Twins are generally identified as identical or non-identical (also called fraternal) though there are some rarer types as well.
Fraternal and identical twins
Dizygotic (DZ) or fraternal twins occur when two separate eggs are released from a woman’s ovaries and each is fertilised by a different sperm cell to form two separate zygotes.
- Both twins develop their own placentas, so are dichorionic
- In about 40% of fraternal twins, the placentas grow together and fuse (due to limited space). A fused placenta can make twins appear identical
- On rare occasions, dizygotic twins can share a placenta (see chimeric twins in Rarer Types of Twinning below)
- Fraternal twins can be the same or different sexes
- Each is as genetically similar as any other sibling
See this diagram
Monozygotic (MZ) or identical twins form when one egg is released and fertilised by a single sperm, forming one zygote. This then splits into two new individuals sometime during the next few days as it develops into an early embryo. These twins are known as monozygotic because they develop from one zygote.
Identical twins can have one or two placentas. The timing of the egg splitting varies, and this is what determines chorionicity (number of placentas).
- If the split occurs before the placenta has started to form (around day 5), then each twin will develop their own placenta (dichorionic). This is what happens in about a third of identical twins. The two placentas can sometimes grow together and may fuse
- In around two-thirds of monozygotic twins, the split takes place after the placenta has already started to form (after day 5), so the twins will share one placenta (monochorionic)
- Rarely, the split takes place even later, when the inner membranes have already formed and the twins then share an amniotic sac
See this diagram
Rarer types of twinning
Mirror imaging is observed in a proportion 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-image twins are they usually have opposite features such as: hair whorls, left/right handedness, the same eye conditions in opposite eyes, and 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. It 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.
People who contain the cells of two or more individuals are called chimeras. Their bodies contain two different sets of DNA. Chimeric twins could occur when developing fraternal twin embryos fuse together and end up sharing a single placenta, an event that usually only happens with identical twins. Although as fraternal twins they do not have the same DNA, they can often share a mixture of genetically-different bloods through the placenta. It is estimated that one in 70 fraternal twins share a placenta and around one in six of these may be chimeric. Chimerism can be identified through DNA testing.
An extremely rare type of twinning is sesquizygotic or semi-identical twins – with only two reported cases in the world, both boy/girl pairs. The twins are identical on their mother’s side but share only part (around 50 percent) of their father’s DNA; hence identified as 75 percent identical. It is thought to occur when the mother’s egg is fertilised simultaneously by two of the father’s genetically different sperm before dividing.
Twins can have different fathers. It happens when the mother ovulates more than one egg and has more than one partner during her fertile period. One egg is fertilised with sperm from one partner, and the other egg with sperm from the second partner. These types of twins are always fraternal or dizygotic.
Superfetation occurs when a woman ovulates more than one egg, but the eggs are released at different times, sometimes up to 24 days apart, and they are fertilised when they are released. The resulting twin pregnancy has different conception dates, so the babies may be quite different in size. Days or weeks may separate the births. It is quite an unusual event. In some cases, the births of twins may be weeks or months apart due to deliberate medical intervention. This is called interval birth.
Unsure if your twins are identical or not?
How can twins be sure of their zygosity i.e. whether they are identical or fraternal twins? Learn more here
Frequency of twinning
In Australia, twins happen in 1 in every 80 births. This means that 1 in 40 Australians is a twin.
The chance of having fraternal twins is higher. Around one third of twins are identical and two-thirds are fraternal.
Over the past decade, multiple birth numbers have remained relatively consistent in Australia. In 2021, 4,248 sets of multiples were born, representing 1.5% of all births (309,996) in Australia. Of the multiples born, 99% were twins and 1% triplets, quadruplets and higher.
The population of Australia is around 25.4 million, so we can estimate there are around 750,000 multiples (based on estimate of 3% of the population is a multiple).
Interestingly, TRA has around 75,000 members, or around 11% of Australia’s multiple-birth population. We thank the multiple-birth community for their wonderful support of our research and we welcome more to join us!
We have resources available for twins (and their families) no matter what their age and stage of life.
Twin pregnancy and early parenting
Find evidence-based information on twin pregnancy, birth and early parenting at TRA’s dedicated website on these topics.
The Australian Multiple Birth Association also has helpful resources for parents and multiples in the early years.
The Psychology of Raising Twins and Multiples looks at the twin dynamic and understanding what it means to be a twin or higher order multiple.
Ready for school
An educational concern that may face parents in the raising of their multiples is whether or not to place them together or separate in the classroom. These resources provide evidence-based guidance:
This resource, published by the International Council of Multiple Birth Organisations, aims to increase understanding of the changes and challenges that multiples face in the teenage years:
Multiple birth siblings as adolescents: A guide for parents of twins and higher order multiples
Dr Nancy Segal is a researcher, psychologist and Director of the Twin Studies Research Centre in the USA. Her articles and books about twin relationships are available at her website including Twin Mythconceptions: False Beliefs, Fables, and Facts About Twins.
Frequently asked questions
Visit our frequently asked questions page here.