Identical vs Fraternal Twins: What’s the Difference?

Team Peanut10 months ago6 min read

So, twins it is. Twice the excitement, twice the fun, twice the love, twice the… tiredness. But let’s get down to the nitty-gritty details of twins. Identical vs fraternal twins – what actually is the difference?

Fraternal twins

You’ll see some twins look like peas in a pod – so similar that even their mamas can’t always tell them apart. Then there are the others: just as cute, but clearly distinct. Which ones are identical and which ones fraternal? Take a guess!

The difference comes down to what happens when the egg and sperm meet. In identical twins (the first one, by the way!), the egg gets fertilized and then splits – making two (or three, why not?) identical babies. In fraternal twins, two different eggs get fertilized, producing two babies with different looks and often different genders.

That’s the long and short of it. But let’s take these two types of twins in a little more detail.

What is the difference between fraternal and identical twins?

So, what’s actually the difference between the two types of twins? Well, mamas, pens at the ready. There are some technical terms approaching.

Monozygotic twins and dizygotic twins

While we all know these two types of twins as identical and fraternal, scientists do things differently. To them, identical twins are better known as monozygotic twins. Fraternal twins, meanwhile, take the name dizygotic twins.

Why? A zygote is the term for the fertilized egg with the sperm inside. In identical twins, there’s one of these (mono meaning ‘one’). In these cases, one fertilized egg splits in two to make two genetically identical babies. In fraternal twins, there are two zygotes (di meaning two) – and the babies produced are genetically different.

This may sound all good and academic. But it’s important. Particularly because identical twins are not necessarily identical. They are genetically identical, yes, but they can sometimes look quite different – and can in rare circumstances even be different genders.

Fundamentally, then, the difference between fraternal and identical twins comes down to those things called zygotes. But this little difference will have an effect on the practical matters of birth.

Because identical twins come from one egg, they may share the same placenta and the same amniotic sac (the bag of fluid that protects the fetus in the womb). Fraternal twins, on the other hand, will have separate placentas and separate amniotic sacs. These things are quite handy to know. Even if, like most mamas of twins, you may opt for a caesarean birth.

Hang on – what are the three types of twins?

Identical vs fraternal twins vs…? Is there a third type of twins? Well, according to researchers, there may well be. And they’ve gone and christened them sesquizygotic twins.

And what are they? The idea is that, in this third type of twins, two different sperm fertilize the same egg at a very specific moment. It’s pretty rare. But it means that, rather than 100% genetically identical (as in identical twins) or about 50% identical (as in fraternal twins), these twins are somewhere in the middle. Cool huh?

By the way, we haven’t forgotten conjoined twins. However, these are technically a type of identical (monozygotic) twins. They occur when the egg doesn’t split fully into two separate parts.

What are more rare, fraternal or identical twins?

Out of the three types of twins, the (checks spelling) sesquizygotic twins are the rarest. But which is rarer between fraternal and identical twins?

Well, all in all, according to the US Center for Disease Control and Prevention, twins account for about 32.6 of every 1000 live births in the US. That means your chances of having twins are about 3.26%. However, this rate changes across the world – and it may be higher if you’re having IVF. According to the NHS, while 1 in 80 births in the UK are of twins, that increases to 1 in 5 after IVF.

Between fraternal and identical, though, it’s the fraternal twins that are more common. And it’s more common for you to have twins of opposite genders than of any other gender pairing. All pregnant women have the same likelihood of giving birth to identical twins. That’s about 1 in 250.

Twins are more likely in mamas who have had twins before – and in those who have other women with twins in the family. Older women too are more likely to give birth to twins – as they are more likely to release more than one egg during ovulation.

Can fraternal twins be identical?

That’s most of the science covered. But how are you to know if you have had identical twins or fraternal twins? And can fraternal twins look identical even if they are not genetically identical?

To start with, fraternal twins can look pretty similar, yes. But they can also look completely different. Fraternal twins follow siblings generally: they can be different genders, have different hair color, eye color, sizes, and facial features. But you’ll almost certainly be able to tell they are siblings.

How to tell if twins are fraternal or identical?

However, you may find that, as newborn babies, you can’t really tell whether your Tweedledum and Tweedledee are identical or not.
To find out after birth, the placenta can be a good place to start. Whether one or two is present can be a good sign of whether you’ve been blessed with identical or fraternal twins (although it is not always an accurate method). Otherwise, health services recommend using a DNA test to see what’s what.

It is generally recommended, though, to find out during pregnancy whether you are dealing with fraternal or identical twins. It’s important to know because, if your twins are sharing the same placenta and amniotic sac, complications are just slightly more likely to occur.

Similarly, knowing whether they are identical or fraternal is important knowledge for later in life too. Identical twins are more likely to have the same illnesses, for example. And, let’s be honest, it is just quite nice to know.

Identical vs fraternal twins

So, there you have it! If there’s anything else you want to know about preparing for double trouble, there’s always a Peanut mama (or two, or three) that’s happy to help.

Read also:
Do Identical Twins Have the Same DNA?
Twin Telepathy: What’s the Truth?
How to Conceive Twins
Can You Take a Paternity Test While Pregnant?
A Short Guide to Identical Triplets