Zygosity Twin Testing Explained

Wondering about twins and twin testing? Here’s some useful info.

Liz Wood Alphabiolabs

By Liz Wood, Health Testing Specialist at at AlphaBiolabs
Last reviewed: 02/26/2024

About twin testing

Let’s be honest: being told that you’re expecting multiple births can be a shock! Some mothers may be thrilled, and even relieved, that they’re carrying healthy babies and have created an instant family unit. Others may dread the implications – not only the increased parenting skills required – but the practicalities of extra demands on finances, logistics and their time.

Multiple births are not as uncommon as you may first think. The most common form of multiple births is twins. In 2016, 131,723 mothers gave birth to twins, which equates to 33.4 out of every 1000 women giving birth. The triplet or higher order birth rate is 101.4 per 100,000 live births. In 2016, 3755 women had triplets, 217 had quadruplet births, and 31 had quintuplets and other higher order births [1].

Thankfully, help is at hand. The National Organization of Mothers of Twins Clubs (also known as Multiples of America) has a network of more than 300 local clubs across the USA, representing more than 25,000-member families with twins, triplets, and quadruplets [2]. Its clubs include Mothers of Twins Club (MOTC), Mothers of Multiples Club (MOMC), or Parents of Multiples Club (POMC). It provides tips for parenting multiples and maintaining a healthy pregnancy, as well as nutritional and educational information. They also have an abundance of resources including educational material, DVDs and online seminars. Blogs and Discussion Boards permit worried parents to share their concerns, ask for advice and hook up with parents in a similar position.

Call Now to order a Twin Zygosity DNA Test: 727-325-2902

Multiple births are increasing

The number of multiple births has been on the increase since 1980. Trends in the increased twin birth rate in the USA between 1980 and 2009 were analyzed in a 2012 study [3]. The research found that in 1980, 1 in 53 babies was a twin; whereas in 2009, 1 in 30 babies was a twin. This represents a 76% increase in the twin birth rate over 30 years. The study estimated that an additional 865,000 more twins were born during this period than if the twin birth rate had not increased during those decades. In other words, in 1980, the twin birth rate was 18.9 out of every 1000 women giving birth; and in 2009, the twin birth rate was 33.3 out of every 1000 women giving birth.

Fascinating facts about twins that are actually true

No-one is certain why multiple births are increasing, but several factors play a role. In particular, the increased use of conception techniques, such as IVF, and mothers waiting later to start their families. Women aged 45 and over are most likely to have a multiple birth. A mother is also more likely to have multiple births if there is a maternal history of multiple pregnancies.

Identical or non-identical?

Once parents have got their heads round the fact that they will soon have more than one baby, the next major question is: will the siblings be identical? Zygosity is an important issue of discussion in the multiples community. The familiar classifications of twins are identical and fraternal, which are commonly used to describe twins based on appearance. However, zygosity is a more important scientific classification for describing twins based on how they formed.

Non-identical siblings are the result of separate fertilized eggs. Non-identical twins from the same birth are known as dizygotic, which means they have two zygotes. Triplets are trizygotic (three zygotes), quadruplets are quadzygotic (four zygotes), and polyzygotic is the umbrella term for multiple zygotes. Because these siblings all grew from separate zygotes, the babies are no more alike than any other brothers or sisters, and may be both male, both female, or one of each. All of these non-identical siblings share DNA in common, as in the case of siblings from different births.

Identical siblings occur in about one-third of multiple pregnancies. If siblings are identical, they are known as monozygotic. This means that they were formed from a single fertilized egg, which went on to split into separate embryos. It also means that their DNA is exactly the same. They may or may not share a placenta. Triplets and higher order multiples may be in different combinations. For example, you may have a set of triplets where two are identical and one is not.


Twin testing (also known Zygosity testing) as is used to determine whether multiple children from the same birth (twins, triplets, quadruplets, etc.) are genetically identical or not.

[1] National Center for Health Statistics: cdc.gov/nchs/fastats/multiple.htm

[2] multiplesofamerica.org

[3] National Center for Health Statistics. Three Decades of Twin Births in the United States, 1980–2009. www.cdc.gov/nchs/data/databriefs/db80.htm

How do you know if your siblings are identical?

Non-identical siblings are the result of separate fertilized eggs. These babies are no more alike than any other brothers or sisters, and may be both male, both female, or one of each. They share DNA in common as in the case of siblings from different births.

Identical siblings are formed from a single fertilized egg, which went on to split into separate embryos. This means their DNA is exactly the same.

The only way to conclusively determine whether siblings are identical or not, is to carry out a DNA test. In utero determination may be incorrect as the placentas of non-identical siblings may fuse together in the womb giving the appearance of a single placenta making them appear to be identical siblings; conversely, approximately one-third of identical twins have separate placentas making them appear non-identical.

Why would you need a zygosity test?

Zygosity twin testing is used to determine whether multiple children from the same birth (twins, triplets, quadruplets, etc.) are genetically identical or not.

It can be useful to know because it can help assess the likelihood of conceiving multiple births again. If a mother has already given birth to non-identical twins, triplets or more, then they are five times more likely to carry multiples in their next pregnancy.

In addition, some illnesses may affect monozygotic children so a zygosity twin test would inform you as to whether both/all children need testing for a particular condition.

An increasing and novel use of a DNA zygosity profile is as a unique gift idea for multiple birth parents and/or their children.

Order a Zygosity Twin DNA Test Online

Do I need to get blood samples?

No blood samples are needed. The test kit includes mouth swabs to obtain tiny amounts of DNA from inside each sibling’s mouth. The swab is rubbed gently and painlessly on the inside of the cheek, so it is not a problem if anyone involved is scared of needles.

What does the test entail?

When we receive your sample in our laboratory, we examine specific markers present in repeat sections of DNA from each sibling. These Short Tandem Repeat (STR) loci include the Amelogenin (gender) locus. A DNA profile is produced for each of the samples provided and the DNA seen at each of these STR loci is compared between the tested individuals to determine zygosity; monozygotic twins will share the same DNA profile whereas dizygotic twins will have different DNA profiles. AlphaBiolabs can examine up to 45 markers.

What’s included in the test kit?

Our DNA twin zygosity test includes a Peace of Mind DNA zygosity test request/consent form, full instructions on how to take the test, a self-addressed envelope to send your DNA samples back to us, two DNA sample collection swab packages containing two swabs in each package and an individual DNA test sample envelope for two siblings.

If you want to test more siblings with the same test, you can add extra siblings to the package when ordering and more swabs will also be provided in the test kit.

How quickly can I get the results?

You can receive your zygosity twin testing results in 3 business days for just $109.

Liz Wood AlphaBiolabs

Liz Wood

Health Testing Specialist at at AlphaBiolabs
Liz joined AlphaBiolabs in 2021 as a Health Testing Specialist. As well as overseeing a range of health tests, she is also the lead on several validation projects for the company’s latest health test offerings. During her time at AlphaBiolabs, Liz has played an active role in the validation of the company’s Genetic Lactose Intolerance Test and Genetic Celiac Disease Test. An advocate for preventative healthcare, Liz’s main scientific interests center around human disease and reproductive health. Her qualifications include a BS in Biology and an MS in Biology of Health and Disease.

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