Identical Twins: Why does the fertilized egg split?

Today I’m putting on my science hat and attacking the question which burns my curiosity as well as many others. Why does a fertilized egg divide to create identical twins? It is well established that this is not a hereditary tendency and yet around 0.3% of babies are born a natural clone. So why does one become two?
The quick answer is… no-one knows. But like all other outstanding questions, there are plenty of scientists trying to find out. So before we consider the theories, here’s a summary of what we do know:


How identical twins form

Unlike fraternal twins (which come from the fertilization of two eggs), identical twins are formed when a single zygote (fertilized egg) splits into two. This natural cloning process will happen at some point in the first ten days of gestation, usually around day 4-5. After a human egg is fertilized, the zygote ‘collapses’ and expands a number of times as the cells multiply/ It’s during one of these ‘collapses’ that the magic moment happens. During a collapse, the cells contained in the embryo divide into two groups, allowing two separate embryos to ‘hatch’ from the protective outer layer of what is now called the blastocyst. These two matching balls of cells keep dividing and multiplying to create two separate, yet identical, babies.

A blastocyst forming two groups of cells within it. Source:

‘Twin Towns’

Allahabad in North-East India is just one of a number of ‘twin towns’. For the past 40years Allahabad has experienced an unusually high number of identical twin births. Locals are sure there is something in the water or soil causing this, as they claim animals such as buffalo are also affected by the phenomenon. However, scientists are yet to find any environmental or social oddities and DNA from Allahabad twins has not given much insight either. Most researchers have written off the theory of twin towns as no more than a statistical fluke. Another twin town, Linha São Pedro in Brazil, reported in the 1990’s that 5% of births were identical twins… that’s quite the fluke.


The Nine Banded Armadillo

Nine-banded armadillos almost always give birth to four identical quadruplets. These quadruplets are created from one single fertilized egg that splits, and then split again. This is thought to be an evolutionary result of the physical constraints imposed by the shape of the armadillo’s uterus. However, while it may be seen as an evolutionary advantage for the Armadillo, twinning of embryos in humans is considered more of a pregnancy fail by scientists, because it holds higher risks of congenital anomalies and premature birth. Still, for armadillos it’s not a random event so something is causing it to happen…

Source: nature,com

Identical twins and IVF

When receiving IVF treatment, the chances of having identical twins rises from 0.3% to 2%. So even if only one embryo is implanted there is a 2% chance it will divide and create two babies. No-one knows why the chances of embryo division is higher, although it is suggested that subtle chemical differences between the lab and the human body are to blame. Maybe it’s just all that prodding and poking?


So… these are all interesting points, but what scientific theories are out there for why identical twins develop from one fertilised egg?

Genetic mutation:

Dr Bruno Reversade has been busy investigating variations in the genomes of families from ’twin towns’ in the hope of finding a twinning gene. He has a candidate region, on chromosome four, and he thinks mutations in a gene here might have been present in the founders of twin towns, and then spread through the population. He speculates that the mutated gene might prevent cells sticking together tightly within the blastocyst, resulting in a split.

Dr Dianna Payne thinks the imperfect environment of the IVF lab is causing some cells to die or weaken at the cell junctions. This leads to the idea that a similar process could be caused in naturally conceived embryos, if they were triggered by faulty genes. It is also possible that subtle differences in the cells could force cells to repel one another, pushing two separated groups of cells to opposite sides of the blastocyst.

Daddy did it:

There is a popular theory (although I have struggled to find any scientific sources) that an enzyme in sperm causes the embryo to split. Many families with twins claim they have an abnormal number of identical twins in their family tree, even though at present science still claims it’s a random event.


It’s all about timing:

Dr Judith Hall suggests twinning depends on the timing of fertilization, explaining why humans twin more than other animals (except the armadillo of course!). Most mammals choose to mate when conditions are perfect, when eggs have been freshly ovulated. Humans on the other hand just do it any old time and an old egg may be more likely to split.


So…there you have it.

I think if you combine Payne and Hall’s ideas to suggest an old egg is ‘faulty’, leading to abnormalities which create a repulsion between the cells, you have a fairly sound theory. After all, most identical twins don’t make it – it has been suggested that 12% of natural conceptions produce identical twins – but the vast majority of embryos are lost. Occasionally this can even result in a ‘vanishing twin’ where one embryo continues to full term while the other is lost and absorbed by the body.

Essentially something has gone wrong with the usual fertilization process to create two babies from one, although as an identical twin mom myself I think it’s just magic. Take your pick, what do you think causes identical twins?


Related Posts:


Cyranoski, D. (2009, April). Developmental biology: Two by two. Retrieved from:

Study: Identical Twins Caused by ‘Embryo Collapse’ (2007, July). Retrieved from:

The extraordinary moment one baby becomes two (2007, July). Retrieved from:


  1. I vote for magic, or miracle, or mystery. I’m like Heather, mysteries yet to be solved! That keeps us humble and identical twins a marvel.
    I really enjoyed learning from your essay. I like the way you gave titles and examples. So very interesting!

  2. An interesting read. In my family, every alternated generation has a twin. SO it looks like its sort of genetic. My granfathers grandfather had a twin. It then moved to my grand father who himself was a twin. My sister then delivered twins.

    But somehow it still intrigues me..

    1. TwinPickle

      Interesting! These were all identical twins? Fraternal (non-identical) twins are indeed hereditary as its the tendency to release multiple eggs that is passed on in the genes. I always think its interesting how things skip generations, I’m sure there’s a reason… maybe I’ll look it up for another post!

    1. TwinPickle

      Fraternal (non-identical) twins do indeed run in families. Some can look really similar, others really different, just like regular brothers and sisters. It’s all so fascinating indeed! Thanks for reading 🙂

    1. TwinPickle

      They do! Sometimes I love the attention and other days I feel for my daughter who is no longer the star attraction. And sometimes I just want to be left alone so I can do my shopping 😂

  3. Wow! This is absolutely fascinating!! I, like a lot of others, find identical twins soooo interesting! When I was little, I always wished so much that I had a twin! Ive never actually looked into why this happens though-the possible explanations outlined here would be so interesting to explore further. I didn’t realise that it’s seen as kind of a body ‘fail,’ and that 12% of conceptions are twins, but so many don’t make it. The old egg theory does seem the most likely. This is all so interesting! My cousin married into a family where there’s so many twins-but the non identical variety, which you’ve said aren’t really mysterious. But her husband is a twin, he also twin sisters, his dad was a twin, and his brother and sisters have all had twins!! It blows my mind a little!!

  4. Kay

    I’ve known several cases of identical twins seeming to run in families so I’ve never bought the claim that it’s completely random. I’ve always wanted to have my own twins (is that totally crazy?) and since my sisters are twins, I still have my fingers crossed that it might happen for me. Really like your blog.

    1. TwinPickle

      Thankyou. And it’s not crazy to want twins at all… they are awesome! Good luck with your quest… maybe there are some good old wives’ tales for weird stuff you can do to improve your chances?! Haha. I will have to look that up for a future blog post 😉

  5. KristenTony

    TwinPickle, i love your articles! My best friends all through school were (well, still are!) identical twins. absolute mirror image until about 8th grade, and starting with one or two features changing! Genetics behind twins are so fascinating. thanks for another great article!!

    1. TwinPickle

      Thanks for your enthusiasm, I’m so glad you find it as fascinating as I do! One of the boys managed to chop the end off his finger a couple of weeks ago so we didn’t quite make it to 8th grade remaining mirror image! 😬😬

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