Identically different: why identical twins are often not

The term “identical twins” is misleading and every day causes arguments between parents of twins and complete strangers. Why? Usually because of the false assumption that identical twins should look identical.

Every identical twin is a human clone. Each pair started life as one fertilised egg, which a few days later split into two, with each embryo then developing as a separate human being. This is why identical twins are often referred to as monozygotic, Greek for “one yolk”. The other kind of twins, known as fraternal, non-identical or dizygotic (two-yolk) started life as two eggs, which were fertilised by two different sperm.

Baby Twins

Identical twins often do not always look or behave identically just as no two cloned animals are alike. This is because, in many cases, twins have different experiences in the womb.

Almost all twins float in separate fluid-filled sacs and have separate umbilical cords. All fraternal twin pairs, and about one third of identical twin pairs, also have their own placenta. As pregnancy progresses, differences can emerge in the size and shape of the sacs, cords and placentas as occurs with all pregnancies.

Furthermore, two thirds of identical twin pairs share the same placenta and compete for the goodies that come from Mum. Sometimes this competition can be so fierce that one twin gets the lion’s share of everything and the other starves.

twin to twin transfusion syndrome

Luckily, obstetricians know about this and constantly monitor mums-to-be who are carrying shared-placenta twins. They can even stop the sharing using a laser to flow of blood from one twin to the other, which can help the twins spend as long as possible in the womb.

In addition to nurture in the womb comes nature – our genetic sequence – and we are always a product of both. Fraternal twins are as genetically-different as any brother or sister but identical twins are usually genetically identical.

But sometimes, very early in development, a random genetic change may occur that causes a specific protein to be absent or change function.  Therefore, as identical twins go through life, they may not always appear identical; in rare cases they can look more different than fraternal twins or even siblings.

How can twins or parents of twins know for sure whether they are identical or fraternal? Firstly, an obvious one; different-sex twins are almost always fraternal except in rare cases in which one twin has a DNA change to a gene that determines sex. Secondly, if an ultrasound scan in the first three months of pregnancy shows that twins are sharing the same placenta, then they will most likely be identical.

However, for all other same-sex twins, and that’s almost half of all twins, a genetic test is needed. This test, often referred to as a zygosity test, produces a set of ‘DNA fingerprints’. If they are all the same, the twins are identical while fraternal twins will have only half in common on average.


Up until recently, fraternal twins often resulted from in vitro fertilisation (IVF) because two or more fertilized embryos were transferred due to the low success rates. Women were frequently surprised because they found themselves carrying twins. This is because, even though the two embryos would result in dizygotic twins, if only one survived, it could still split and end up as a pair of identical twins.

Although most of the time, our guesses about twins’ identity based on hair colour, eye colour and height are correct, genetic fingerprints will always be more accurate. As the tests usually cost between $100 and $200 (currently $139 for members of the Australian Twin Registry) , it’s not too expensive to have them done; all you need is a cheek swab from each twin.

So why do we need to know whether twins are identical or not? Twins themselves will tell you that having an identical twin means that if their twin gets a serious illness, they wonder about their chances of also developing it. Parents of twins will tell you that they are sick of having arguments with strangers who think they know best that their different-looking twins cannot be identical. Furthermore, most twins say they would love to know out of plain curiosity.

So next time you see a pair of twins in public, you can discuss not so much whether they are identical or not but how do they know!

22 thoughts on “Identically different: why identical twins are often not”

  1. I have just read this and I am so relieved to know that I am in fact correct, and my twins are identical even though they don’t look exactly alike. :-). I have twin girls, aged 6, and we were told right from the start that they were identical, but they “aren’t” and have to constantly explain that yes they are! And now I can tell people why! I can’t remember the term but my obstetrician said they one twin was eating all the food and the other twin wasn’t getting as much. I explain this to people and say well if they weren’t identical, this wouldn’t happen. Am I correct in saying that? It seems to quiet people, so that’s why I have been saying that, but now I have more information and I’m so grateful. :-).
    Jos O’Halloran

    1. Yes you are correct to say that. Twin-to-Twin-Transfusion Syndrome (where one baby gets alot more nutrients than the other baby) can only happen if the twins share the same placenta, meaning they came from the same egg and are infact identical twins 🙂

  2. Thanks Jos. The technical term is twin-to-twin transfusion syndrome. You are correct and I know of other cases in which a mother was correct and doubted by others. In fact this only happens IF the twins are identical.

    1. TTTS is an extremely serious condition with a mortality rate of 80 to 100%. Please be careful about minimising the seriousness of the condition, it is not merely a case of one getting more than the other. If you’re twins had TTTS (sounds like it might have been low grade TTTS) your Dr should have taken extreme caution during your pregnancy and you are very lucky they bboth survived, most dont…

      1. The reason I emphasise this is women pregnant with monochorionic twins need to be on high alert for TTTS not just dismiss it as “they won’t look alike”

      2. I totally agree. Often from the first trimester, a shared placenta can be identified and as a result, the pregnancy should be more closely monitored. As for risk, I know that laser surgery is now often an option but a woman should consult her medical professional for information on her particular risk. There are also many twins that are born different sizes that have not had twin to twin transfusion syndrome and we usually guess that they have had a slightly different share of Mum’s “goodies”. More often than not, both are healthy. Again, the best person to talk to is your medical practitioner.

      3. Thank you so much for that information!!!! I will be showing your comment to my sister and my now ex-husband, as they are the biggest non-believers (if that is ok of course).
        Yes, I was extremely lucky!! I had obstetrician appts every 2 weeks instead of the usual 4 weeks, and had a c-section at 35 weeks. Kennedy was 7lb 4oz, and Ella was 4lb. So yes, low grade, but definitely a difference! :-). Ella is still 1cm shorter and 1kg lighter, and did have a few issues to start with but is a fighter!!!! :-). You really have made my day!!! Honestly!!!! :-). Thank you so so much!!!!! :-).

      4. I almost lost my twins to TTTS and spend a lot of time explaining to people why they aren’t the same size – hence reliving the day they were born over and over! It’s not to be minimized at all.

  3. My twin sister and I were always told we are fraternal, I have darker skin, eyes and hair. However as we have gotten older (33) people are confusing us more and more. Is it possible to be identical while still having different features such as the ones I have named?

  4. MY girls twins are 22 this year and I had the “Zygosity” test done when they were quite young as the doctors told me they were not Identical , but I was sure they were , and I was right they are , although they often don’t seem that way , one always looked bigger than the other , and different ideas on dressing from the very start

  5. I recently became pregnant via a single embryo transfer via IVF that resulted in a twin pregnancy. We assume that the embryo split and the twins were identical. At my 7 week scan there was a size difference in the sacs which became larger as the pregnancy progressed. The babies had separate sacs and placentas. At my 22 week scan the sacs differed in size by 7 weeks. An amnio revealed that the small twin which showed many abnormalities had triploidy. The amnio ruptured the membrane of the small twin. He died at 26 weeks and I delivered the other twin at 29 weeks and he is perfect. So, were they identical? I am 47 and have never fallen pregnant naturally so the chances of the 2nd twin being conceived by me at the same time as the embryo was implanted are slim.

    1. Rebecca, firstly I am sorry to hear what happened. I am no obstetrician but my guess would be that yours is a rare example of a genetic change happening to just one of a pair of identical twins early in development. One third of identical twins have separate placentas.

  6. There is one other very important reason that parents of twins, or older twins themselves, might want to undergo the twin zygosity test. Many extremely valuable scientific studies focus exclusively on identical twins, and some on only fraternal twins. Knowing whether or not twins are genetically identical can enable to them to include or exclude themselves from such studies and make a valuable contribution to our body of scientific knowledge on many genetic issues.

  7. I had identical triplets. They were born in Brisbane at the mater Mothes hospital1993. A placenta histology was performed and came back saying that they are identical triplets. Sometimes they all look so much the same and then other times they look so different to me.

  8. This is a very interesting read! I had no idea that one third of IDs have their own sac and placenta. My GBB triplets are 1 and we’ve often wondered whether we should have the boys tested. At an ultrasound at 7 weeks I was told they looked like it had initially been one sac that had split. They look so different to me, though I know part of that is definitely nurture (one was very squished in my uterus). I hope you can answer this question. I thought IDs would do things at a similar time, even down to teething. One boy has more teeth than the other. One has dark brown eyes, the other’s are still changing (though they’re getting darker and more similar to his brother). If they were identical, wouldn’t their eye colour change at a similar rate, and their teeth cut at similar times?

  9. Always great to hear other parents’ experiences! We have twin boys, age 23. We were told during the pregnancy they were not identical, and then had it ‘confirmed’ at delivery with 2 separate placentas. However, due to their similarity, we weren’t convinced! Being in the ‘Teeth and faces of young twins’ study through Adelaide Uni, the boys had cheek scrapings done when they were around 8 years old….which confirmed our suspicion, they were indeed identical!

    1. Jess, my four year old girls are “identical” twins and one has significantly darker hair,eyes, and skin than her sister. She also has a wider face. People still say they can’t tell who is who but once you learn these differences it’s easy. So yes, you might well be an ID twin.
      Regarding TTTS, the larger twin is actually more at risk, not the smaller. His/her system gets overloaded with blood, fluid, and nutrients, and this can have severe consequences. I was monitored every two weeks, was an anxious mess, but thank God all went well.

  10. My twin girls shared a placenta and a sac (monochorionic monoamniotic or MCMA twins) so definately identical even though strangers still argue with me!

  11. my fraternal girls shared a was bigger and the other smaller.they are almost one and am still struggling with the other one to gain weight

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