Why Do So Many Identical Twins Think They’re Fraternal?

It’s a phenomenon that baffles many twin researchers: Why do so many twins think they’re fraternal (DZ) when in fact they’re identical (MZ)?

For starters, many identical twins and their families are very in tune to their physical and social differences—however subtle—rather than their greater similarities. They believe that if one is more outgoing or an inch taller, for instance, this is proof enough to classify them as fraternal because…aren’t all identical twins identical?

Well, no.

Although MZ twins have the exact same DNA, there are differences in their epignome or the way that their genes are “organized” within a cell, accounting for variations in both their physical appearance and personality.

Zygosity misdiagnosis often begins with the medical community where many parents are given incorrect information regarding their twins’ zygosity either during a routine ultrasound during the pregnancy or at the birth. Why is that? Many technicians and obstetricians simply don’t know the physiology of twinning. Although a careful examination of the placenta is helpful it’s not one hundred percent accurate. Approximately 20 to 25 percent of identical twins can have separate placentas! On the other hand, two separate placentas from a set of fraternal twins can fuse together giving the appearance of one. It’s estimated that 55 percent of twin zygosity can be determined through the sex of the twins and placentation; the remaining 45 percent requires further DNA testing.

identical twin boys with red hairNo wonder parents then carry the misdiagnosis into their homes where it’s perpetuated for years to come. It doesn’t help that at birth, identical twins can look vastly different. One twin can be significantly smaller due to a number of factors or their head shapes can be different from each other. So the incorrect zygosity makes sense to the whole family. Even as these twins age and they begin to look more alike, their zygosity is no longer questioned. It’s difficult to believe otherwise.

Furthermore, when you really think about it on a very basic level, it’s only natural for a twin, especially one who is older and aware of his own feelings, to believe that he is very different from a cotwin. As he grows into a young man and his quest for autonomy and individuality increases, it’s a normal reaction to think he is unique and different from all other human beings on the planet, even from his own cotwin.

The only true way of determing twin zygosity is through DNA testing. These days, home testing is not only reliable but relatively inexpensive running about $150. Many researchers believe that it’s vastly important for all twins to be tested at birth to determine their true zygosity.

How did you determine the zygosity of your twins?


15 thoughts on “Why Do So Many Identical Twins Think They’re Fraternal?

  1. Alyssa

    We had boy/girl twins so zygosity was not an issue. We knew they were fraternal! But to add they started out with two plcentas but about 4 wks or so before they were born at 36 wks 2 days they fused. Whent they were delivered there was only one placenta. Our twin B a girl also developed SEVERE HYPOglycemia and spent 3 weeks in the NICU on glucose through IVs. She should have died 3 times her levels dropped so low. No explation on why she had no energy reserves. Curiosty has sparked but there is no real research on whether or not fused placentas can develop into twin to twin transfusion, which could explain our daughters condition at birth. Alas there is no research though.

      1. jessica lowenthal

        Hi – I am currently pregnant with boy/girl twins via a 5 day IVF transfer if 2 blasts. My RE initially said they were identical based on the 6 week and 9 week ultrasounds. She did not see a membrane at the first U/S and saw a thin one at the second. Any thoughts on how you might explain a definite diagnosis of identical twins resulting in boy/girl twins? Is it possible that the embryos fused after transfer?

        1. Christina Baglivi Tinglof

          wow! Fascinating! But so many questions: You say they’re b/g twins but identical? Very, very, very, very rare. Was the zygosity diagnosis based on an ultrasound? If so, mistakes can easily be made. The only true way to confirm zygosity is through DNA testing (amnio or other such procedure). Maybe they aren’t b/g and that would explain it all.

      2. jessica

        Well, I think they are probably not identical because gender is very clear…normal looking boy and girl. I think the identical piece was the incorrect part, but the doctor was so confident so I am just trying to figure out what could account for them looking like they were in 1 sac. There is a membrane visible but the perinatologist says it looks so thin, more like identicals than fraternals. Have you ever heard of a case where embryos fused, thus looking like they shared a sac?
        Thanks for your feedback.

        1. Christina Baglivi Tinglof

          Yes (but I don’t think fuse is the right word), from what I understand, as a woman progresses through pregnancy, the membranes begin to thin and it’s harder to differentiate between the two separate sacs.

  2. Nanner

    AWESOME post! Is there research behind this? (it’s late and my vision is fuzzy, sorry if I missed it!)
    I’ve been claiming for a while now that I think the current statistics on identical twinning are completely inaccurate – partly for this reason. Having 2 sets of identical twins, and talking to other mothers of twins, I’ve realized how many of them absolutely repel the idea that their twins are MZ because they are ‘so different’. Yet I know MZ twins with different sizes, hair colors and builds. Andy yes, I was told more than once by medical professionals that my twins (either set) was not identical or was most likely not identical.

    I think someday we’ll perhaps discover that MZ twinning is much more common that previously thought, and in some cases less ‘random’ than previously thought. But having 2 sets of MZ twins, I am slightly biased!

  3. Elizabeth M.

    Great subject! I’ve also been surprised by the ignorance surrounding zygosity- by the medical community and twins/parents of twins themselves.

    Adding to the confusion is the common assumption that identical twins are somehow “natural” and not possible to conceive via IVF. Quite the opposite is true- in fact there is an increased rate of identical twinning in IVF pregnancies. But in an odd twist the majority of identical twins born via IVF are transferred with another blast, so the result is triplets. Data isn’t too clear on this, but the rate of identical twinning per embryo transferred in IVF may be around 2-5% instead of 0.4% in a spontaneous conception.

    Of course all of those MZ twins are mono/di or mo/mo because IVF transfers are done at 3 or 5 days and di/di MZ cleavage occurs between 1-3 days.

    Our first big clue that my di/di boys were MZ was that their blood type is the same. At their 4 mo checkup their pediatrician said that she thought they were starting to look alike and recommended that we do the DNA test if we were curious.

    I’m glad we did the test. There are practical reasons for knowing if they are identical- we’ll be able to tell if one of them falls behind developmentally, we know they are cool with being on the same schedule (naps, feeding, etc) and tend to like the same things. It’s amazing how their teeth came in in the same order, they started crawling within days of each other, and even started walking within an 45 minutes of each other.

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  5. Sarah

    Interesting….my 9 month old, boy IVF twins are fraternal. We put two eggs in and had two placentas…..although they look quite different (more to us than others), I wonder what the chances are that they’re identical? I did not realize there was so much grey area.

    1. Christina Baglivi Tinglof

      I agree. I keep saying I want to have all three of my children DNA tested (twins and a singleton) just out of curiosity. Would the fraternal twins have more genetic markers in common than the singleton? Or would one twin have more in common with his singleton brother? All so very fascinating.

      1. Elizabeth

        I think the best value is to use https://www.23andme.com for genetic testing. I paid $160 for a DNA test that only checked the zygosity of my boys. 23 and Me will identify their zygosity and tell you much, much more about their health & ancestry. It’s a really cool service and only costs around $200.

        1. Christina Baglivi Tinglof

          I’ve heard of this company (I think I saw a segment on the Today Show). They do tell you a lot including what part of the world your ancestors are from. BUT, experts say, some of the information they offer may be too much information. For instance, you mention “health.” Some people may get disturbing news and undergo expensive and invasive testing only to find out that they’re perfectly fine. It may be a double edge sword. (Still, it is tempting, isn’t it?)

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