We usually describe twins according to:
- how many eggs they develop from – one or two
- whether they share a placenta in their mother’s womb.
The main types of twins are fraternal twins and identical twins.
All pregnancies start when a sperm fertilises an egg. This fertilised egg is called a zygote.
Sometimes a woman’s ovaries release two eggs, and two separate sperm fertilise each egg. This forms twins. These twins are called fraternal twins, dizygotic twins (meaning two zygotes) or non-identical twins.
During pregnancy, the developing babies get oxygen and food from their mother through the placenta and umbilical cord. Fraternal twins have separate placentas and umbilical cords. The technical name for this is dichorionic.
Fraternal twins can be the same or opposite sex and their genes are as different as any other brother and sister. Often, same-sex fraternal twins look different – for example, they might have different hair or eye colour. Occasionally they look quite similar.
Fraternal twins happen in around 1 in 40 pregnancies in Australia.
Sometimes a fertilised egg splits within a few days of conception to produce genetically identical twins. Because these twins come from one zygote, they are also known as monozygotic. Identical twins are the same sex.
There are three types of identical twins.
About one-third of identical twins split soon after fertilisation and form completely separate twins. Like fraternal twins, these twins have separate placentas.
The other two-thirds split after they attach to the wall of the womb. As a result, they share a placenta. The technical name for this is monochorionic.
In a very small number of identical twins, splitting might happen even later. In this case, both twins share an inner sac, called the amnion, in addition to sharing a placenta. The technical name for this is monoamniotic twins. They’re often called MoMo twins.
Identical twins often look different. This can lead to identical twins being wrongly thought of as fraternal.
Identical twins happen in around 1 in 250 pregnancies in Australia.
Sharing a placenta or inner sac: why it’s important to know
Sharing a placenta means that twins share a blood supply during pregnancy. Sometimes the blood supply is shared unequally, which can cause health problems for both twins.
Women who are pregnant with identical twins sharing a placenta need to be checked more often than women with twins with separate placentas. Frequent checks can pick up early on any potential complications.
Twins sharing an inner sac (monoamniotic) are also at a higher risk of complications during pregnancy because of the chance that their umbilical cords might tangle and cut off their blood supply. These twins are checked even more closely. Medical professionals often recommend that these twins are born earlier than other types of twins.
Fraternal or identical: why it’s important to know
Same-sex twins with separate placentas can be fraternal or identical.
You can do a genetic test, called a zygosity test, after your babies are born, to find out whether same-sex twins are identical or fraternal. The test doesn’t hurt and involves collecting a sample of cheek cells by rubbing the inside of your babies’ cheeks with a soft applicator (like a cotton bud). The cost of this test starts at $120 (including both twins) in Australia.
It’s a good idea for twins to know their zygosity for health reasons, because every health condition has a genetic part to it. Identical twins are more likely than fraternal twins to get the same illness.
If one of a pair of identical twins is diagnosed with a particular disease or health condition, like high blood pressure, the other has a very high chance of developing the same condition. This means that the other twin should be checked often for early symptoms.
Because of their genetic make-up, identical twins will always be compatible for organ transplantation, if they ever need it. Fraternal twins are compatible only sometimes.
Other reasons that twins and parents of twins have for wanting to know whether they’re fraternal or identical include:
- wanting to confirm or find out what type of twins they are
- trying to work out the chance of having more twins in future pregnancies (only fraternal twins run in families)
- making sure they have the right information about their genetic make-up
- being able to answer questions from family, friends and others
- being involved in twin research
- just being curious!
Some identical twins are mirror twins – for example, their hair parts on opposite sides, they are oppositely handed, or they have birthmarks on opposite sides of their body. In some rare cases, their internal organs can be mirror images of each other. It’s not known why some twins are like this.
In very rare cases, twins can be born physically joined together in different ways. These twins, called conjoined twins (previously called Siamese twins), can happen if the fertilised egg splits quite late after fertilisation.
Triplets (1 in 5000 births) and quadruplets (fewer than 1 in 100 000 births) can develop as a result of combinations of fraternal and identical twinning. There are no reliable figures for quintuplets (five) and sextuplets (six).
More facts about twins
In Australia, twins happen in 1 in every 70 births. This means that 1 in 35 Australians is a twin.
One-third of twins are identical and two-thirds are fraternal.
The birth rate of identical twins is the same around the world and doesn’t vary with the mother’s age.
In contrast, the birth rate of fraternal twins varies widely across countries and can be influenced by the mother’s age. Women aged over 35 years are the most likely to have fraternal twins, because their ovaries are more likely to release more than one egg at a time.
More twins were born from the 1990s to the mid-2000s. This is because it was common to transfer more than one embryo during in-vitro fertilisation (IVF) treatments. The rate of twins from IVF is now lower, because it’s now usual practice to transfer only a single embryo.