Identical Twins: What Are Monozygotic Twins?

Identical twins have been intriguing human beings for millennia.

From Greek mythology to Shakespeare to the absolute classic that is The Parent Trap, the idea of having a real-life doppelganger for a relative is perfect fodder for a good story.

But first off, if you’re on the road to having identical twins yourself, congratulations!

And congratulations again!

Before you know it, seeing double will just be part of your daily routine.

Now, your next job is to find out all about the different types of twins, and what having identical twins means for you and your family.

In this article: 📝

What does identical twins mean?

If you aim to impress people at dinner parties, whip out this term to describe your identical twosome: monozygotic twins.

While it sounds like it belongs to a delightful duo of sci-fi characters, “monozygotic twins” breaks down to provide a pretty good description of how identical twins are formed.

Mono means one.

Zygote is the fertilized ovum that is to become your baby.

Put that together, and monozygotic twins mean twins formed from just one zygote (and therefore just one egg).

Voilà, identical twins!

So why is this significant?

Well, it marks the key difference between the main types of twins.

And what is it that makes twins identical?

Well, it’s because they’re the product of the same zygote, simple as that!

When do identical twins split?

So just when does the identical twins’ monozygote split?

Very early on, it turns out ‒ around day 2-6 of your pregnancy, so within the first week.

If division takes place after day 12 this can lead to conjoined twins.

Are identical twins in the same sac?

Do identical twins share the same amniotic sac?

No usually, no ‒ identical twins (or any type of twins, for that point), rarely share an amniotic sac.

In fact, around just 1% of twins share an amniotic sac.

Are di/di twins identical?

Di/di twins means dichorionic diamniotic twins ‒ both babies have their own amniotic and chorionic sacs, and are the result of two separate eggs being fertilised.

Can di/di twins be identical?

Yes, they certainly can.

This happens when one egg is fertilised and splits very early in development.

But not all di/di twins are identical twins.

They can also be fraternal (non-identical) twins.

How to tell identical twins apart

If you’ve just found out you’re expecting identical twins, or you’ve got newborn identical twins, a big question on your mind is probably how to tell identical twins apart?

If they’re true identical twins, they will look… identical.

But our mamas of twins on Peanut have these top four suggestions on how to tell identical twins apart:

• Dress them differently: Different colored outfits for each of your identical twins (or triplets). They can still be matching outfits, just in different colors!
  
• Watch their facial expressions: Even though your identical twins look alike, they’re each their own individual person. Keep an eye on their faces and movements to tell your identical twins apart.
  
• Draw a colored dot on their feet: Just after their baths, draw a colored dot on one of each of their feet ‒ just be sure to use non-permanent, skin-friendly pens. Not keen on colored dots? How about a child-friendly nail polish on one of their toes?
  
• Pick a side: When taking pictures, make sure you have separate sides for your identical twins ‒ one always on the lefthand side, one always on the right. Then, it’ll be much easier to tell them apart when you’re looking at photos in the future!

What are the 3 types of twins?

There are three main types of twins (well, technically seven, but that’s where it gets very complicated!).

Let’s break the big three down:

1. Identical Twins

Yeah, we know… this is old news to you.

Another word for identical twins?

Monozygotic twins.

How are identical twins formed?

Well, identical twins are formed when a single egg is fertilized, becomes a zygote, and then splits into two.

Each side of the split forms into an embryo and then, as time goes on, someone who looks uncannily like their sibling.

They often share a placenta in the womb.

2. Fraternal Twins

The next type is fraternal twins, also known as dizygotic twins.

If monozygotic twins are formed from a single egg, it follows that dizygotic twins be formed from two different eggs.

How does this happen?

When the mama-to-be ovulates, two eggs are released.

Some very productive work happens and both are fertilized.

The result? Two zygotes! Hence: dizygotic.

Fraternal twins won’t be identical twins ‒ rather like two siblings that share a womb, usually a birthday (unless they choose to be born on either side of midnight) and probably a room.

So those are the two main types of twins.

3. Semi-identical twins

To start, semi-identical twins are rare, as in super-duper rare.

They are referred to as, wait for it, sesquizygotic twins.

(To be fair, simply getting that word out of your mouth is probably just as rare.)

They happen when your babies somehow share a placenta (like identical twins might) but are not identical.

While not conclusive, possible theories are that semi-identical twins are formed when a single egg is fertilized by two different sperm.

Now, this is something seemingly impossible because usually after an egg is fertilized, it’s like, “Thanks, I’m done!” and shuts the doors for all other sperm to enter.

But there you have it. Magic.

The research is still quite new in this arena, and not altogether conclusive, but one thing we know is that something like semi-identical twins seem to exist. Mind-boggling.

So those are the three types of twins.

(It’s important to note that there are some other anomalies—adding up to a full seven types of twins.

These include identical mirror-image twins, mixed chromosome twins, and twins that result from superfecundation and superfetation (whatever that is!).

What are mirror twins?

Identical, mirror-image twins are twins that split over a week after conception.

With this ‘late’ splitting, your identical twins will each look like the asymmetrical identical twin of the other.

What is the difference between identical twins and non-identical twins?

Simply put, identical twins look… identical!

Right down to birthmarks.

Non-identical twins may look similar ‒ even near-identical, but they might have some differences.

When is the identical twins ultrasound?

Just when is the identical twins ultrasound? Usually around 8 weeks.

That’s your first trimester ultrasound ‒ basically, the first time you’ll ‘see’ your babies.

This is because the point at which your identical twin babies’ monozygote will split is usually between days 2-6 of your pregnancy.

How rare is it to be an identical twin?

It’s pretty rare.

In about every 1000 twin births, about 3 to 5 will be identical twins—and the chances of you having twins that can very accurately pretend to be each other is not correlated directly to your family history.

In fact, environmental factors and sheer blind luck seem to play a larger role in determining whether you spend the years that follow feeling like you’re living in The Shining.

However, when it comes to IVF identical twins, that’s a more common occurrence ‒ around 0.95% of all IVF treatment pregnancies.

Are identical twins 100% the same?

Yes, yes, yes, identical twins do share the same DNA! 100%.

Remember the part about them being formed by the same fertilized egg?

They really are two parts of the same whole.

Fraternal twins on the other hand only share 50% of their DNA.

That means that, in contrast, they can look pretty different and be of different genders.

Are identical twins genetically the same? Yup, they certainly are!

Do identical twins have the same fingerprints?

Very interesting question.

So, we know that identical twins are the same in pretty much all respects.

However, they are still very much unique individuals and one of the defining characteristics of this is their different fingerprints.

While their fingerprints may bear a huge resemblance to one another, they are ultimately different.

But why, you ask?

Well, fingerprints are not a simple product of our DNA.

Rather, we develop them in the womb—and the two little beings inside you are going to have a slightly different time in there.

That means, no sharing of fingerprint-enabled devices—and no taking the fall for one another if up against the law.

Do identical twins have the same blood type?

Yes, identical twins do have the same blood type.

This is because they’re genetically the same in every way.

Can you have black and white identical twins?

No, it’s not possible to have identical twins of different races.

But you can have fraternal (non-identical) twins with different skin colors.

Can identical twins look different?

What about identical twins that look nothing alike?

They’re fraternal twins, not identical twins.

Can identical twins be of different sexes?

No, identical twins can’t be different sexes.

So if you’ve got boy-girl twins, or girl-boy twins, they’re fraternal twins

But not all same-sex twins are identical twins.

Finally, if you are having identical twins, it’s important that you keep up with regular check-ups and talk to your healthcare provider if anything feels a little off.

Because the process of growing identical twins is extraordinary, the demands it takes on your body are upped significantly.

For this reason, twins are often born premature—around 37 weeks.

Motherhood is one heck of an adventure.

And motherhood when it involves identical twins?

Well. It’s the equivalent of ordering a double.

We wish you double the love on this adventure.

You’ve got this, mama!