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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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.

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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 can be mirrored; but not the appendix or heart or other internal organs as far as we know. We have some examples of our own mirroring: 

• We are opposite handed (Flame is left and Lake right)
• We tend to have mirror image dental problems
• Our cowlicks (tuft of hair at fringe) are on opposite sides
• Where people often have 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 all 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 really seem to experience mirror image aptitudes and have instead have very similar interests and skill levels.

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

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

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"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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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. 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.

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