Types of twins

It’s exciting finding out that you’re a mum-to-be, but what if it turns out you’re expecting not one, but two babies? Having twins can mean double the joy, but you’ve probably got twice as many questions too! Read on to learn about the differences between identical and fraternal twins, and what the other differences are between the common types of twin pregnancy.

Fraternal Twins (Dizygotic)

Fraternal twins, which are also called dizygotic or non-identical twins, are the result of two separate eggs being fertilised by two separate sperm. This can happen because the ovaries occasionally release not one, but two eggs in the same menstrual cycle – if both are fertilised, it can lead to a twin pregnancy. Fraternal twins can be the same sex (two boys or two girls) or different (one girl and one boy). Each foetus is nourished by its own placenta and protected by a separate amniotic sac. Fraternal twins are as alike as any siblings from two separate pregnancies. They share the same amount of genetic material as any two brothers, sisters or a brother and sister. Fraternal twins are the most common type of twins, accounting for around two thirds of all twin pregnancies.

In Summary

Fraternal twins are the most common type of twins, formed when two eggs are fertilised by two separate sperm. They have as much in common as siblings who were born separately.

Identical Twins (Monozygotic)

Identical twins, also known as monozygotic twins, form when a single fertilised egg splits into two and grows into two separate embryos. The twins are the same gender, have the same blood type and share physical traits such as eye colour, hair colour and facial features.

Identical twins may or may not share a placenta and/or amniotic sac. This is where things get a little bit more complicated, but don’t worry – the general principles aren’t that difficult to understand. We’ll explain this side of things in more detail a little further down.

Identical twins are less common than fraternal twins – around one third of all twins are identical. If you’re pregnant, your overall chance of having identical twins is about 1 in 250. It’s not always possible to tell if you have identical or fraternal twins while you’re pregnant, although sometimes an ultrasound scan showing the placenta (or placentas) – if it’s performed before 14 weeks pregnant – can offer a strong clue. The most accurate way of determining whether your twins are identical or fraternal is a DNA test, which can be performed after your babies are born. Of course, you might decide this isn’t necessary if your newborn twins look very different from each other.

Types of Twins – identical vs fraternal

In Summary

Identical twins are formed when a single egg is fertilised, splits into two and grows into two separate embryos. They are less common than fraternal twins, and – as the name suggests – identical twins share the same eye and hair colour, facial features and other physical traits.

Conjoined Twins

In rare cases the embryo only partially separates, and two identical twin foetuses remain partially connected as they grow. Conjoined twins are extremely rare. Sadly, many conjoined twins do not make it to full term or live only a short while after birth. Conjoined twins who do survive are sometimes successfully separated by surgery.

The Different Types of Twin Pregnancy

Whether twins are identical or not is only one of the differences between the types of twin pregnancy. Other distinctions are made based on whether the twins share a placenta and/or the membranes of the amniotic sac.

These differences are important because each type has different risks, which need to be managed by your midwife or doctor to ensure the safest possibly pregnancy for you and your little ones. To understand the three main types of twin pregnancy, it helps to know that the sac of fluid surrounding and protecting a foetus is made up of two membranes:

  • An outer membrane – called the ‘chorion’

  • An inner membrane – known as the ‘amnion’.

Both membranes are often referred to together as the ‘amniotic sac’ or simply the ‘membranes’. However, when talking about the different kinds of twin pregnancy you might also hear the inner membrane by itself called the amniotic sac – as distinct from the outer membrane, the chorion. So, for example, if your midwife or doctor says that your twins share a placenta and chorion but have their own amniotic sacs, it means they share the outer membrane but not the inner one. This may seem a little complicated, but although there are a few other factors that can vary, broadly speaking, twin pregnancies fall into three main categories. The three main types of twin pregnancy are:

Dichorionic Diamniotic (DCDA) Twins

This is where each twin has a separate chorion, amnion and placenta. All fraternal twins and around a third of all identical twins are this type. This is the most common type of twin pregnancy, and it also carries the lowest risks. You’ll still be monitored more closely than a single pregnancy though, and it’s important to attend all your antenatal appointments and scans so any problems can be spotted and treated in good time if necessary.

Monochorionic Diamniotic (MCDA) Twins

These types of twins are enclosed in a single chorion and share the same placenta, but each is snugly housed in a separate amnion (the inner membrane). Around two thirds of identical twins are this type. Because of the shared placenta, the risks of this type of twin pregnancy are a little higher, so you’ll likely be offered more frequent scans and monitored more closely. You may also be referred to a specialist clinic.

Monochorionic Monoamniotic Twins

These types of twins share the same chorion, placenta and amnion. This is the rarest type of twin pregnancy – only affecting 1 in every 100 sets of identical twins – and it comes with a higher risk of complications because the foetuses can get tangled in their umbilical cords. If you are pregnant with monochorionic-monoamniotic twins, your pregnancy will be monitored closely. It’s also likely that you’ll be offered a planned birth earlier than you would in the case of DCDA or MCDA twins.

In Summary

All non-identical twins and around a third of identical twins each have their own placenta and inner amniotic membrane. The remaining two thirds of identical twins share a placenta and outer membrane, but each have a separate inner membrane. Very rarely, identical twins share a placenta and both membranes of the amniotic sac. As a general rule, a twin pregnancy where the placenta and/or inner membranes are shared need to be monitored more closely.

How Common Is Having Twins?

Multiple births – which include twins, triplets, quadruplets and more – have become more common in recent years, partly because more couples are using fertility treatments. Another factor is that on average mums-to-be are becoming pregnant later, which can also increase the likelihood of a multiple birth. Today in the UK around 1 in every 65 births are twins or other multiples, compared to 1 in every 100 births in 1984.

FAQS AT A GLANCE

  • Fraternal twins, also called non-identical twins, are the most common type of twins – two thirds of all twins are non-identical.

  • If one twin is a boy and the other is a girl, the twins are always fraternal (not identical) twins. Girl-girl twins and boy-boy twins may be identical or fraternal.

  • An ultrasound scan of your placenta could offer some clues, but the most accurate way to tell if your twins are identical or fraternal (non-identical) is with a DNA test performed after your babies are born.

  • Yes, identical twins can have separate amniotic sacs. In fact, around one third of all identical twins have separate placentas and amniotic sacs.

  • Identical twins form when a single fertilised egg splits into two. Fraternal twins happen when two separate eggs are fertilised by two separate sperm. Fraternal twins are as alike as any two siblings. Identical twins share common traits such as gender, physical appearance, hair and eye colour.

The Bottom Line

It isn’t always easy to understand the biology behind twins, but whatever kind of twins or multiples you have they’re sure to be a miracle of nature. In the meantime, if you thought choosing one baby name was going to be tricky, now you’ve got to come up with two! Check out our list of 100+ twin baby names for some inspiration.

How we wrote this article
The information in this article is based on the expert advice found in trusted medical and government sources, such as the National Health Service (NHS). You can find a full list of sources used for this article below. The content on this page should not replace professional medical advice. Always consult medical professionals for full diagnosis and treatment.