys apart, so we decided to arrange a Twin DNA (Zygosity) Test with AlphaBiolabs to clarify the matter once and for all.

“To our surprise, the results confirmed that the boys are identical. This has ended years of speculation within the family and has provided us with conclusive scientific proof.”

Need more information on our Twin DNA Test? Call our Customer Services team on 727 325 2902 or email info@alphabiolabsusa.com.

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Casey Randall AlphaBiolabs

Casey Randall

Head of Genetics at AlphaBiolabs
Casey joined AlphaBiolabs in 2012 and heads up both the Genetics and Health testing teams. An expert in DNA analysis and a member of the International Society for Forensic Genetics (ISFG), Casey holds an MS degree in DNA Profiling and a BS degree in Forensic Science. Casey is responsible for maintaining the highest quality testing standards, as well as looking for ways to further enhance the service that AlphaBiolabs provides and exploring new and innovative techniques in DNA analysis.

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