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!