Twins & Multiples

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Mirror Twins: Causes, and What It Means for Your Family

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Have you wondered why some twins who look so identical to each other are so completely opposite? Or looked at two children who seemed identical, but there was something unique about each one that you just couldn’t put your finger on?

It turns out that there might be a scientific reason for it!

They might be something called ‘mirror twins’ or ‘mirror-image twins,’ a phenomenon that scientists have only recently started to look into.

Even more shockingly, it turns out that as many as 1 in 4 identical twins could actually be mirror image twins, without even realising it.

But what does this mean? What is a mirror twin, and how are they different from normal identical twins?

twin crib

What Causes Mirror Image Twins?

Normal identical twins are caused when a single fertilised egg splits between the fourth and eighth day into two identical embryos, then each embryo goes on to grow into a healthy baby.

Because both babies come from the same fertilised egg, each twin has the same genes, so they end up growing up ‘identical’.

However, for these twins, it’s thought that the egg splits later, between days nine and twelve, and this somehow causes the twins to act as mirrors of each other.

Are you expecting twins? Are you the new parent of twins? Join our community here and join the conversation.

How Are Mirror Twins Different From Identical Twins?

Whilst normal identical twins are, well, identical, mirror image twins look the same, except reversed. Like you’re looking at them in a mirror.

It can be physical, with simple things like hair parting different ways or moles and birthmarks appearing on opposite sides.

It can manifest as one twin being right handed and the other being left. (Which is really common, by the way, so if you have identical twins who are differently handed, they might be mirror twins!)

It can even come through in really odd ways. One pair from the UK both lost their first tooth at about the same time, and they both lost one of the same type of tooth, an incisor. Except one girl lost her bottom left tooth, and the other lost her bottom right!

In the most extreme cases, one mirror twin will have internal organs in different places! Their heart and vein structure will be flipped, as if their entire body has been reversed! Thankfully, whilst this is a bizarre syndrome, it’s not harmful in any way!

Mirror Image Twins With Mirror Image Personalities​

For some reason, many also have mirrored personalities.

For example, the twins we mentioned earlier who both lost a tooth at the same time? They’re described by their parents as ‘the quiet one’ and ‘the lunatic.’

This is pretty normal. Another pair, even though they’re a little older now, realised they were mirror twins in the middle of a college psychology class. For their whole lives they’d wondered why they were so alike in looks but but so far apart in personality, and now they knew!

Finally, these twins shared all of the usual signs. One was left handed, the other was right, they both needed braces when they were children, but their tooth placements were a reflection of each others, but when it came to personalities, both kids couldn’t have been more different.

One girl, Erin, describes herself as the quiet one, and this is something she loves, as it’s one obvious way that they’re different, whilst her sister Ash is incredibly outgoing and will talk to anyone, no matter where they are.

But this doesn’t happen every time. In this book on twins, written by recognised behavioural specialist and twin expert Dr Nancy Segal, two twins who were separated at birth and only met for the first time after 39 years were found to have a huge amount of similarities, far beyond what could be coincidence!

For example, both twins suffered from tension headaches. Weird, but something you can understand. But both twins also bit their fingernails, even as adults. Both smoked, and even smoked the same brand of cigarettes! Both shared hobbies, and as a finale, even though neither of them knew the other, both liked the same sort of holidays, and regularly took holidays on exactly the same beach in Florida! If that’s not a signifier of the special bond twins can share, I don’t know what is!

It’s not just interesting, though. This phenomenon is incredibly helpful to scientists studying genetics, because of the way mirror image twins manifest the same things, but on different sides of their bodies.

This proves that there’s a massive genetic component to things like personality, likes and dislikes. On top of that, because they share tooth placement and even little things like where and when wisdom teeth come through, it means things like that are written into our bodies when we’re first conceived!

By studying twins, it enables scientists to work out exactly what is and isn’t determined by our genetics, and work to cure the things that aren’t helpful, like diseases and even cancers! All because of twins!cute twins

What Does This Mean For You And Your Twins?

If you’re having identical twins, there’s a pretty good chance that you might be lucky enough to have mirror twins!

They are nothing to concern yourself about. In fact, it’s just another way for your twin treasures to be unique.

It can also be helpful. For example, if you have them and one starts teething early, you can be pretty sure the other one will too. This will carry on all their lives, and it means you can use one as the gauge for the other. Obviously, both of your children are going to be individuals, with their own unique likes and dislikes, but it’s a good start to go on, and more than most new mothers get, especially the first timers out there!

The only real issue, and this is something we’ve heard multiple times, is working out who is who, especially when they’re a little way away from you! It might be worth investing in color coded clothes or unique hair decs now!

Have you been told you’re going to have mirror twins? Maybe you’re already blessed enough to have a set in your life? Either way, we want to hear your story in the comments below!


  1. I gave birth to twins o 4/20/12..on is left and one is right handed…they r very opposite in personalities and i have always said that if u put them together they would b like 1 whole person…im curious if they r mirror twins..

  2. I have identical twins born in July 2013, and I suspect they are mirror twins too. One is left handed and the other is right handed. Not much else to go by in terms of physical traits, but every step of their development has been in lock-step from when they were infants: teething, walking, talking, etc.

  3. My twin girls couldn’t be any more different. They are identical twins born 2012. One is right handed, the other left handed. One has always said she wanted to be a boy (boy clothes, toys, activities, even chooses to play the male part in pretend playing with friends, has mostly best friends that are boys). The other twin is super girly.

  4. We have mirror twin boys, and they have corresponding moles on opposite sides, one is right-handed and the other left-handed, and their hair swirls in opposite directions. They say identical twins don’t run in families, but I disagree. My grandpa had mirror twin brothers!

  5. I have a set of 8 year old twin girls. They look just alike one has a birth mark on her right side of shoulder and the other on her left. Their curls go in opposite directions. One is a lefty the other a righty. And the funny thing is I am the first in my family to have twins and on Dad’s side as well. But the mirror image things gets me all the time. And they are so close. They make me wish I was a twin.

  6. My sister and I are mirror twins:

    1. Our hair parts on the opposite side.
    2. We both never had wisdom teeth, just an identical
    impacted upper one that never came through, mine on
    the right, my sister’s on the left.
    3. We sleep on opposite sides on our beds.
    4. We fold our arms the opposite way.
    5. I’m straight, my sister’s gay.
    6. We have an identical quirky toe on the opposite feet.
    7. I like creative writing, my sister likes drawing.

    And so on and so on.

  7. I have identical mirror twins. They are very similar and compatible. One is left handed with a left dimple and one is right handed with a right dimple. They are best friends.

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