The human body performs some incredible works of magic, and identical twins conception is one of them. Why does a fertilized egg split to create identical twins? It is well established that embryo splitting is not a hereditary tendency and yet around 0.3% of babies are born a natural clone. So what causes identical twins?
The quick answer is… no-one knows. But like all wonderous outstanding questions, there are plenty of scientists trying to figure it out. These guys are busy looking at when the egg splits to form twins, environmental factors and unusual occurrences of repeat identical twins… are we really sure it doesn’t run in the family?
Before we consider the scientific theories below, 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, but the exact time the embryos split varies, creating different ‘types’ of identical twins.
The different types of identical twins:
- If the split happens early in day 1 after fertilization the twins will form separate placentas and amniotic sacs, these are medically called Dichorionic/Diamniotic (di/di) twins.
- It is more likely the split with occur around days 4-5. By this time the twins will have time to form separate sacs but will end up sharing a placenta. These, the most common type of identical twins are called Monochorionic-Diamniotic (mono/di) twins.
- 1% of the time the split happens around day 9-10 meaning the babies end up sharing a sac. Monochorionic-Monoamniotic (mono/mono or mo/mo) twins are sometimes ‘mirror twins’, causing quirks such as first teeth cropping up on opposite sides of the mouth, and in some cases, even internal organ positions are mirrored.
- Very rarely, the embryo splits around day 13-15, making it impossible for the twins to separate fully, causing the babies to be born conjoined.
The embryo splitting process:
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.
Strange facts studied to try and explain why the fertilized egg splits…
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 splits 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 armadillo, it’s not a random event so something is causing it to happen…
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?
SCIENTIFIC THEORIES AS TO Why does the fertilized egg splitS…
We’ve looked at when it happens, but what scientific theories are out there for why identical twins develop from one fertilised egg?
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.
An enzyme in sperm:
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. Twin dads like to consider themselves carriers of super sperm… maybe they’re right?
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?
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Cyranoski, D. (2009, April). Developmental biology: Two by two. Retrieved from: http://www.nature.com/news/2009/090415/full/458826a.html
Study: Identical Twins Caused by ‘Embryo Collapse’ (2007, July). Retrieved from: http://www.foxnews.com/story/2007/07/05/study-identical-twins-caused-by-embryo-collapse.html
The extraordinary moment one baby becomes two (2007, July). Retrieved from: http://www.dailymail.co.uk/sciencetech/article-465788/The-extraordinary-moment-baby-two.html