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Identical Twin DNA Testing

A twin DNA test, also known as a twin zygosity test, is a DNA test for determining whether your twins, triplets or quadruplets, etc. are identical or fraternal and identifies the type of cell union that occurred at the time of conception.

There are technically three types of twins. They can be either identical, fraternal or semi-identical, however, the most common are identical twins or fraternal twins.

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Monozygotic, which means “one zygote” refers to Identical Twins. Identical twins form when a single fertilized egg splits into 2 genetically identical parts.

Identical twins share the same DNA set, which means they might look identical and have very similar traits such as health and personality. However, identical twins can also look very different from each other due to the fact that physical appearance is also influenced by environmental factors and not just genetics. Identical twins are always the same sex sets such as boy/boy or girl/girl.

Types of twins

Dizygotic means “two zygotes” and refers to Fraternal twins. Fraternal twins are created when 2 separate eggs are fertilized by two separate sperm and implant into the uterus. The genetics are pretty much the same as siblings born at separate times. They might look somewhat similar or they may look nothing at all alike.


Semi Identical happened in one known case back in 2007 where the twins were half identical and half fraternal.  The researchers of this rare case hypothesized two different theories about how this might have happened. In the first hypothesis, the mother’s egg divided itself and was then fertilized by two separate sperm – one with a Y (boy) chromosome and one with an X (girl) chromosome. In the second hypothesis, the mother’s egg was fertilized by two separate sperm and then divided into two separate embryos.  Semi-identical twins are so rare there is currently only one known case in the world so it is not typically tested for.

It is not uncommon for doctors to misclassify what type of twins were born confusing fraternal with identical. The reason for this is that not all identical twins are developed in one membrane. Two different placentas and two sets of membranes are formed if the fertilized egg splits prior to 2 days after conception. The percentage rate of this happening is about 25-33% percent of the time.

When the delivering doctor examines the membranes surrounding the twins correctly and makes a determination that the twins were monochorionic, the twins are always identical regardless if the amniotic membrane is shared (i.e. monoamniotic) or non-shared (i.e. diamniotic) and a twin zygosity test is not called for. Sometimes however, the placental assessment is not performed or difficult to make by the delivery, especially if there was a c-section.  A twin DNA test is the best way to know for certain if same sexed twins are identical or fraternal if chorionicity is uncertain or not known.

When Would You Want a Twin Zygosity Test?

Many people want a zygosity test to appease curiosity and know for certain the relationship status of their twins.  Additionally for medical reasons there are some health conditions where it would be very helpful to know if a condition experienced by one twin will be most likely to be experienced by the other twin. Identical twins can donate organs to each other in some situations. For making important medical decisions, we highly recommend getting a test with an accredited laboratory which includes monitored sample collection and a chain of custody.

Since physical looks alone isn’t a trusted guide in identifying twin type, how can parents know for certain their twins’ zygosity? In some cases, it will be evident during pregnancy. Some identical twins form in a single sac, sharing a single placenta and amniotic membrane. However, the number of placentas isn’t always a clear indication either. The separate placentas of fraternal twins can fuse together and give the appearance they are one. And identical twins might develop with totally different placentas and sacs. The best way to know for certain is through DNA testing.

A Zygosity DNA test is an easy test that can be performed at any point after the twins are born.

Twin DNA Test Procedure

Our twin DNA test is performed by using 24 specific DNA markers instead of the industry standard 16 markers for the highest accuracy possible. If the twins are identical they will match on all 24 markers. Anything less than all 24 markers matching would mean they are not identical twins.  If the mother doesn’t know for sure whether the two children are from the same father (which can happen, even if they were born together), we would suggest also doing a sibling DNA test which we could do in conjunction with the twin zygosity test.

Order Your Twin Zygosity Test Now!

identical twin DNA test

there are three types of twins

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Your Identical Twin Testing Kit Includes:

  • Discreet Shipping
  • Easy to use kit with detailed instructions
  • Fast results: 1 business day after all samples are received by the lab. (5 business days for non-standard samples)
  • Results are private and confidential
  • One low price include kits + results
  • Easy online ordering without salespeople pressuring you

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Commonly Asked Questions About Identical Twin Testing

We test 24 markers in our twin zygosity DNA tests instead of the industry standard 16 marker for the highest accuracy in your test results.

Although quite rare, this has happened in some occasions where the mother had sex with more than one man in the same day and the sperm from different fathers fertilized separate eggs. A separate sibling test would need to be performed to determine whether or not the fraternal twins are also half siblings or a direct paternity test could be done as well.

The test data shows some evidence that fraternal twins may be somewhat hereditary but there is less evidence showing that identical twins are hereditary.

No. It is actually preferable to do a cheek swab rather than a blood test as blood stems cells can mix together during maternal circulation and if there are different fathers involved some of the blood stem cells could mix with the child not fathered so the blood test would not be as accurate as cheek cells which would not be affected the same way.

Yes. We know not everyone has a degree in genetics, and therefore we make sure the results are easy to read and understand. We are always available for questions after your results are ready as well.

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