About Twins

Monozygotic (MZ) or identical twins

Identical twins occur when one fertilised embryo splits to form two individuals.

Identical twins are genetically the same; however, this does not mean that everything about them will be exactly the same. For example, while the fingerprints of identical twins are more alike than the prints of two completely unrelated people, they are not identical.

Identical twins are always the same sex; they usually are the same height, weight and have similar hair color. In relation to the genetic differences in MZ twins, different genes can be turned on or off which is why one identical twin may have a condition or disease that the other doesn’t. The differences are due to a number of natural chemical modifications that can influence changes to DNA – called epigenetics. Researchers have found that epigenetic changes in twins' genomes (their entire hereditary information) increase as the twins' age and become greater the longer they live apart. Common differences relate to features such as birthmarks, moles, hair patterns and teeth development. Aside from physical differences, there can be even greater differences in their personalities.  

Dizygotic (DZ) or non-identical twins

Non-identical twins occur when two separate eggs are released by the mother at the same time and are fertilised by two different sperm. These two fertilised eggs then implant independently in the uterus. DZ twins are genetically similar – just like normal brothers/sisters/siblings in that they share approximately 50% of their genetic material. DZ twins are still very special because they share the same womb and have very similar environments as they grow throughout life. 

One of the most common questions relating to identical twins is the issue of mirror imaging.

What is it?

‘Mirror image’ is a type of identical twinning. It can happen in any type of identical twins. When the split occurs late - more than a week after conception - the twins can develop reverse asymmetric features. This term is not really a type of twin, just a way to describe their physical features. Twins that split later than this can result in conjoined twins.


The major characteristics of mirror imaged twins are they usually have opposite features such as

  • hair whirls
  • left/right handedness
  • mirror twins can have the same eye conditions in opposite eyes
  • opposite teeth eruption
  • some mirror image twins cross their legs opposite to each other
  • in extreme cases, twins can even have reversed organs

Mirror imaging is never the same for every set of twins – it is a matter of degrees. Not every set of mirror twins is opposite handed and not every set has different hair whirls.


Many assume that mirror imaging is a separate type of twin. They’ll list “identical” ”non-identical” or “mirror image” when completing forms. However, mirror imaging is simply an example of identical twinning.

Identical twins do not have to be identical in every way –and can in fact exhibit more differences as they age.

Ascertaining if a twin is mirror-image or not cannot be determined via a DNA test – this will only confirm if they are identical or not; it can only be done by looking at the twins themselves. 

Polar body twinning is thought to occur when an egg splits - and each half is fertilised by a different sperm. This results in twins who appear very much alike but share approximately 75% of their DNA. 

This was thought to be impossible however a pair of twins were discovered that are identical on their mother's side, but share only half their genes on their father's side.

They are the result of two sperm cells fertilising a single egg, which then divided to form two embryos - and each sperm contributed genes to each child.

Each stage is unlikely, and scientists believe the twins are probably unique. 


Parents of twins have routinely been told that if their twins were born with the one placenta that they were identical. However in recent years there have been a few reports of twins who were told they were identical for precisely this reason - only to later find out that they are in fact non-identical. This is due to something called 'chimerism' - when an individual is composed of two genetically different types of cells. It is thought that approximately 8% of non-identical twins are chimeras; the rise in IVF is considered to be a contributing factor. 

This is a condition affecting identical twins when the babies share a common placenta while in the womb. This can lead to problems in both twins as one twin can receive too much blood (the ‘recipient’ twin), and the other too little (the ‘donor’). The ‘donor’ twin is usually much smaller than the ‘recipient’ twin and can have complications such as anaemia, growth retardation and dehydration. The extra blood received by the ‘recipient’ twin can place strain on their heart. They also have excessive urinary output resulting in increased amniotic fluid, while their co-twin suffers the opposite fate. Early detection is key to ensuring best outcomes for both babies. 

‘Monochorionic/monoamniotic’ literally means one chorion (the outer membrane) and one amniotic sac (the fluid surrounding the foetus). MoMo twins are identical and occur in approximately 1% of pregnancies about day nine after fertilisation. Monoamniotic triplets and higher order multiples are possible, but extremely rare. The only way to detect MoMo pregnancies is via ultrasound;early diagnosis is important as complications can arise which warrant careful, regular monitoring. The most significant dangers relate to cord entanglement and/or compression and twin to twin transfusion.

Twins Research Australia

Address: 3/207 Bouverie St
Carlton, Vic 3010

Email: info@twins.org.au

ABN: 84 002 705 224

Twins Research Australia has received continuous funding from the National Health and Medical Research Council (NHMRC) since 1981, most recently through a Centre of Research Excellence Grant (2015-2022). TRA is administered by the University of Melbourne.

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