Zygosity Twin Testing Explained

Being told that you’re expecting multiple births can be a shock to many parents. Some mothers may be thrilled, and even relieved, that they are 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.

Thankfully, help is at hand. Multiple births are not as uncommon as you may first think. In 2015, 11,073 mothers had a multiple birth, which equates to 16.1 out of every 1000 women giving birth. Of these, 10,901 women had twins, 169 had triplets and three had quadruplets and above.1 The most common form of multiple births is twins. Twins occur 90% of the time with the other 10% divided between triplets, quadruplets, and other higher multiples. Organisations, such as TAMBA (Twins & Multiple Births Association),2 The Multiple Births Foundation3 and Twinsclub4 all provide reassurance and practical advice. They also have an abundance of resources including downloadable guides and online videos and seminars. Message boards and online forums permit worried parents to share their concerns, ask for advice and hook up with parents in a similar position.

The number of multiple births has been on the increase since 1980. No-one is certain why this is, 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: in 2015, 102.4 out of every 1000 women giving birth in this age group had a multiple birth.1 A mother is also more likely to have multiple births if there is a maternal history of multiple pregnancies.

 

Types of twins

In medical terms,5 there are three types of twins:

  • dichorionic diamniotic (DCDA) twins: each twin has their own separate placenta with its own separate inner membrane (amnion) and outer membrane (chorion)
  • monochorionic diamniotic (MCDA) twins: each twin shares a single placenta with a single outer membrane and two inner membranes
  • monochorionic monoamniotic (MCMA) twins: both share the inner and outer membranes

 

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? All non-identical twins are DCDA, and one-third of identical twins are DCDA. The other two-thirds of identical twins are MCDA, and just 1% of identical twins are MCMA.

Whether or not the babies are identical depends on how the babies were formed. Non-identical siblings are the result of separate fertilised 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 fertilised 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.

zygosity twin test?

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

Zygosity Twin Testing FAQs

How do you know if your siblings are identical?
Non-identical siblings are the result of separate fertilised 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 fertilised 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 whether multiple siblings are identical 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.

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 24 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.

What’s included in the test kit?
Our DNA 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 the next day for £149, or upgrade to same day results for an extra £50. Please note that we will need to receive your samples in our laboratory before 10 am.