7 Different Types of Twins (Monozygotic vs Dizygotic)

Wait a minute… there are 7 different types of twins?! If you’re expecting twin peanuts, first off, congratulations, mama! Now let’s see what kind of twins you’ve got!

You’re about to experience everything from cute matching outfits to the inevitable case of mistaken identity when they pretend to be one another. Double strollers, double the toys, double (we’re afraid to say this) the amount of poop – and, of course, double the love.

Knowing the different types of twins is the first step on this wild journey with your quickly expanding family. We’re going to delve into all things twins.

Right, let’s get twinning!

In this article: 📝

What are the 3 main types of twins?

The two different types of twins that are the most common are fraternal and identical twins.

The difference between them?

Well, it all has to do with a little word that describes the earliest stages of human life: zygotes. A zygote is what happens when an egg is fertilized by a sperm. [1]

In the case of twins, this process gets a little more complicated, however. Either, your body gets super ambitious and releases two eggs to be fertilized, or your one fertilized egg splits into two.

And then there are some third, more complicated options, which we’ll get to later. That’s how you get different types of twins.

But which one is which, and what does any of it mean for you and your family?

Fraternal twins, or dizygotic twins:

Dizygotic or fraternal twins are the result of two fertilized eggs (each fertilized by a separate sperm) growing into two babies at the same time in the same place. That’s two zygotes, one womb. (Sounds like the beginning of an oddball TV sitcom.) The two puts the di in dizygotic. [2]

This means that fraternal twins have different placentas and different umbilical cords, both nourished by the best Café de Maman has to offer.

Fraternal twins are the most common of the different types of twins. Think of them as siblings who decided to be born at the same time, rather than two halves of the same whole. They can be of the same or opposite genders and either look quite different from one another, or pretty damn similar.

Identical twins, or monozygotic twins

Identical twins or monozygotic twins are formed when a fertilized egg splits into two. So that’s one zygote (monozygotic) forming two humans. [3]

And here’s the interesting part: this split can happen at different stages after fertilization. If the egg splits early, within the first 3 days of fertilization, each twin gets their own placenta and sac — everyone has their own space.

If it splits between days 4 and 8, they usually share a placenta but have separate sacs. And if it happens after day 8, they share both the placenta and the same sac, which is a closer setup and needs extra doctor checkups. This is a slightly more rare kind when it comes to different types of twins and accounts for about one in three of every twin set that is born.

If you are pregnant with identical twins, ensure that you schedule regular health checkups as the sharing of the placenta causes a pretty snug setup inside you.

Can you get male and female identical twins?

Why? Quite simply, because they share the same DNA. To make a girl, you need a double set of X chromosomes, one from the mama and one from the papa. To make a boy, you need an XY set of chromosomes (acquired in the same way, of course).

So, because monozygotic twins are the result of the splitting of an egg already fertilized (and thus replete with its chromo-combo), both sides of that split (ie. both babies) are going to have the same sex. And because things are never really cut and dry, there are actually a whole lot more options when it comes to different types of twins.

But you can have fraternal (non-identical) twins that look very similar, with different sexes.

What are the 7 types of twins?

Um, what? Yes. Hold onto your seats. So now you know the first two on the list:

1. Fraternal twins
2. Identical twins

But what are the other 5 different types of twins? Let’s take a look:

1. Half-identical twins: Some researchers think this is what happens when an egg splits and only then gets fertilized. So that means they’re sort of an amalgamation between fraternal and identical twins, sharing all their mom’s DNA but only half of the dad’s. [4]
2. Mirror-image twins: Now, this is something that feels straight out of Wonderland. Mirror-image twins are exactly as they sound. Their features are the mirror image of one another. This happens when the fertilized egg splits later than usual, when the embryo has already developed a right and left side, so when it splits, each twin ends up as a kind of mirror reflection of the other. [5]
3. Mixed chromosome twins: This is a pretty interesting one. They begin their existence as fraternal twins (from two fertilized eggs) which then merge into one. That result? A mixed bag of DNA. [6]
4. Superfetation: Imagine you got pregnant while you were already pregnant. Double-whammy. That’s what superfetation is. This is one of the rarest types of twins. [7]
5. Superfecundation: Now imagine your twins have different fathers. This is known as superfecundation: where woman becomes pregnant while already pregnant with the baby of another man – and gives some serious fodder to soap opera writers. This is another of the rarest types of twins. [8]

Then there are a few other anomalies to watch out for, like conjoined and “parasitic” twins. In these cases, your healthcare provider will talk you through the different options available to you.

What are Didi twins?

“Didi” or “di/di” twins are dichorionic-diamniotic twins. What types of twins are di/di?

All fraternal twin pregnancies (non-identical twins) are di/di, but in some rare cases, paternal twins (identical twins) can also be di/di. [9]

What are MCDA twins?

Monochorionic, diamniotic twins (MCDA twins) are a type of identical (paternal) twins — made from one single fertilized egg. MCDA twins share a placenta, but have separate amniotic sacs.

But this can also be a bit tricky. Identical twins with this setup require more monitoring because sharing a placenta can lead to tricky issues like twin-to-twin transfusion syndrome. [10]

What is the difference between monozygotic and dizygotic twins?

Monozygotic twins come from one fertilized egg splitting, so they will be identical twins, right down to their DNA. Dizygotic twins come from two separate fertilized eggs, so they will not be identical (but they may look alike).

What are rare types of twins?

So what's the rarest type of twins? At the moment, doctors think that half-identical twins are the rarest types of twins. [11]

We say “at the moment”, because there aren’t enough studies done on this particular type of twins. Probably because they’re so rare.

Why it matters to know the different types of twins

Is it even necessary that you know the different types of twins? Turns out, it’s for medical reasons.

Once you know that you are going to be a mama to twins, your healthcare provider will likely recommend a zygosity test to determine what type of twin pregnancy you’ll experience. There are a few main reasons why this is important to determine: [12]

• Identical twins may have identical diseases. That’s because they share the same DNA makeup.
• Identical twins are a sure match for organ transplants, while fraternal twins may or may not be.
• There are some conditions that impact certain types of twins ‒ such as unequal placental sharing (UPS) for twins that share a placenta. [13]
• Plus, knowing the zygosity can help doctors plan delivery and prenatal care better. For instance, babies from monochorionic pregnancies often require more frequent checkups than those from dichorionic pregnancies.

Plus, you’ll likely be super-curious about the little beings cohabitating in your belly. Truly pretty damn amazing.

What is the most common twin pair?

Fraternal twins (twins that don’t look alike) are the most common types of twins. This is also how you can have boy and girl twin babies, or even half-identical twins. They’re basically normal siblings, just sharing a womb.

What are the different types of identical twins?

All identical twins are paternal twins (not fraternal). True identical twins will be the same gender and be identical in every way. Mirror-image twins are another type of identical twins ‒ one of the rarest types of twins in the world!

Can twins have different blood types?

It depends on what types of twins you have. Only fraternal twins (dizygotic twins) can have different blood types.

So if you have identical twins (paternal or monozygotic twins), they will have the same blood type.

What are the chances of having twins with IVF?

Having fertility treatments can increase your chances of having multiple babies at a time ‒ twins, triplets, quadruplets, quintuplets…

This is partly because if you’re going through ovulation induction (where medication stimulates your ovaries), leading to more eggs being released, increasing the chance that more than one fertilizes naturally. But with IVF, especially when doctors do a frozen embryo transfer, they carefully control how many embryos get placed in your uterus.

It’s quite standard for clinics to usually transfer just one or two embryos to reduce the risks that tag along with multiple pregnancies. When it comes to having IVF twins, the percentage chance can be as much as 12% for women under 35, 9% for women from 35-37, and 5% for women over 38.

When can you tell what types of twins you have?

When it comes to determining your types of twins, an ultrasound can only take you so far. To tell what types of twins you have, you’ll have to wait until after birth.

You can either have a twin zygosity DNA test (to tell whether twins are identical or fraternal from their DNA), or your midwife will check your twins’ placenta(s). Identical twins tend to share a placenta, while non-identical twins usually have two separate placentas.

🔎 Dig deeper: Are There Early Signs of Twin Pregnancy in the First 2 Weeks?

There you go! That’s everything there is to know about the 7 different types of twins.

Expecting your own twosome? Check out our Peanut community of mamas to multiples!