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 about 70% of twin births in Australia.
Very rarely, fraternal twins share a placenta. These types of twins are called chimaeric twins.
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 happen in around 1 in 250 pregnancies in Australia.
Although identical twins have the same genes, they don’t always look the same. This is because children’s health and development are shaped not only by genes but also by experiences in the womb and after birth. For example, a twin who receives less blood from a shared placenta might weigh less at birth.
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 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.
Medical professionals use ultrasound to work out how many placentas twins have. The earlier the ultrasound, the more accurately it can say how many placentas there are. It gets harder to work out later in pregnancy. After the birth, the placentas should be looked at to confirm or determine what type of twins they are.
Fraternal or identical: why it’s important to know
Same-sex twins with separate placentas can be fraternal or identical. For health reasons, it’s good to know whether your twins are fraternal or identical.
To find out whether twins are identical or fraternal, you can ask for a genetic test after your babies are born. This is called a zygosity test. 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.
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 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 why twins and parents of twins might want to know whether they’re fraternal or identical include:
- 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 (less 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 about twins
In Australia, twins happen in 1 in every 80 births. This means that 1 in 40 Australians is a twin.
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.