Twins FAQ
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interesting twin facts

How many twins are there?

There are approximately 100 million twins worldwide.
The twin birth rate for fraternal twins varies slightly from country to country; identical twin birth rates are pretty much constant all over the world.

In Australia the twin birth rate is currently about 1 set of twins for every 80 births; there are a total of approx half a million twins in Australia.

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Types of twins

Basically there are two different types of twins; identical (monozygotic - MZ) or fraternal (dizygotic - DZ)

Monozygotic twins develop when a single egg is fertilised by a single sperm and at some stage in the first two weeks the developing embryo splits in two, with the result that two, GENETICALLY IDENTICAL babies develop.

Dizygotic twins occur when two separate eggs are fertilised by two different sperm. These two fertilised eggs then develop independently. Dizygotic twins share the same type of genetic relationship as non-twin siblings, hence the term fraternal, meaning brotherly.

Very rarely, the two eggs are fertilised by sperm from different men, leading to cases where DZ twins can be half-brothers or sisters, sharing only their mother's genes.

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Mirror Image Twins

If you are mirror image twins it means that you are also identical twins. Mirror image twins are created when the fertilised egg splits quite late (around days 9-12), any later and twins can be co-joined.

Mirror image twins have small mirror image differences but are actually genetically identical; this means you have the exact same DNA!

Some parts of the body are commonly mirrored; mirroring of appendix or heart or other internal organs is rare. We have some examples of our own mirroring: 

  • We are opposite handed (Flame is left-handed and Lake right)
  • We tend to have mirror image dental problems
  • Our cowlicks (tuft of hair at fringe) are on opposite sides
  • People often have two different size feet; with us the opposite foot is larger
  • We look very similar to each other but the resemblance is even stronger if we stand facing each other (as though in a mirror) rather than side by side. Or get both of us facing into a mirror and things start to look very confusing.
  • Our finger prints don't seem to conform to being mirror image instead they are nearly identical (only one pair of fingers has any major difference)

We've had emails from many other mirror image twins who tell us they also have these types of physical similarities. We don't seem to experience mirror image aptitudes and instead we have very similar interests and skill levels.

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Separation at School

The most common 'school' question - whether to separate your twins at school or not? and if so when should it be done?

We believe that this issue should be judged separately for each set of twins depending on their individual needs. If twins don't want to be separated and are forced apart, it can be a very distressing time for all involved. If twins are happy to be in separate classes than it can be a good chance for them to develop independently.

Be aware that the needs of your twins may change, and they may wish to be together when they are younger and later not want to share any classes at all! Also, sometimes you have the difficult situation arise where one twin wants to be together with the other, and the other twin is keen to be in a different class.

Don't be slow to admit if you make a mistake, and the first choice doesn't work out - most schools are ready to let twins find out what is right for them.

Overall the most important thing is to be guided by your own twins unique situation and do what feels right.

See our Twin Stories page for details about our experiences with separation at school.

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Identical or not? (Zygosity testing)

"Are you identical?" is probably the most common question asked of twins. Twins belonging to a male/female pair obviously have a definite answer (no!) but twin pairs of the same sex can't always respond with confidence.

The most objective way to assess zygosity is to use DNA fingerprinting, that is, testing the twins' blood or another form of physical sample, such as cheek cells, for a range of genetic markers and identifying matches and differences between the samples. If you're in Australia and interested in finding out about testing you can contact The Australian Twin Registry for more information (phone 1800 037 021).

QIMR research has found that in the absence of DNA testing, a good way to assess zygosity is to ask the twins the following questions:
Do you have the same eye colour?
Do you have similar height, weight and natural hair colour and texture?
Were you usually mistaken for one another by non-family members as children?
If the answer to these three questions is "yes", the pair is quite likely monozygotic (identical).

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Twin Research

Most 'twin research' is really medical research performed on twins, rather than research about twins. Because of our genetic similarities identical twins are subjects with a ready-made control group. Identicals share 100% of their genes with their twin.

No matter how carefully a scientist tries to find someone of the same weight, gender, age and other variables, an identical twin is a much closer match for a test subject than someone selected from the general population. This close matching makes research on identical twin subjects and controls very valuable to science.

As the twins in the control group aren't given the medicine in question (or don't take exercise, or smoke, or whatever is being studied), it is more likely that any results noted can be put down to the test topic. Otherwise, different results could be caused by one of the many differences that always exist between members of non-twin test groups, no matter how skillfully chosen.

During the 2nd World War, the Nazi Dr Mengele performed horrible experiments on identical twins - such as infecting one of a pair with a fatal illness. For a long time afterwards memories of those experiments made people shy away from ideas about the importance of genes - that idea was seen as a Nazi idea, popular with racists. It became more acceptable to believe that environment was very important in a person's development. These days, research being performed on twins has helped lead scientists back to the belief that genes are equally important with the environment in predicting such things as a person's health, or IQ, or their talents.

Fraternal twins, who share 50% of their genes are also valuable to researchers. Just like other brothers and sisters, (assuming they have the same mother and father!) they share 50% of their genes and their environment is very similar. However, fraternal twins are the same age, where brothers and sisters may have years between them, and perhaps they experienced different types of family life.

Families change and develop over time, and their diets and other facts of family life also alter over the years. All the changing factors introduce variations in the sibling subjects. Matching between the subject group and the control group becomes less close. Some studies, such as the Australian one into number and distribution of moles on the skin, are performed on fraternal twins specifically to test how much of the result is genetic and how much is related to the environment, in this case factors such as exposure to the sun.

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Do twins run in the family?

A common question females ask is: "My mother (or sister, aunt or grandmother) had twins, what is the chance of me having twins too?"

Fraternal (dizygotic) twins do tend to run in families. This is probably because there are genetic influences which make multiple ovulation more likely. Women who have fraternal twins tend to produce eggs from both ovaries in the same month, rather than from only one ovary at a time.

There is no medical evidence of identical twins running in the family however there is some anecdotal evidence in support of it. Women who conceive when they are over 30 years of age also have an increased chance of having twins.

If your mother, sister, aunt or other close female relative is a twin or has twins your chances are roughly doubled - in Australia for example this would give you around 1 chance in 35.

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For more information check out our Twin Stories page or try our Other Twin Links

Know any fun twin trivia
you'd like included on our site?

In the meantime check out this cool Facts About Multiples site

page last updated April 2002 back to home