TWINS: FRATERNAL, IDENTICAL AND OTHER TYPES OF TWINS

Most twins are born healthy, but a twin pregnancy can mean a higher chance of health complications for you and your babies so you'll need extra check-ups.

Twins that share a placenta or inner sac need to be checked more closely.

It’s good to know whether twins are fraternal or identical for health reasons later in life. We usually describe twins based on:

• 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. Fraternal 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, zygotic 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. 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. Very rarely, fraternal twins share a placenta.

Identical 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: 1. About one-third of identical twins split soon after fertilisation and form completely separate twins. Like fraternal twins, these twins have separate placentas.

2. The other two-thirds split after they attach to the wall of the womb. As a result, they share a placenta. 3. 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. 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 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 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. 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. 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).

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.

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).

Did you find this helpful? Share your story with us!