What Are Fraternal Twins (or Dizygotic Twins?)

You’ve just had your first-trimester 8 weeks ultrasound, and it’s BIG news! Fraternal twins!

While you’re scrambling to get yourself together and educate yourself on the ins and outs of paternal vs fraternal twins, we thought we’d step in to give you the lowdown.

What are fraternal twins?

Are fraternal twins genetic?

And, seriously, what the heck are dizygotic twins?

Let’s jump in.

In this article: 📝

What are fraternal twins?

You’ve heard the term tossed around a lot in your life—but what does “fraternal twins” mean?

Well, here it is, the fraternal twins definition:

Fraternal twins occur when two eggs are fertilized.

Another word for fraternal twins? Dizygotic twins.

How do fraternal twins form?

Since they’re dizygotic twins, fraternal twins form with two eggs.

So that’s two different eggs, two different sperm, two different tiny zygotes (AKA fertilized eggs), two different amniotic sacs and placentas.

See? Even as zygotes, they are trying so desperately to differentiate themselves

(Spoiler alert: this is the first of many attempts at individuality.)

Fraternal twins are but one type of twin. If you’d believe it, there are (possibly) three kinds.

Yes, of course, we’re going to tell you what they are.

We wouldn’t leave you hanging.

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

What are the 3 types of twins?

Hold on. Three types?

Paternal twins. Fraternal twins. And… something in-between.

The notion of the third type of twin is somewhat contentious.

Medical researchers are divided (like fraternal zygotes, if you catch our drift).

Some believe that a third type indeed exists and it is a mixture between the paternal and fraternal twin types.

This third type occurs when two sperm cells fertilize the same maternal egg, resulting in different sets of genes from dad but identical genes from mom.

So the third type then, if it exists, is somewhat of a hybrid form of the two other types.

What’s the difference between paternal and fraternal twins?

So, identical vs fraternal twins—what’s what?

As mentioned, fraternal twins are dizygotic, meaning they are the product of two separate fertilized eggs.

Paternal twins, on the other hand, are a little different.

Paternal twins are monozygotic and result from one fertilized egg dividing into two.

These are the totally identical type and may even share the same amniotic sac.

They are (almost) inseparable.

Learn more ➡️ Different Types of Twins: Monozygotic & Dizygotic

Can fraternal twins actually be identical?

Yes and no, depending on your definition of ‘identical’.

In the sense that they have a strong chance of looking like each other, sure.

In the sense that they are formed from the same fertilized egg and therefore have the exact same DNA, not really.

If you’re wondering why do fraternal twins look different, the answer is because they are made from two different fertilized eggs.

When it comes to paternal (or identical) twins, this difference wouldn’t be possible as they are the result of a splitting fertilized egg (with the same chromosomes) rather than two separate products of fertilization.

So, the basic differentiator between paternal and fraternal twins? DNA.

Identical twins = identical DNA.

Fraternal twins = non-identical DNA.

How much DNA do fraternal twins share?

Simple: 50%.

Because they’re the product of two different eggs being fertilized.

What are the chances of having fraternal twins?

Well, in the US, the 2021 figures from the CDC show that the twin birth rate was 31.2 per 1,000 live births (although that’s not specific to fraternal twins).

And around the world, the chances of having twins is going up, generally.

So that’s all well and good, but it does beg the question, what are the factors that increase your chances of being a mama to fraternal twins?

Turns out, there are quite a few…

1. They look good in your genes

Are fraternal twins genetic?

Yes, hereditary genes definitely play a role here.

If you’re a fraternal twin, you may beget fraternal twins.

(Luckily, you’ll have some experience with the politics of being a twosome and should be able to parent accordingly.)

Chances are also increased if your siblings are fraternal twins or (gasp) you’ve already had a set of fraternal twins.

2. Your age

If you’re a mama who has amassed a few years of life experience under your belt, it may feel like the bit about your age plays like a stuck record when it comes to pregnancy.

“It’s because you’re older”, they seem to say over and over again about every question asked.

And yes, we’re about to push that ol’ chestnut down your throat again.

Fraternal twins can indeed be the result of being older, because, as biomedical scientist Kellie Leonard explains, “as we age, the amount of FSH increases in order to counteract the reduction in eggs within the ovaries”.

This phenomenon may just push an extra egg out of those ovaries at ovulation.

3. If you’ve had fertility treatment

Many fertility drugs, like letrozole and clomiphene (Clomid) serve to stimulate the ovaries.

The ovaries may get a little too excited by the whole thing and land up producing more than one egg at a time.

With more than one egg being released at a time, your chances of carrying twins increases.

4. You’re already a mama

This is an interesting one—the more babies you have, the higher your chances of conceiving fraternal twins.

Which is a little cruel of nature, one would think, when space is already at a premium.

And if you’ve already had fraternal twins?

Well, the chances of having fraternal twins twice are even higher.

5. Your weight

Strangely enough, the chances of having fraternal twins can increase if you have a higher BMI.

According to this study, “the odds of monozygous twinning were not significantly related to BMI, but the odds of dizygous twinning were significantly related to increased BMI”.

So identical twin pregnancies weren’t linked to BMI, but non-identical twin pregnancies were.

How rare are fraternal twins?

Still pretty rare, although they’re the most common type of twin!

23 in 1000 pregnancies will result in fraternal twins.

But if you have fraternal twins in your family, or you’re a fraternal twin, your chances could increase to 1 in 60.

Do fraternal twins have the same blood type?

Not always.

Sometimes, fraternal twins have different blood types.

Sometimes, they have the same blood type ‒ it’s just the same chances as two non-twin siblings having the same blood type.

Can fraternal twins have different eye colors?

Yes, it can happen ‒ after all, they’re siblings, but not necessarily identical.

Just as any two siblings can have different eye colors (or the same), so, too, can fraternal twins.

But can fraternal twins have different hair colors?

We’ll give you just the same answer ‒ sometimes.

Do fraternal twins have the same father?

It’s pretty rare, but “fraternal twins, different dads” is something that happens to a small percentage of mamas each year.

There’s no conclusive evidence to say just how many fraternal twins have different dads, although it could be as many as 1 in 400 fraternal twin pregnancies.

Although, usually, most fraternal twins have the same father.

Can fraternal twins be the same gender?

Yes, fraternal twins can be the same sex, but you can’t rely on having fraternal twins as a gender predictor ‒ there’s just as much chance of having fraternal twins with two different sexes.

Boy-girl, boy-boy, girl-girl ‒ your fraternal twins could be any of these combos!

Are the Olsen twins fraternal twins?

Ah, possibly the most famous twins in the world! So, are the Olsen twins identical or fraternal?

It might surprise you to learn that Mary-Kate and Ashley are fraternal twins ‒ not identical (or paternal).

They look similar because they’re siblings, not because they’re twins.

Which is the same reason why their younger sister, Elizabeth Olsen, also looks pretty similar.

Do fraternal twins share a placenta?

No, fraternal twins don’t share a placenta.

They have separate placentas and umbilical cords ‒ pretty much like they are their own individual pregnancy.

When do you know you’re having fraternal twins?

Your first-trimester ultrasound (8 weeks) will show fraternal twins.

You’ll see two fetuses, likely both in their own gestational sac, snuggled up together.

So cute. And so the fraternal twins bond begins!

How long do fraternal twin pregnancies last?

As you embark on this journey to becoming the mama of fraternal twins, you may find that the gestation period is usually shorter—around 32-38 weeks rather than the more typical 40 weeks.

This is because, seriously, there’s only so much your body can handle before it yells, “Everybody out!”.

Right now, you’re probably experiencing a beautifully-mixed bag of all the feels.

While you may be thrilled to hear the pitter-patter of little feet, that’s a whole lot of little feet to find shoes for.

While the thought of an extra mouth to feed may initially seem a little overwhelming, fraternal twins are a real genuine gift.

Your babies come with a built-in best friend.

Nothing will be able to break that fraternal twins bond.

They’ll know a bond stronger than any other—after all, they shared a womb before they even shared a room—and that is pretty damn special.

And if you’re after more support from fellow moms of multiples, or maybe even some twin baby name inspo, there’s a whole Community of twin mamas on Peanut.

And we think you and your duo will fit right in. 💕