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DNA Testing for Twins
Updated on January 4, 2023
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DNA Testing
DNA Testing for Twins

A DNA test can prove that two people are related and the degree to which they are related. To prove that two people are twins, they’d need to undergo a twin zygosity DNA test.

DNA stands for deoxyribonucleic acid. It carries genetic information, which is something each person shares with their ancestors.

DNA shows a link between more-distance relatives, including:

  • Cousins
  • Grandparents and grandchildren
  • Aunts and uncles and nieces and nephews
  • Siblings
DNA Testing for Twins 2

Are DNA test results different if siblings are twins?

It depends on whether they are fraternal twins or identical twins.

Identical Twins

Identical twins or monozygotic twins are two different people who grew from a single fertilized egg and one sperm.

Once the sperm reaches the egg, the fertilized egg splits into two identical cells, which eventually grow into physically identical and nearly genetically identical people. As they grow, they are an exact match of one another.

Fraternal Twins

Fraternal twins are two eggs fertilized by two sperm. They are not identical and aside from being in utero and born at the same time, they are biologically no closer than any other siblings.

If twins are identical twins DNA will show this. Identical twins' DNA profiles are identical. They look the same, they are the same sex, and they have the same DNA.

If they are fraternal or non identical twins, their DNA sample will show only that they are siblings. Fraternal twins' DNA profiles are very similar, but no more similar than siblings born separately.

Not only can a twin DNA test help you determine your twin relationship, but it can also help an expectant mother determine if her twins are fraternal or identical while she is pregnant.

Can a DNA Test Give a False Positive?

A DNA test can give a false positive reading.

A test performed early in pregnancy can show that a mother is carrying twin babies. But due to “vanishing twin syndrome,” the test can produce a false positive.

This occurs when there is a second baby at the time the test is administered, but a miscarriage causes only one baby to grow to term.

Additionally, undetected tumors and mosaicism might also trigger a false positive in a DNA test. 

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Can Twins Have Different Fathers?

Yes, but it is extremely rare. And in most cases, nobody will ever know that there are two different fathers.

If two different sperm fertilize two different eggs at approximately the same time, it can result in two babies that appear to be fraternal twins.

DNA testing would show them to be only half-siblings. 

Keep in mind, an at-home DNA testing is not a “legal test.”

It’s legal to take the test and it can provide you with information. However, the results are not recognized by the court system. A legal test must be performed by a doctor and/or at a DNA testing clinic.

If you’re interested in taking a DNA test for legal reasons, speak to an attorney or contact the court to determine what constitutes a legal test in your situation.

How Early Can Twins Be Detected?

Twins are detectable very early in a pregnancy.

In the case of fraternal twins, there will always be two fertilized eggs. With identical twins, they would not be detected until the egg splits, but this happens very early in the development process.

Twins can be:

  • Di-di
  • Mo-di
  • Mo-mo 

Di-di Twins

Di-di twins, which are twins with their own separate inner and outer sacs and placentas, are detectable at week five of the pregnancy. Heartbeats might not be discoverable yet, but the sacs have formed and are visible.

Mo-di or Mo-Mo Twins

Mo-di or mo-mo twins aren’t detectable until after six weeks. These types of twins share a single sac and are identifiable only by their two heartbeats.

Di-di, mo-di, and mo-mo is not always an indication as to whether twins are identical or fraternal.

For instance, dichorionic diamniotic (di-di) have their own inner and outer sacs and placentas. 

This type of twins can be fraternal or identical, though identical di-di twins are sometimes mistaken for fraternal twins. All fraternal and a third of all identical twins are di-di.

Mo-di and mo-mo twins are always identical.

Monochorionic diamniotic (mo-di) share one placenta and one outer membrane, but each has its own inner membrane. This is the most common type of identical twins.

Monochorionic monoamniotic (mo-mo) twins share a single placenta and outer and inner membranes. This is a very rare occurrence and accounts for only about one percent of all identical twins.

Keep in mind, two placentas sometimes fuse into one, which makes it seem as if fraternal twins are identical. A DNA test would prove otherwise.

How are After-Birth Twin DNA Tests Performed?

If it was impossible to tell for sure during the pregnancy that twins are identical, DNA testing is performed after birth.

This can be done using a blood sample from the placenta(s) or using by swabbing the cheek cells from each baby.

Samples can be taken from the comfort of your own home and sent to a lab for analysis.

Why Take an Identical Twin DNA Test?

Sometimes the reason for taking a twins DNA test to prove twins are identical is mere curiosity.

Twins can look very much alike without being identical – the same as siblings born at different times. Confirming that twins are indeed identical, if it was not determined in utero, lets them know they are of a rare variety.

Another reason to take an identical twins DNA test is that the twins want to participate in a scientific study. The test shows that the twins originally split from one egg, so identical twins are able to provide information based on their genetics.

Twin studies are very common, but participants must prove they are identical or fraternal before they are admitted into most studies.

Some mothers choose to undergo a twin DNA test during pregnancy to determine if their twins are identical because this increases the risk of complications.

Why is it important to know you’re pregnant with identical twins?

Twin-to-Twin Transfusion Syndrome 

Approximately 15 percent of monochorionic twins experience twin-to-twin transfusion syndrome, in which one twin gets too much blood and the other does not get enough.

Frequent Doctor Check-ups

Mothers pregnant with monochorionic twins see their doctor more often than they would in other types of pregnancies.

Knowing if your twins are identical or fraternal helps you plan and recognize issues early.

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Updated on January 4, 2023
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2 sources cited
Updated on January 4, 2023
  1. “Twins with Different Fathers: Is It Possible?” Flo.Health - #1 Mobile Product for Women’s Health, Accessed 1 Dec. 2020.

  2. JoNel Aleccia. “UW Experts Shed Light on False Positives in Prenatal Tests.” The Seattle Times, Apr. 2015, Accessed 1 Dec. 2020.

Kelly Jamrozy
Kelly Jamrozy
Content Contributor
Kelly has experience working with clients in a variety of industries, including legal, medical, marketing, and travel. Her goal is to share important information that people can use to make decisions about their health and the health of their loved ones. From choosing the best treatment programs to improving dental and vision health to finding the best method for helping anyone who is struggling with health issues, she hopes to share what she learns through informative content.
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